TY - JOUR A1 - de Carvalho, Sidney J. A1 - Metzler, Ralf A1 - Cherstvy, Andrey G. T1 - Critical adsorption of polyelectrolytes onto planar and convex highly charged surfaces BT - the nonlinear Poisson–Boltzmann approach JF - New journal of physics : the open-access journal for physics N2 - We study the adsorption–desorption transition of polyelectrolyte chains onto planar, cylindrical and spherical surfaces with arbitrarily high surface charge densities by massive Monte Carlo computer simulations. We examine in detail how the well known scaling relations for the threshold transition—demarcating the adsorbed and desorbed domains of a polyelectrolyte near weakly charged surfaces—are altered for highly charged interfaces. In virtue of high surface potentials and large surface charge densities, the Debye–Hückel approximation is often not feasible and the nonlinear Poisson–Boltzmann approach should be implemented. At low salt conditions, for instance, the electrostatic potential from the nonlinear Poisson–Boltzmann equation is smaller than the Debye–Hückel result, such that the required critical surface charge density for polyelectrolyte adsorption σc increases. The nonlinear relation between the surface charge density and electrostatic potential leads to a sharply increasing critical surface charge density with growing ionic strength, imposing an additional limit to the critical salt concentration above which no polyelectrolyte adsorption occurs at all. We contrast our simulations findings with the known scaling results for weak critical polyelectrolyte adsorption onto oppositely charged surfaces for the three standard geometries. Finally, we discuss some applications of our results for some physical–chemical and biophysical systems. KW - polyelectrolyte adsorption KW - electrostatic interactions KW - critical phenomena KW - Debye screening Y1 - 2016 U6 - https://doi.org/10.1088/1367-2630/18/8/083037 SN - 1367-2630 VL - 18 PB - IOP Publ. CY - London ER - TY - GEN A1 - de Carvalho, Sidney J. A1 - Metzler, Ralf A1 - Cherstvy, Andrey G. T1 - Critical adsorption of polyelectrolytes onto planar and convex highly charged surfaces BT - the nonlinear Poisson–Boltzmann approach N2 - We study the adsorption–desorption transition of polyelectrolyte chains onto planar, cylindrical and spherical surfaces with arbitrarily high surface charge densities by massive Monte Carlo computer simulations. We examine in detail how the well known scaling relations for the threshold transition—demarcating the adsorbed and desorbed domains of a polyelectrolyte near weakly charged surfaces—are altered for highly charged interfaces. In virtue of high surface potentials and large surface charge densities, the Debye–Hückel approximation is often not feasible and the nonlinear Poisson–Boltzmann approach should be implemented. At low salt conditions, for instance, the electrostatic potential from the nonlinear Poisson–Boltzmann equation is smaller than the Debye–Hückel result, such that the required critical surface charge density for polyelectrolyte adsorption σc increases. The nonlinear relation between the surface charge density and electrostatic potential leads to a sharply increasing critical surface charge density with growing ionic strength, imposing an additional limit to the critical salt concentration above which no polyelectrolyte adsorption occurs at all. We contrast our simulations findings with the known scaling results for weak critical polyelectrolyte adsorption onto oppositely charged surfaces for the three standard geometries. Finally, we discuss some applications of our results for some physical–chemical and biophysical systems. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 292 KW - polyelectrolyte adsorption KW - electrostatic interactions KW - critical phenomena KW - Debye screening Y1 - 2016 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-100295 ER - TY - JOUR A1 - Caetano, Daniel L. Z. A1 - Carvalho, Sidney Jurado de A1 - Metzler, Ralf A1 - Cherstvy, Andrey G. T1 - Critical adsorption of multiple polyelectrolytes onto a nanosphere BT - splitting the adsorption-desorption transition boundary JF - Interface : journal of the Royal Society N2 - Employing extensive Monte Carlo computer simulations, we investigate in detail the properties of multichain adsorption of charged flexible polyelectrolytes (PEs) onto oppositely charged spherical nanoparticles (SNPs). We quantify the conditions of critical adsorption-the phase-separation curve between the adsorbed and desorbed states of the PEs-as a function of the SNP surface-charge density and the concentration of added salt. We study the degree of fluctuations of the PE-SNP electrostatic binding energy, which we use to quantify the emergence of the phase subtransitions, including a series of partially adsorbed PE configurations. We demonstrate how the phase-separation adsorption-desorption boundary shifts and splits into multiple subtransitions at low-salt conditions, thereby generalizing and extending the results for critical adsorption of a single PE onto the SNP. The current findings are relevant for finite concentrations of PEs around the attracting SNP, such as the conditions for PE adsorption onto globular proteins carrying opposite electric charges. KW - nanoparticles KW - polyelectrolytes KW - electrostatics KW - critical adsorption KW - phase-transition boundary Y1 - 2020 U6 - https://doi.org/10.1098/rsif.2020.0199 SN - 1742-5689 SN - 1742-5662 VL - 17 IS - 167 PB - Royal Society CY - London ER - TY - THES A1 - Balzer, Arnim T1 - Crab flare observations with H.E.S.S. phase II T1 - Crab Flare Observations with H.E.S.S. Phase II N2 - The H.E.S.S. array is a third generation Imaging Atmospheric Cherenkov Telescope (IACT) array. It is located in the Khomas Highland in Namibia, and measures very high energy (VHE) gamma-rays. In Phase I, the array started data taking in 2004 with its four identical 13 m telescopes. Since then, H.E.S.S. has emerged as the most successful IACT experiment to date. Among the almost 150 sources of VHE gamma-ray radiation found so far, even the oldest detection, the Crab Nebula, keeps surprising the scientific community with unexplained phenomena such as the recently discovered very energetic flares of high energy gamma-ray radiation. During its most recent flare, which was detected by the Fermi satellite in March 2013, the Crab Nebula was simultaneously observed with the H.E.S.S. array for six nights. The results of the observations will be discussed in detail during the course of this work. During the nights of the flare, the new 24 m × 32 m H.E.S.S. II telescope was still being commissioned, but participated in the data taking for one night. To be able to reconstruct and analyze the data of the H.E.S.S. Phase II array, the algorithms and software used by the H.E.S.S. Phase I array had to be adapted. The most prominent advanced shower reconstruction technique developed by de Naurois and Rolland, the template-based model analysis, compares real shower images taken by the Cherenkov telescope cameras with shower templates obtained using a semi-analytical model. To find the best fitting image, and, therefore, the relevant parameters that describe the air shower best, a pixel-wise log-likelihood fit is done. The adaptation of this advanced shower reconstruction technique to the heterogeneous H.E.S.S. Phase II array for stereo events (i.e. air showers seen by at least two telescopes of any kind), its performance using MonteCarlo simulations as well as its application to real data will be described. N2 - Das H.E.S.S. Experiment misst sehr hochenergetische Gammastrahlung im Khomas Hochland von Namibia. Es ist ein sogenanntes abbildendes atmosphärisches Cherenkov-Teleskopsystem welches in der 1. Phase, die im Jahr 2004 mit der Datennahme begann, aus vier identischen 13 m Spiegelteleskopen bestand. Seitdem hat sich H.E.S.S. als das erfolgreichstes Experiment in der bodengebundenen Gammastrahlungsastronomie etabliert. Selbst die älteste der mittlerweile fast 150 entdeckten Quellen von sehr hochenergetischer Gammastrahlung, der Krebsnebel, fasziniert immernoch Wissenschaftler mit neuen bisher unbekannten und unerwarteten Phänomenen. Ein Beispiel dafür sind die vor kurzem entdeckten sehr energiereichen Ausbrüche von hochenergetischer Gammastrahlung. Bei dem letzten deratigen Ausbruch des Krebsnebels im März 2013 hat das H.E.S.S. Experiment für sechs Nächte simultan mit dem Fermi-Satelliten, welcher den Ausbruch entdeckte, Daten genommen. Die Analyse der Daten, deren Ergebnis und deren Interpretation werden im Detail in dieser Arbeit vorgestellt. Während dieser Beobachtungen befand sich ein neues 24 m × 32 m großes Spiegelteleskop, das H.E.S.S. II- Teleskop, noch in seiner Inbetriebnahme, trotzdem hat es für eine dieser sechs Nächte an der Datennahme des gesamten Teleskopsystems teilgenommen. Um die Daten rekonstruieren und analysieren zu können, mussten die für die 1. Phase des Experiments entwickelten Algorithmen und die Software des H.E.S.S.- Experiments angepasst werden. Die fortschrittlichste Schauerrekonstruktionsmethode, welche von de Naurois und Rolland entwickelt wurde, basiert auf dem Vergleich von echten Schauerbildern, die mit Hilfe der Cherenkov-Kameras der einzelnen Teleskope aufgenommen wurden, mit Schauerschablonen die mit Hilfe eines semianalytischen Modells erzeugt wurden. Das am besten passende Bild und damit auch alle relevanten Schauerparameter, wird mit Hilfe einer pixelweisen Loglikelihood-Anpassung ermittelt. Die nötigen Änderungen um Multiteleskopereignisse, welche vom heterogenen H.E.S.S. Phase II Detektor gemessen wurden, mit Hilfe dieser fortschrittlichen Schauerrekonstruktionsmethode analysieren zu können, sowie die resultierenden Ergebnisse von MonteCarlo-Simulationen, als auch die Anwendung auf echte Daten, werden im Rahmen dieser Arbeit präsentiert. KW - Gammastrahlungsastronomie KW - Rekonstruktionsmethoden KW - Datenanalyse KW - Krebsnebel KW - gamma-ray astronomy KW - reconstruction methods KW - data analysis KW - Crab Nebula Y1 - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-72545 ER - TY - JOUR A1 - Jay, Raphael Martin A1 - Eckert, Sebastian A1 - Vaz da Cruz, Vinicius A1 - Fondell, Mattis A1 - Mitzner, Rolf A1 - Föhlisch, Alexander T1 - Covalency-driven preservation of local charge densities in a metal-to-ligand charge-transfer excited iron photosensitizer JF - Angewandte Chemie : a journal of the Gesellschaft Deutscher Chemiker ; International edition N2 - Charge-density rearrangements after metal-to-ligand charge-transfer excitation in an iron photosensitizer are investigated by R. M Jay, A. Fohlisch et al. in their Communication (DOI: 10.1002/anie.201904761). By using time-resolved X-ray absorption spectroscopy, surprising covalency-effects are revealed that inhibit charge-separation at the intra-molecular level. Furthermore, the underlying mechanism is proposed to be generally in effect for all commonly used photosensitizers in light-harvesting applications, which challenges the common perception of electronic charge-transfer. KW - charge-transfer KW - density functional calculations KW - iron KW - photochemistry KW - X-ray absorption spectroscopy Y1 - 2019 U6 - https://doi.org/10.1002/anie.201904761 SN - 1433-7851 SN - 1521-3773 VL - 58 IS - 31 SP - 10742 EP - 10746 PB - Wiley-VCH CY - Weinheim ER - TY - THES A1 - Ganesan, Lakshmi Meena T1 - Coupling of the electrical, mechanical and optical response in polymer/liquid-crystal composites T1 - Kopplung von elektrischen, mechanischen und optischen Effekten in einem Polymer/Flüssigkristall-Komposit N2 - Micrometer-sized liquid-crystal (LC) droplets embedded in a polymer matrix may enable optical switching in the composite film through the alignment of the LC director along an external electric field. When a ferroelectric material is used as host polymer, the electric field generated by the piezoelectric effect can orient the director of the LC under an applied mechanical stress, making these materials interesting candidates for piezo-optical devices. In this work, polymer-dispersed liquid crystals (PDLCs) are prepared from poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) and a nematic liquid crystal (LC). The anchoring effect is studied by means of dielectric relaxation spectroscopy. Two dispersion regions are observed in the dielectric spectra of the pure P(VDF-TrFE) film. They are related to the glass transition and to a charge-carrier relaxation, respectively. In PDLC films containing 10 and 60 wt% LC, an additional, bias-field-dependent relaxation peak is found that can be attributed to the motion of LC molecules. Due to the anchoring effect of the LC molecules, this relaxation process is slowed down considerably, when compared with the related process in the pure LC. The electro-optical and piezo-optical behavior of PDLC films containing 10 and 60 wt% LCs is investigated. In addition to the refractive-index mismatch between the polymer matrix and the LC molecules, the interaction between the polymer dipoles and the LC molecules at the droplet interface influences the light-scattering behavior of the PDLC films. For the first time, it was shown that the electric field generated by the application of a mechanical stress may lead to changes in the transmittance of a PDLC film. Such a piezo-optical PDLC material may be useful e.g. in sensing and visualization applications. Compared to a non-polar matrix polymer, the polar matrix polymer exhibits a strong interaction with the LC molecules at the polymer/LC interface which affects the electro-optical effect of the PDLC films and prevents a larger increase in optical transmission. N2 - Mikrometer-große, in eine Polymermatrix eingebettete Flüssigkristall-Tröpfchen können als elektro-optische Lichtventile fungieren, da die Ausrichtung der Flüssigkristalle durch ein externes elektrisches Feld verändert werden kann. Wird nun ein ferroelektrisches Polymer als Matrix verwendet, so kann das durch den piezoelektrischen Effekt erzeugte und von der äußeren mechanischen Spannung abhängige elektrische Feld den Flüssigkristall ausrichten. Solche Materialien können daher als piezo-optische Lichtventile eingesetzt werden. Im Rahmen dieser Arbeit wurden PDLCs (polymer-dispersed liquid crystals) durch Einbettung von nematischen Flüssigkristallen in Poly(Vinylidenefluoride-Trifluorethylene) (P(VDF-TrFE)) erzeugt. Die Wechselwirkungen an der Grenzfläche zwischen Flüssigkristall und Polymer wurden mittels dielektrischer Spektroskopie untersucht. Im dielektrischen Spektrum des reinen P(VDF-TrFE) wurden zwei Dispersions-Regionen beobachtet, welche vom Glasübergang und einer Ladungsträgerrelaxation des Polymers herrühren. PDLC Folien mit unterschiedlichen Anteilen von Flüssigkristall-Tröpfchen (10 bzw. 60 Gewichtsprozente) zeigten beim Anlegen eines elektrischen Wechselfelds zusatzliche Relaxationseffekte, welche der Bewegung der eingebetteten Flüssigkristall-Moleküle zugeordnet werden konnten. Durch die Einlagerung der Flüssigkristall-Moleküle weist die Struktur eine Relaxation auf, die gegenüber vergleichbaren Prozessen im reinen Flüssigkristall deutlich verlangsamt ist. Des weiteren wurde das elektrooptische und piezo-optische Verhalten der mit 10 und 60 Gewichtsprozent Flüssigkristall geladenen Folien untersucht. Die Lichtstreuung hängt dabei ab von der Fehlanpassung der Brechungsindizes von Polymermatrix und Flüssigkristallen sowie von den Wechselwirkungen der Polymerdipole mit den Flüssigkristall-Molekülen an der Tröpfchenoberfläche. Es konnte erstmalig gezeigt werden, dass die Lichtdurchlässigkeit der PDLC-Folien durch eine externe mechanische Spannung gesteuert werden kann. Dieser Effekt macht das piezo-optische PDLC-Material für die Verwendung in Optik- und Sensoranwendungen interessant. Im Vergleich mit unpolaren Wirtspolymeren zeigen polare Wirtsmaterialien eine deutlich stärkere Wechselwirkung zwischen den Flüssigkristall-Molekülen an der Polymer/Flüssigkristall-Grenzfläche, welche den elektrooptischen Effekt beeinflusst und so die maximale transmissions änderung reduziert. KW - PDLC KW - P(VDF-TrFE) KW - nematic LC KW - piezo-optisch KW - elektro-optisch KW - PDLC KW - P(VDF-TrFE) KW - nematic LC KW - piezo-optical KW - electro-optical Y1 - 2010 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-41572 ER - TY - THES A1 - Sahlmann, Hanno T1 - Coupling matter to loop quantum gravity N2 - Motiviert durch neuere Vorschläge zur experimentellen Untersuchung von Quantengravitationseffekten werden in der vorliegenden Arbeit Annahmen und Methoden untersucht, die für die Vorhersagen solcher Effekte im Rahmen der Loop-Quantengravitation verwendet werden können. Dazu wird als Modellsystem ein skalares Feld, gekoppelt an das Gravitationsfeld, betrachtet. Zunächst wird unter bestimmten Annahmen über die Dynamik des gekoppelten Systems eine Quantentheorie für das Skalarfeld vorgeschlagen. Unter der Annahme, dass sich das Gravitationsfeld in einem semiklassischen Zustand befindet, wird dann ein "QFT auf gekrümmter Raumzeit-Limes" dieser Theorie definiert. Im Gegensatz zur gewöhnlichen Quantenfeldtheorie auf gekrümmter Raumzeit beschreibt die Theorie in diesem Grenzfall jedoch ein quantisiertes Skalarfeld, das auf einem (klassisch beschriebenen) Zufallsgitter propagiert. Sodann werden Methoden vorgeschlagen, den Niederenergieliemes einer solchen Gittertheorie, vor allem hinsichtlich der resultierenden modifizierten Dispersonsrelation, zu berechnen. Diese Methoden werden anhand von einfachen Modellsystemen untersucht. Schließlich werden die entwickelten Methoden unter vereinfachenden Annahmen und der Benutzung einer speziellen Klasse von semiklassischen Zuständen angewandt, um Korrekturen zur Dispersionsrelation des skalaren und des elektromagnetischen Feldes im Rahmen der Loop-Quantengravitation zu berechnen. Diese Rechnungen haben vorläufigen Charakter, da viele Annahmen eingehen, deren Gültigkeit genauer untersucht werden muss. Zumindest zeigen sie aber Probleme und Möglichkeiten auf, im Rahmen der Loop-Quantengravitation Vorhersagen zu machen, die sich im Prinzip experimentell verifizieren lassen. N2 - Motivated by recent proposals on the experimental detectability of quantum gravity effects, the present thesis investigates assumptions and methods which might be used for the prediction of such effects within the framework of loop quantum gravity. To this end, a scalar field coupled to gravity is considered as a model system. Starting from certain assumptions about the dynamics of the coupled gravity-matter system, a quantum theory for the scalar field is proposed. Then, assuming that the gravitational field is in a semiclassical state, a "QFT on curved space-time limit" of this theory is defined. In contrast to ordinary quantum field theory on curved space-time however, in this limit the theory describes a quantum scalar field propagating on a (classical) random lattice. Then, methods to obtain the low energy limit of such a lattice theory, especially regarding the resulting modified dispersion relations, are discussed and applied to simple model systems. Finally, under certain simplifying assumptions, using the methods developed before as well as a specific class of semiclassical states, corrections to the dispersion relations for the scalar and the electromagnetic field are computed within the framework of loop quantum gravity. These calculations are of preliminary character, as many assumptions enter whose validity remains to be studied more thoroughly. However they exemplify the problems and possibilities of making predictions based on loop quantum gravity that are in principle testable by experiment. KW - Quantengravitation KW - Loop-Quantengravitation KW - semiklassische Zustände KW - Dispersionsrelationen KW - quantum gravity KW - loop quantum gravity KW - semiclassical states KW - dispersion relations Y1 - 2002 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-0000602 ER - TY - JOUR A1 - Sander, Andreas Alexander Christoph A1 - Fürst, F. A1 - Kretschmar, P. A1 - Oskinova, Lidia M. A1 - Todt, Helge Tobias A1 - Hainich, Rainer A1 - Shenar, Tomer A1 - Hamann, Wolf-Rainer T1 - Coupling hydrodynamics with comoving frame radiative transfer BT - Stellar wind stratification in the high-mass X-ray binary Vela X-1 JF - Astronomy and astrophysics : an international weekly journal N2 - Aims. To gain a realistic picture of the donor star in Vela X-1, we constructed a hydrodynamically consistent atmosphere model describing the wind stratification while properly reproducing the observed donor spectrum. To investigate how X-ray illumination affects the stellar wind, we calculated additional models for different X-ray luminosity regimes. Methods. We used the recently updated version of the Potsdam Wolf-Rayet code to consistently solve the hydrodynamic equation together with the statistical equations and the radiative transfer. Results. The wind flow in Vela X-1 is driven by ions from various elements, with Fe III and S III leading in the outer wind. The model-predicted mass-loss rate is in line with earlier empirical studies. The mass-loss rate is almost unaffected by the presence of the accreting NS in the wind. The terminal wind velocity is confirmed at u(infinity) approximate to 600 km s(-1). On the other hand, the wind velocity in the inner region where the NS is located is only approximate to 100 km s(-1), which is not expected on the basis of a standard beta-velocity law. In models with an enhanced level of X-rays, the velocity field in the outer wind can be altered. If the X-ray flux is too high, the acceleration breaks down because the ionization increases. Conclusions. Accounting for radiation hydrodynamics, our Vela X-1 donor atmosphere model reveals a low wind speed at the NS location, and it provides quantitative information on wind driving in this important HMXB. KW - stars: mass-loss KW - stars: winds, outflows KW - stars: early-type KW - stars: atmospheres KW - stars: massive KW - X-rays: binaries Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201731575 SN - 1432-0746 VL - 610 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Nemati, Somayyeh A1 - Henkel, Carsten A1 - Anders, Janet T1 - Coupling function from bath density of states JF - epl : a letters journal exploring the frontiers of physics N2 - Modelling of an open quantum system requires knowledge of parameters that specify how it couples to its environment. However, beyond relaxation rates, realistic parameters for specific environments and materials are rarely known. Here we present a method of inferring the coupling between a generic system and its bosonic (e.g., phononic) environment from the experimentally measurable density of states (DOS). With it we confirm that the DOS of the well-known Debye model for three-dimensional solids is physically equivalent to choosing an Ohmic bath. We further match a real phonon DOS to a series of Lorentzian coupling functions, allowing us to determine coupling parameters for gold, yttrium iron garnet (YIG) and iron as examples. The results illustrate how to obtain material-specific dynamical properties, such as memory kernels. The proposed method opens the door to more accurate modelling of relaxation dynamics, for example for phonon-dominated spin damping in magnetic materials. Y1 - 2022 U6 - https://doi.org/10.1209/0295-5075/ac7b42 SN - 0295-5075 SN - 1286-4854 VL - 139 IS - 3 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Gong, Chen Chris A1 - Tönjes, Ralf A1 - Pikovsky, Arkady T1 - Coupled Möbius maps as a tool to model Kuramoto phase synchronization JF - Physical review : E, Statistical, nonlinear and soft matter physics N2 - We propose Mobius maps as a tool to model synchronization phenomena in coupled phase oscillators. Not only does the map provide fast computation of phase synchronization, it also reflects the underlying group structure of the sinusoidally coupled continuous phase dynamics. We study map versions of various known continuous-time collective dynamics, such as the synchronization transition in the Kuramoto-Sakaguchi model of nonidentical oscillators, chimeras in two coupled populations of identical phase oscillators, and Kuramoto-Battogtokh chimeras on a ring, and demonstrate similarities and differences between the iterated map models and their known continuous-time counterparts. Y1 - 2020 U6 - https://doi.org/10.1103/PhysRevE.102.022206 SN - 2470-0045 SN - 2470-0053 SN - 1063-651X SN - 2470-0061 SN - 1550-2376 VL - 102 IS - 2 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Vafin, Sergei A1 - Riazantseva, Maria A1 - Pohl, Martin T1 - Coulomb collisions as a candidate for temperature anisotropy constraints in the solar wind JF - The astrophysical journal : an international review of spectroscopy and astronomical physics ; Part 2, Letters N2 - Many solar wind observations at 1 au indicate that the proton (as well as electron) temperature anisotropy is limited. The data distribution in the (A(a), beta(a),(parallel to))-plane have a rhombic-shaped form around beta(a),(parallel to) similar to 1. The boundaries of the temperature anisotropy at beta(a),(parallel to) > 1 can be well explained by the threshold conditions of the mirror (whistler) and oblique proton (electron) firehose instabilities in a bi-Maxwellian plasma, whereas the physical mechanism of the similar restriction at beta(a),(parallel to) < 1 is still under debate. One possible option is Coulomb collisions, which we revisit in the current work. We derive the relaxation rate nu(A)(aa) of the temperature anisotropy in a bi-Maxwellian plasma that we then study analytically and by observed proton data from WIND. We found that nu(A)(pp) increases toward small beta(p),(parallel to) < 1. We matched the data distribution in the (A(p), beta(p),(parallel to))-plane with the constant contour nu(A)(pp) = 2.8 . 10(-6) s(-1), corresponding to the minimum value for collisions to play a role. This contour fits rather well the left boundary of the rhombic-shaped data distribution in the (A(p), beta(p),(parallel to))-plane. Thus, Coulomb collisions are an interesting candidate for explaining the limitations of the temperature anisotropy in the solar wind with small beta(a),(parallel to) < 1 at 1 au. KW - instabilities KW - plasmas KW - scattering KW - solar wind KW - Sun: heliosphere Y1 - 2019 U6 - https://doi.org/10.3847/2041-8213/aafb11 SN - 2041-8205 SN - 2041-8213 VL - 871 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - THES A1 - Gil-Merino Rubio, Rodrigo T1 - Cosmology through gravitational lenses N2 - In dieser Dissertation nutze ich den Gravitationslinseneffekt, um eine Reihe von kosmologischen Fragen zu untersuchen. Der Laufzeitunterschied des Gravitationslinsensystems HE1104-1805 wurde mit unterschiedlichen Methoden bestimmt. Zwischen den beiden Komponenten erhalte ich einen Unterschied von Delta_t(A-B) = -310 +-20 Tagen (2 sigma Konfidenzintervall). Außerdem nutze ich eine dreijährige Beobachtungskampagne, um den Doppelquasar Q0957+561 zu untersuchen. Die beobachteten Fluktuationen in den Differenzlichtkurven lassen sich durch Rauschen erklären, ein Mikrogravitationslinseneffekt wird zur Erklärung nicht benötigt. Am Vierfachquasar Q2237+0305 untersuchte ich den Mikrogravitationslinseneffekt anhand der Daten der GLITP-Kollaboration (Okt. 1999-Feb. 2000). Durch die Abwesenheit eines starken Mikrogravitationslinsensignals konnte ich eine obere Grenze von v=600 km/s f für die effektive Transversalgeschwindigkeit der Linsengalaxie bestimmen (unter der Annahme von Mikrolinsen mit 0.1 Sonnenmassen). Im zweiten Teil der Arbeit untersuchte ich die Verteilung der Dunklen Materie in Galaxienhaufen. Für den Galaxienhaufen Cl0024+1654 erhalte ich ein Masse-Leuchtkraft-Verhältnis von M/L = 200 M_sun/L_sun (innerhalb eines Radius von 3 Bogenminuten). Im Galaxienhaufen RBS380 finde ich eine relativ geringe Röntgenleuchtkraft von L =2*10^(44) erg/s, obwohl im optischen eine große Anzahl von Galaxien gefunden wurde. N2 - In this thesis the gravitational lensing effect is used to explore a number of cosmological topics. We determine the time delay in the gravitationally lensed quasar system HE1104-1805 using different techniques. We obtain a time delay Delta_t(A-B) Delta_t(A-B) =-310 +- 20 days (2 sigma errors) between the two components. We also study the double quasar Q0957+561 during a three years monitoring campaign. The fluctuations we find in the difference light curves are completely consistent with noise and no microlensing is needed to explain these fluctuations. Microlensing is also studied in the quadruple quasar Q2237+0305 during the GLITP collaboration (Oct.1999-Feb.2000). We use the absence of a strong microlensing signal to obtain an upper limit of v=600 km/s for the effective transverse velocity of the lens galaxy (considering microlenses with 0.1 solar masses). The distribution of dark matter in galaxy clusters is also studied in the second part of the thesis. In the cluster of galaxies Cl0024+1654 we obtain a mass-to-light ratio of M/L = 200 M_sun/L_sun (within a radius of 3 arcminutes). In the galaxy cluster RBS380 we find a relatively low X-ray luminosity for a massive cluster of L =2*10^(44) erg/s, but a rich distribution of galaxies in the optical band. KW - Gravitationslinsen KW - Mikrolinsen KW - Quasar KW - gravitational lensing KW - microlensing KW - quasar Y1 - 2003 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-0001030 ER - TY - THES A1 - Schmidt, Robert W. T1 - Cosmological applications of gravitational lensing N2 - In dieser Arbeit benutze ich den Gravitationslinseneffekt als ein Werkzeug, um zwei recht unterschiedliche kosmologische Fragestellungen zu bearbeiten: die Natur der dunklen Materie in Galaxienhalos und die Rotation des Universums. Zuerst untersuche ich den Mikrolinseneffekt in den Gravitationlinsensystemen Q0957+561 und Q2237+0305. In diesen Systemen scheint das Licht eines Quasars durch die Linsengalaxie hindurch. Aufgrund der Relativbewegung zwischen Quasar, Linsengalaxie und Beobachter verursachen kompakte Objekte innerhalb der Galaxie oder dem Galaxienhalo Helligkeitsfluktuationen des Hintergrundquasars. Ich vergleiche die am 3.5m Teleskop des Apache Point Observatory zwischen 1995 und 1998 gewonnene Lichtkurve des Doppelquasars Q0957+561 (Colley, Kundic & Turner 2000) mit numerischen Simulationen, um zu untersuchen, ob der Halo der Linsengalaxie aus massiven kompakten Objekten (MACHOs) besteht. Dieser Test wurde zuerst von Gott (1981) vorgeschlagen. Ich kann MACHO-Massen von 10^-6 M_sun bis zu 10^-2 M_sun ausschliessen, sofern der Quasar kleiner ist als 3x10^14 h_60^-0.5 cm und MACHOs mehr als 50% des dunklen Halos ausmachen. Im zweiten Teil der Arbeit stelle ich neue Beobachtungsdaten fuer den Vierfachquasar Q2237+0305 vor, die am 3.5m Teleskop des Apache Point Observatory zwischen Juni 1995 und Januar 1998 gewonnen wurden. Obwohl die Daten bei veraenderlichen, oft schlechten Seeing Bedingungen und grober Pixelaufloesung aufgenommen wurden, ist die Photometrie der beiden helleren Quasarbilder A und B mit Hilfe von HST-Beobachtungen moeglich. Ich finde ein Helligkeitsmaximum in Bild A mit einer Amplitude von 0.4 bis 0.5 mag und einer Dauer von wenigstens 100 Tagen. Dies zeigt, dass in der Linsengalaxie der Mikrolinseneffekt stattgefunden hat. Im abschliessenden Teil der Arbeit benutze ich dann den schwachen Gravitationslinseneffekt, um Grenzen fuer eine Klasse von rotierenden Kosmologien vom Goedel-Typ zu ermitteln, die von Korotky & Obukhov (1996) beschrieben wurde. In Studien des schwachen Linseneffektes werden die Formen von tausenden von Hintergrundgalaxien vermessen und gemittelt. Dabei werden kohaerente Verzerrungen der Galaxienformen gemessen, die von Massenverteilungen im Vordergrund oder von der grossraeumigen Struktur der Raumzeit selbst verursacht werden. Ich berechne die vorhergesagte Scherung als Funktion der Rotverschiebung in rotierenden Kosmologien vom Goedel-Typ und vergleiche diese mit der oberen Grenze fuer die kosmische Scherung gamma_limit von 0.04, die in Studien des schwachen Linseneffektes gewonnen wurde. Dieser Vergleich zeigt, dass Modelle vom Goedel-Typ keine groesseren Rotationen omega als H_0=6.1x10^-11 h_60/Jahr haben koennen, wenn die Grenze fuer die kosmische Scherung fuer den ganzen Himmel gilt. N2 - In this thesis we use the gravitational lensing effect as a tool to tackle two rather different cosmological topics: the nature of the dark matter in galaxy halos, and the rotation of the universe. Firstly, we study the microlensing effect in the gravitational lens systems Q0957+561 and Q2237+0305. In these systems the light from the quasar shines directly through the lensing galaxy. Due to the relative motion of the quasar, the lensing galaxy, and the observer compact objects in the galaxy or galaxy halo cause brightness fluctuations of the light from the background quasar. We compare light curve data from a monitoring program of the double quasar Q0957+561 at the 3.5m telescope at Apache Point Observatory from 1995 to 1998 (Colley, Kundic & Turner 2000) with numerical simulations to test whether the halo of the lensing galaxy consists of massive compact objects (MACHOs). This test was first proposed by Gott (1981). We can exclude MACHO masses from 10^-6 M_sun up to 10^-2 M_sun for quasar sizes of less than 3x10^14 h_60^-0.5 cm if the MACHOs make up at least 50% of the dark halo. Secondly, we present new light curve data for the gravitationally lensed quadruple quasar Q2237+0305 taken at the 3.5m telescope at Apache Point Observatory from June 1995 to January 1998. Although the images were taken under variable, often poor seeing conditions and with coarse pixel sampling, photometry is possible for the two brighter quasar images A and B with the help from HST observations. We find independent evidence for a brightness peak in image A of 0.4 to 0.5 mag with a duration of at least 100 days, which indicates that microlensing has taken place in the lensing galaxy. Finally, we use the weak gravitational lensing effect to put limits on a class of Goedel-type rotating cosmologies described by Korotky & Obukhov (1996). In weak lensing studies the shapes of thousands of background galaxies are measured and averaged to reveal coherent gravitational distortions of the galaxy shapes by foreground matter distributions, or by the large-scale structure of space-time itself. We calculate the predicted shear as a function of redshift in Goedel-type rotating cosmologies and compare this to the upper limit on cosmic shear gamma_limit of approximately 0.04 from weak lensing studies. We find that Goedel-type models cannot have larger rotations omega than H_0=6.1x10^-11 h_60/year if this shear limit is valid for the whole sky. KW - Gravitationslinseneffekt KW - Quasare KW - Kosmologie KW - dunkle Materie Y1 - 2000 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-0000261 ER - TY - JOUR A1 - Saha, Sourav A1 - Owen, Lewis A. A1 - Orr, Elizabeth N. A1 - Caffee, Marc W. T1 - Cosmogenic Be-10 and equilibrium-line altitude dataset of Holocene glacier advances in the Himalayan-Tibetan orogen JF - Data in brief N2 - A comprehensive analysis of the variable temporal and spatial responses of tropical-subtropical high-altitude glaciers to climate change is critical for successful model predictions and environmental risk assessment in the Himalayan-Tibetan orogen. High-frequency Holocene glacier chronostratigraphies are therefore reconstructed in 79 glaciated valleys across the orogen using 519 published and 16 new terrestrial cosmogenic 10Be exposure age dataset. Published 10Be ages are compiled only for moraine boulders (excluding bedrock ages). These ages are recalculated using the latest ICE-D production rate calibration database and the scaling scheme models. Outliers for the individual moraine are detected using the Chauvenet's criterion. In addition, past equilibrium-line altitudes (ELAs) are determined using the area-altitude (AA), area accumulation ratio (AAR), and toe-headwall accumulation ratio (THAR) methods for each glacier advance. The modern maximum elevations of lateral moraines (MELM) are also used to estimate modern ELAs and as an independent check on mean ELAs derived using the above three methods. These data may serve as an essential archive for future studies focusing on the cryospheric and environmental changes in the Himalayan-Tibetan orogen. A more comprehensive analysis of the published and new 10Be ages and ELA results and a list of references are presented in Saha et al. (2019, High-frequency Holocene glacier fluctuations in the Himalayan-Tibetan orogen. Quaternary Science Reviews, 220, 372–400). KW - Cosmogenic nuclides KW - Equilibrium-line altitudes KW - Holocene KW - Central asia KW - Glaciation Y1 - 2019 U6 - https://doi.org/10.1016/j.dib.2019.104412 SN - 2352-3409 VL - 26 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Maiti, Snehanshu A1 - Makwana, Kirit A1 - Zhang, Heshou A1 - Yan, Huirong T1 - Cosmic-ray transport in magnetohydrodynamic turbulence JF - The astrophysical journal : an international review of spectroscopy and astronomical physics / part 1 N2 - This paper studies cosmic-ray (CR) transport in magnetohydrodynamic (MHD) turbulence. CR transport is strongly dependent on the properties of the magnetic turbulence. We perform test particle simulations to study the interactions of CR with both total MHD turbulence and decomposed MHD modes. The spatial diffusion coefficients and the pitch angle scattering diffusion coefficients are calculated from the test particle trajectories in turbulence. Our results confirm that the fast modes dominate the CR propagation, whereas Alfven and slow modes are much less efficient and have shown similar pitch-angle scattering rates. We investigate the cross field transport on large and small scales. On large/global scales, normal diffusion is observed and the diffusion coefficient is suppressed by M-A(zeta) compared to the parallel diffusion coefficients, with zeta closer to 4 in Alfven modes than that in total turbulence, as theoretically expected. For the CR transport on scales smaller than the turbulence injection scale, both the local and global magnetic reference frames are adopted. Superdiffusion is observed on such small scales in all the cases. Particularly, CR transport in Alfven modes show clear Richardson diffusion in the local reference frame. The diffusion transitions smoothly from the Richardson's one with index 1.5 to normal diffusion as the particle mean free path decreases from lambda(parallel to) >> L to lambda(parallel to) << L, where L is the injection/coherence length of turbulence. Our results have broad applications to CRs in various astrophysical environments. Y1 - 2022 U6 - https://doi.org/10.3847/1538-4357/ac46c8 SN - 1538-4357 VL - 926 IS - 1 PB - Institute of Physics Publ. CY - London ER - TY - THES A1 - Shaw, Vasundhara T1 - Cosmic-ray transport and signatures in their local environment N2 - The origin and structure of magnetic fields in the Galaxy are largely unknown. What is known is that they are essential for several astrophysical processes, in particular the propagation of cosmic rays. Our ability to describe the propagation of cosmic rays through the Galaxy is severely limited by the lack of observational data needed to probe the structure of the Galactic magnetic field on many different length scales. This is particularly true for modelling the propagation of cosmic rays into the Galactic halo, where our knowledge of the magnetic field is particularly poor. In the last decade, observations of the Galactic halo in different frequency regimes have revealed the existence of out-of-plane bubble emission in the Galactic halo. In gamma rays these bubbles have been termed Fermi bubbles with a radial extent of ≈ 3 kpc and an azimuthal height of ≈ 6 kpc. The radio counterparts of the Fermi bubbles were seen by both the S-PASS telescopes and the Planck satellite, and showed a clear spatial overlap. The X-ray counterparts of the Fermi bubbles were named eROSITA bubbles after the eROSITA satellite, with a radial width of ≈ 7 kpc and an azimuthal height of ≈ 14 kpc. Taken together, these observations suggest the presence of large extended Galactic Halo Bubbles (GHB) and have stimulated interest in exploring the less explored Galactic halo. In this thesis, a new toy model (GHB model) for the magnetic field and non-thermal electron distribution in the Galactic halo has been proposed. The new toy model has been used to produce polarised synchrotron emission sky maps. Chi-square analysis was used to compare the synthetic skymaps with the Planck 30 GHz polarised skymaps. The obtained constraints on the strength and azimuthal height were found to be in agreement with the S-PASS radio observations. The upper, lower and best-fit values obtained from the above chi-squared analysis were used to generate three separate toy models. These three models were used to propagate ultra-high energy cosmic rays. This study was carried out for two potential sources, Centaurus A and NGC 253, to produce magnification maps and arrival direction skymaps. The simulated arrival direction skymaps were found to be consistent with the hotspots of Centaurus A and NGC 253 as seen in the observed arrival direction skymaps provided by the Pierre Auger Observatory (PAO). The turbulent magnetic field component of the GHB model was also used to investigate the extragalactic dipole suppression seen by PAO. UHECRs with an extragalactic dipole were forward-tracked through the turbulent GHB model at different field strengths. The suppression in the dipole due to the varying diffusion coefficient from the simulations was noted. The results could also be compared with an analytical analogy of electrostatics. The simulations of the extragalactic dipole suppression were in agreement with similar studies carried out for galactic cosmic rays. N2 - Unsere Galaxie wird ständig von hochenergetischen geladenen Teilchen unterschiedlicher Energie bombardiert, die als kosmische Strahlung bezeichnet werden und deren Ursprung nicht bekannt ist. Satelliten- und erdgestützte Messungen haben bisher ergeben, dass es in unserer Galaxie Beschleuniger für kosmische Strahlung gibt, z. B. die Überreste explodierender Sterne (Supernova-Überreste), aber bei den höchsten kosmischen Strahlungsenergien bleiben die Quellen ein Rätsel. Fortschritte zu erzielen ist eine Herausforderung, auch weil die kosmische Strahlung durch Magnetfelder abgelenkt wird, was bedeutet, dass die beobachtete Richtung mit der Richtung der Quelle übereinstimmen kann oder auch nicht. Unsere Galaxie weist starke Magnetfelder auf, deren Beschaffenheit noch nicht gut verstanden ist, insbesondere in der Komponente außerhalb der Scheibe (dem galaktischen Halo). Darüber hinaus haben Beobachtungen in den letzten zehn Jahren blasenartige Strukturen im galaktischen Halo mit enormen Gesamtenergien aufgedeckt, die auch als galaktische Halo-Blasen bezeichnet werden. All dies motiviert uns, den galaktischen Halo zu untersuchen. In meiner Doktorarbeit schlagen wir ein neues, vereinfachtes Magnetfeldmodell für galaktische Halo-Blasen vor. Das Modell umfasst sowohl strukturierte als auch turbulente Komponenten des Magnetfelds. Das vereinfachte Modell wurde mit Beobachtungsdaten verglichen, um den am besten passenden Parametersatz zusammen mit den Unsicherheiten zu erhalten. Ich untersuchte die Propagation der ultrahochenergetischen kosmischen Strahlung durch das vereinfachte Modell und untersuchte dessen Auswirkungen auf die Ankunftsrichtungen der ultrahochenergetischen kosmischen Strahlung für zwei potenzielle Quellen, Centaurus~A und NGC~253. Außerdem habe ich sowohl numerisch als auch analytisch abgeschätzt, wie stark ein Dipol der extragalaktischen kosmischen Strahlung durch verschiedene Konfigurationen der turbulenten Magnetfelder des vereinfachten Modells unterdrückt wird. Die Ergebnisse all dieser Arbeiten werden in dieser Arbeit im Detail vorgestellt. KW - galactic magnetic fields KW - synchrotron radiation KW - non-thermal radiation KW - ultra-high energy cosmic rays KW - cosmic ray propagation KW - Ausbreitung der kosmischen Strahlung KW - galaktische Magnetfelder KW - nicht-thermische Strahlung KW - Synchrotronstrahlung KW - ultrahochenergetische kosmische Strahlung Y1 - 2024 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-620198 ER - TY - JOUR A1 - Husemann, Bernd A1 - Bielby, R. A1 - Jahnke, K. A1 - Arrigoni-Battaia, F. A1 - Worseck, Gabor A1 - Shanks, T. A1 - Wardlow, J. A1 - Scholtz, J. T1 - Cosmic dance at z similar to 3 BT - Detecting the host galaxies of the dual AGN system LBQS 0302-0019 and Jil with HAWK-I plus GRAAL JF - Astronomy and astrophysics : an international weekly journal N2 - We recently discovered that the luminous radio-quiet (QSO) LBQS 0302-0019 at z=3.286 is likely accompanied by an obscured AGN at 20 kpc projected distance, which we dubbed Jil. It represents the tightest candidate obscured/unobscured dual AGN system at z >3. To verify the dual AGN scenario we obtained deep K-s band (rest-frame V band) imaging with the VLT/HAWK-I+GRAAL instrument at 0 '.4 resolution during science verification in January 2018. Indeed, we detect the individual host galaxies of the QSO and Jil with estimated stellar masses of log(M-*/M-circle dot)=11.4 +/- 0.5 and log(M-*/M-circle dot)=10.9 +/- 0.5, respectively. Near-IR spectra obtained with VLT-KMOS reveal a clear [O-III] lambda 5007 line detection at the location of Jil which does not contribute significantly to the Ks band flux. Both observations therefore corroborate the dual AGN scenario. A comparison to Illustris cosmological simulations suggests a parent halo mass of log(M-halo/M-*)=13.2 +/- 0.5 for this interacting galaxy system, corresponding to a very massive dark matter halo at that epoch. KW - Galaxies: interactions KW - Galaxies: high-redshift KW - large-scale structure of Universe KW - instrumentation: adaptive optics KW - quasars: individual: LBQS 0302-0018 Y1 - 2018 U6 - https://doi.org/10.1051/0004-6361/201833363 SN - 1432-0746 VL - 614 PB - EDP Sciences CY - Les Ulis ER - TY - JOUR A1 - Muzahid, S. A1 - Fonseca, G. A1 - Roberts, A. A1 - Rosenwasser, B. A1 - Richter, Philipp A1 - Narayanan, A. A1 - Churchill, C. A1 - Charlton, J. T1 - COS-Weak: probing the CGM using analogues of weak Mg II absorbers at z < 0.3 JF - Monthly notices of the Royal Astronomical Society N2 - We present a sample of 34 weak metal line absorbers at z < 0.3 selected by the simultaneous >3σ detections of the Si iiλ1260 and C iiλ1334 absorption lines, with Wr(SiII)<0.2 Å and Wr(CII)<0.3 Å, in archival HST/COS spectra. Our sample increases the number of known low-z ‘weak absorbers’ by a factor of >5. The column densities of H i and low-ionization metal lines obtained from Voigt profile fitting are used to build simple photoionization models. The inferred densities and line-of-sight thicknesses of the absorbers are in the ranges of −3.3 < log nH/cm−3 < −2.4 and ∼1 pc–50 kpc (median ≈500 pc), respectively. Most importantly, 85 per cent (50 per cent) of these absorbers show a metallicity of [Si/H]>−1.0(0.0)⁠. The fraction of systems showing near-/supersolar metallicity in our sample is significantly higher than in the H i-selected sample of Wotta et al., and the galaxy-selected sample of Prochaska et al., of absorbers probing the circum-galactic medium at similar redshift. A search for galaxies has revealed a significant galaxy-overdensity around these weak absorbers compared to random positions with a median impact parameter of 166 kpc from the nearest galaxy. Moreover, we find the presence of multiple galaxies in ≈80 per cent of the cases, suggesting group environments. The observed dN/dz of 0.8 ± 0.2 indicates that such metal-enriched, compact, dense structures are ubiquitous in the haloes of low-z group galaxies. We suggest that these are transient structures that are related to galactic outflows and/or stripping of metal-rich gas from galaxies. KW - galaxies: formation KW - galaxies: haloes KW - quasar: absorption line Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty529 SN - 0035-8711 SN - 1365-2966 VL - 476 IS - 4 SP - 4965 EP - 4986 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Pikovskij, Arkadij A1 - Dolmatova, A. A1 - Goldobin, Denis S. T1 - Correlations of the States of Non-Entrained Oscillators in the Kuramoto Ensemble with Noise in the Mean Field JF - Radiophysics and Quantum Electronics N2 - We consider the dynamics of the Kuramoto ensemble oscillators not included in a common synchronized cluster, where the mean field is subject to fluctuations. The fluctuations can be either related to the finite size of the ensemble or superimposed on the mean field in the form of common noise due to the constructive features of the system. It is shown that the states of such oscillators with close natural frequencies appear correlated with each other, since the mean-field fluctuations act as common noise. We quantify the effect with the synchronization index of two oscillators, which is calculated numerically and analytically as a function of the frequency difference and noise intensity. The results are rigorous for large ensembles with additional noise superimposed on the mean field and are qualitatively true for the systems where the mean-field fluctuations are due to the finite size of the ensemble. In the latter case, the effect is found to be independent of the number of oscillators in the ensemble. Y1 - 2019 U6 - https://doi.org/10.1007/s11141-019-09927-4 SN - 0033-8443 SN - 1573-9120 VL - 61 IS - 8-9 SP - 672 EP - 680 PB - Springer CY - New York ER - TY - THES A1 - Schemenz, Victoria T1 - Correlations between osteocyte lacuno-canalicular network and material characteristics in bone adaptation and regeneration T1 - Korrelationen zwischen dem lakuno-kanalikulären Netzwerk der Osteozyten und Materialeigenschaften bei der Knochenanpassung und -regeneration N2 - The complex hierarchical structure of bone undergoes a lifelong remodeling process, where it adapts to mechanical needs. Hereby, bone resorption by osteoclasts and bone formation by osteoblasts have to be balanced to sustain a healthy and stable organ. Osteocytes orchestrate this interplay by sensing mechanical strains and translating them into biochemical signals. The osteocytes are located in lacunae and are connected to one another and other bone cells via cell processes through small channels, the canaliculi. Lacunae and canaliculi form a network (LCN) of extracellular spaces that is able to transport ions and enables cell-to-cell communication. Osteocytes might also contribute to mineral homeostasis by direct interactions with the surrounding matrix. If the LCN is acting as a transport system, this should be reflected in the mineralization pattern. The central hypothesis of this thesis is that osteocytes are actively changing their material environment. Characterization methods of material science are used to achieve the aim of detecting traces of this interaction between osteocytes and the extracellular matrix. First, healthy murine bones were characterized. The properties analyzed were then compared with three murine model systems: 1) a loading model, where a bone of the mouse was loaded during its life time; 2) a healing model, where a bone of the mouse was cut to induce a healing response; and 3) a disease model, where the Fbn1 gene is dysfunctional causing defects in the formation of the extracellular tissue. The measurement strategy included routines that make it possible to analyze the organization of the LCN and the material components (i.e., the organic collagen matrix and the mineral particles) in the same bone volumes and compare the spatial distribution of different data sets. The three-dimensional network architecture of the LCN is visualized by confocal laser scanning microscopy (CLSM) after rhodamine staining and is then subsequently quantified. The calcium content is determined via quantitative backscattered electron imaging (qBEI), while small- and wide-angle X-ray scattering (SAXS and WAXS) are employed to determine the thickness and length of local mineral particles. First, tibiae cortices of healthy mice were characterized to investigate how changes in LCN architecture can be attributed to interactions of osteocytes with the surrounding bone matrix. The tibial mid-shaft cross-sections showed two main regions, consisting of a band with unordered LCN surrounded by a region with ordered LCN. The unordered region is a remnant of early bone formation and exhibited short and thin mineral particles. The surrounding, more aligned bone showed ordered and dense LCN as well as thicker and longer mineral particles. The calcium content was unchanged between the two regions. In the mouse loading model, the left tibia underwent two weeks of mechanical stimulation, which results in increased bone formation and decreased resorption in skeletally mature mice. Here the specific research question addressed was how do bone material characteristics change at (re)modeling sites? The new bone formed in response to mechanical stimulation showed similar properties in terms of the mineral particles, like the ordered calcium region but lower calcium content compared to the right, non-loaded control bone of the same mice. There was a clear, recognizable border between mature and newly formed bone. Nevertheless, some canaliculi went through this border connecting the LCN of mature and newly formed bone. Additionally, the question should be answered whether the LCN topology and the bone matrix material properties adapt to loading. Although, mechanically stimulated bones did not show differences in calcium content compared to controls, different correlations were found between the local LCN density and the local Ca content depending on whether the bone was loaded or not. These results suggest that the LCN may serve as a mineral reservoir. For the healing model, the femurs of mice underwent an osteotomy, stabilized with an external fixator and were allowed to heal for 21 days. Thus, the spatial variations in the LCN topology with mineral properties within different tissue types and their interfaces, namely calcified cartilage, bony callus and cortex, could be simultaneously visualized and compared in this model. All tissue types showed structural differences across multiple length scales. Calcium content increased and became more homogeneous from calcified cartilage to bony callus to lamellar cortical bone. The degree of LCN organization increased as well, while the lacunae became smaller, as did the lacunar density between these different tissue types that make up the callus. In the calcified cartilage, the mineral particles were short and thin. The newly formed callus exhibited thicker mineral particles, which still had a low degree of orientation. While most of the callus had a woven-like structure, it also served as a scaffold for more lamellar tissue at the edges. The lamelar bone callus showed thinner mineral particles, but a higher degree of alignment in both, mineral particles and the LCN. The cortex showed the highest values for mineral length, thickness and degree of orientation. At the same time, the lacunae number density was 34% lower and the lacunar volume 40% smaller compared to bony callus. The transition zone between cortical and callus regions showed a continuous convergence of bone mineral properties and lacunae shape. Although only a few canaliculi connected callus and the cortical region, this indicates that communication between osteocytes of both tissues should be possible. The presented correlations between LCN architecture and mineral properties across tissue types may suggest that osteocytes have an active role in mineralization processes of healing. A mouse model for the disease marfan syndrome, which includes a genetic defect in the fibrillin-1 gene, was investigated. In humans, Marfan syndrome is characterized by a range of clinical symptoms such as long bone overgrowth, loose joints, reduced bone mineral density, compromised bone microarchitecture, and increased fracture rates. Thus, fibrillin-1 seems to play a role in the skeletal homeostasis. Therefore, the present work studied how marfan syndrome alters LCN architecture and the surrounding bone matrix. The mice with marfan syndrome showed longer tibiae than their healthy littermates from an age of seven weeks onwards. In contrast, the cortical development appeared retarded, which was observed across all measured characteristics, i. e. lower endocortical bone formation, looser and less organized lacuno-canalicular network, less collagen orientation, thinner and shorter mineral particles. In each of the three model systems, this study found that changes in the LCN architecture spatially correlated with bone matrix material parameters. While not knowing the exact mechanism, these results provide indications that osteocytes can actively manipulate a mineral reservoir located around the canaliculi to make a quickly accessible contribution to mineral homeostasis. However, this interaction is most likely not one-sided, but could be understood as an interplay between osteocytes and extra-cellular matrix, since the bone matrix contains biochemical signaling molecules (e.g. non-collagenous proteins) that can change osteocyte behavior. Bone (re)modeling can therefore not only be understood as a method for removing defects or adapting to external mechanical stimuli, but also for increasing the efficiency of possible osteocyte-mineral interactions during bone homeostasis. With these findings, it seems reasonable to consider osteocytes as a target for drug development related to bone diseases that cause changes in bone composition and mechanical properties. It will most likely require the combined effort of materials scientists, cell biologists, and molecular biologists to gain a deeper understanding of how bone cells respond to their material environment. N2 - Knochen haben eine komplexe hierarchische Struktur, die einen lebenslangen Umbauprozess durchläuft, bei dem der Knochen sich seinen mechanischen Anforderungen anpasst. Um ein gesundes und stabiles Organ zu erhalten müssen Knochenresorption (durch Osteoklasten) und Knochenbildung (durch Osteoblasten) ausgewogen sein. Osteozyten lenken dieses Wechselspiel, indem sie mechanische Belastungen wahrnehmen und sie in biochemische Signale übersetzen. Die Osteozyten sitzen in Lakunen und sind durch Kanälchen untereinander und mit anderen Knochenzellen über ein Netzwerk (LCN) verbunden, das in der Lage ist, Ionen zu transportieren und eine Kommunikation von Zelle zu Zelle zu ermöglichen. Außerdem vermutet man, dass Osteozyten auch durch direkte Wechselwirkungen mit der umgebenden Matrix zur Mineralhomöostase beitragen könnten. Im Mittelpunkt dieser Arbeit steht die Frage, ob Osteozyten ihre materielle Umgebung aktiv verändern können. Um Spuren dieser Wechselwirkung zwischen Osteozyten und der extrazellulären Matrix nachzuweisen, werden materialwissenschaftliche Charakterisierungsmethoden eingesetzt. Zunächst wurden gesunde Mäuseknochen charakterisiert. Die erworbenen Ergebnisse wurden dann mit drei murinen Modellsystemen verglichen: 1) einem Belastungsmodell; 2) ein Heilungsmodell und 3) ein Krankheitsmodell, bei dem das Fbn1-Gen dysfunktional ist und Defekte in der Bildung des extrazellulären Gewebes verursacht werden. Die Messstrategie umfasste Routinen, die es ermöglichen, die Organisation des LCN und der Materialkomponenten (d.h. die organische Kollagenmatrix und die mineralischen Partikel) in denselben Knochenvolumina zu analysieren und die räumliche Verteilung der verschiedenen Datensätze zu vergleichen. Die dreidimensionale Netzwerkarchitektur des LCN wird durch konfokale Laser-Scanning-Mikroskopie nach Rhodamin-Färbung gemessen und anschließend quantifiziert. Der Kalziumgehalt wird mittels quantitativer Rückstreuelektronenbildgebung bestimmt, während Klein- und Weitwinkel-Röntgenstreuung verwendet werden, um die Dicke und Länge der Mineralpartikel zu bestimmen. Zunächst wurden Querschnitte der Unterschenkel von gesunden Mäusen charakterisiert, um zu untersuchen, ob Veränderungen in der LCN-Architektur auf Wechselwirkungen von Osteozyten mit der umgebenden Knochenmatrix zurückgeführt werden können. Die Kortizes zeigten zwei Hauptregionen, ein Band mit ungeordneter LCN-Architektur, umgeben von einer Region mit geordneter LCN. Die ungeordnete Region ist ein Überbleibsel der frühen Knochenbildung und wies kurze und dünne Mineralpartikel auf. Der umgebende, stärker ausgerichtete Knochen zeigte ein geordnetes und dichtes LCN, sowie dickere und längere Mineralpartikel. Der Kalziumgehalt blieb bei beiden Regionen unverändert. Im Mausbelastungsmodell wurde das linke Schienbein zwei Wochen lang mechanisch stimuliert, was zu einer erhöhten Knochenbildung führt. Hier sollte die Forschungsfrage beantwortet werden, wie sich Knochenmaterialeigenschaften an (Re-)Modellierungsstellen aufgrund von Belastung ändern. Der, als Reaktion auf die mechanische Stimulation, gebildete neue Knochen zeigte ähnliche Eigenschaften in Bezug auf die Mineralpartikel, wie die geordnete Kortexregion. Es gab eine klar erkennbare Grenze zwischen reifem und neu gebildetem Knochen. Trotzdem gingen einige Kanälchen durch diese Grenze, die die LCN aus reifem und neu gebildetem Knochen verband. Für das Heilungsmodell wurden die Oberschenkel von Mäusen einer Osteotomie unterzogen (einer Operation, bei der durch einen Schnitt in der Diaphyse ein Bruch erzeugt wird). Danach konnte die Fraktur 21 Tage heilen. Dadurch konnten in diesem Modell gleichzeitig verkalkter Knorpel, knöcherner Kallus und Kortex untersucht werden. Dafür wurde die räumliche Verteilung der LCN-Topologie sowie die Mineraleigenschaften der verschiedenen Gewebetypen und ihrer Grenzflächen visualisiert und verglichen. Alle Gewebetypen zeigten strukturelle Unterschiede über mehrere Längenskalen hinweg. Der Kalziumgehalt nahm von kalzifiziertem Knorpel zu knöchernem Kallus zu lamellarem kortikalem Knochen zu und wurde homogener. Der Grad der LCN-Organisation nahm ebenfalls zu, während die Lakunen vom Kallus zum Kortexgewebe kleiner wurden, ebenso wie die Lakunendichte. Im verkalkten Knorpel waren die Mineralpartikel kurz und dünn. Der größte Teil des Kallus wies eine Geflechtsknochenstruktur auf und diente als Gerüst für lamellares Gewebe, das dünnere Mineralpartikel, aber einen höheren Grad an Ausrichtung sowohl in den Mineralpartikeln als auch im LCN aufwies. Der Kortex zeigte die höchsten Werte für Minerallänge, Dicke und Orientierungsgrad. Obwohl nur wenige Kanälchen den Kallus und kortikale Regionen verbinden, weist dies darauf hin, dass eine Kommunikation zwischen Osteozyten beider Gewebe möglich sein sollte. Es wurde auch ein Mausmodell für das Marfan-Syndrom untersucht, das einen Gendefekt im Fibrillin-1-Gen beinhaltet. Beim Menschen ist das Marfan-Syndrom durch eine Reihe klinischer Symptome gekennzeichnet, wie z. B. übermäßiges Wachstum der Gliedmaßen, überstreckbare Gelenke, verringerte Knochenmineraldichte, beeinträchtigte Knochenmikroarchitektur und erhöhte Frakturraten. Somit scheint Fibrillin-1 eine Rolle in der Skeletthomöostase zu spielen. Deswegen untersuchte die vorliegende Arbeit, ob und wie das Marfan-Syndrom die LCN-Architektur und die umgebende Knochenmatrix verändert. Die Mäuse mit Marfan-Syndrom zeigten bereits ab einem Alter von sieben Wochen längere Schienbeine als ihre gesunden Wurfgeschwister. Im Gegensatz dazu erschien die kortikale Entwicklung verzögert, was über alle gemessenen Merkmale hinweg beobachtet wurde, d.h. niedrigere endokortikale Knochenbildung, lockereres und weniger organisiertes LCN, geringerer Grad an Kollagenorientierung sowie ein Trend zu dünneren und kürzeren Mineralpartikel. In jedem der drei Modellsysteme fand diese Studie, dass Änderungen in der LCN-Architektur räumlich mit Parametern des Knochenmatrixmaterials korrelierten. Obwohl der genaue Mechanismus nicht bekannt ist, liefern diese Ergebnisse Hinweise darauf, dass Osteozyten ein Mineralreservoir aktiv manipulieren können. Dieses Reservoir befindet sich um die Kanälchen herum und dieser Prozess würde es ermöglichen, einen schnell zugänglichen Beitrag zur Mineralhomöostase zu leisten. Diese Interaktion ist jedoch höchstwahrscheinlich nicht einseitig, sondern könnte als Wechselspiel zwischen Osteozyten und extrazellulärer Matrix verstanden werden, da die Knochenmatrix biochemische Signalmoleküle enthält, die das Verhalten von Osteozyten verändern können. Knochen(re)modellierung kann daher nicht nur als Methode zur Defektbeseitigung oder Anpassung an äußere mechanische Reize verstanden werden, sondern auch zur Effizienzsteigerung möglicher Osteozyten-Mineral-Interaktionen während der Knochenhomöostase. Angesichts dieser Ergebnisse erscheint es sinnvoll, Osteozyten als Ziel für die Arzneimittelentwicklung im Zusammenhang mit Knochenerkrankungen in Betracht zu ziehen, die Veränderungen der Knochenzusammensetzung und deren mechanischen Eigenschaften verursachen. KW - bone KW - lacunae KW - mineralization KW - SAXS KW - lacuno-canalicular network KW - µCT KW - CLSM KW - konfokales Laser-Scanning-Mikroskop KW - Kleinwinkelröntgenstreuung KW - Knochen KW - Lakunen KW - lakuno-kanaliculäres Netzwerk KW - Mineralisierung KW - µCT Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-559593 ER - TY - JOUR A1 - Teomy, Eial A1 - Metzler, Ralf T1 - Correlations and transport in exclusion processes with general finite memory JF - Journal of statistical mechanics: theory and experiment KW - Brownian motion KW - exclusion processes Y1 - 2019 U6 - https://doi.org/10.1088/1742-5468/ab47fb SN - 1742-5468 VL - 2019 IS - 10 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Dörries, Timo A1 - Loos, Sarah Anna Marie A1 - Klapp, Sabine H. L. T1 - Correlation functions of non-Markovian systems out of equilibrium BT - analytical expressions beyond single-exponential memory JF - Journal of statistical mechanics: theory and experiment : JSTAT N2 - This paper is concerned with correlation functions of stochastic systems with memory, a prominent example being a molecule or colloid moving through a complex (e.g. viscoelastic) fluid environment. Analytical investigations of such systems based on non-Markovian stochastic equations are notoriously difficult. A common approximation is that of a single-exponential memory, corresponding to the introduction of one auxiliary variable coupled to the Markovian dynamics of the main variable. As a generalization, we here investigate a class of 'toy' models with altogether three degrees of freedom, giving rise to more complex forms of memory. Specifically, we consider, mainly on an analytical basis, the under- and overdamped motion of a colloidal particle coupled linearly to two auxiliary variables, where the coupling between variables can be either reciprocal or non-reciprocal. Projecting out the auxiliary variables, we obtain non-Markovian Langevin equations with friction kernels and colored noise, whose structure is similar to that of a generalized Langevin equation. For the present systems, however, the non-Markovian equations may violate the fluctuation-dissipation relation as well as detailed balance, indicating that the systems are out of equilibrium. We then study systematically the connection between the coupling topology of the underlying Markovian system and various autocorrelation functions. We demonstrate that already two auxiliary variables can generate surprisingly complex (e.g. non-monotonic or oscillatory) memory and correlation functions. Finally, we show that a minimal overdamped model with two auxiliary variables and suitable non-reciprocal coupling yields correlation functions resembling those describing hydrodynamic backflow in an optical trap. KW - correlation functions KW - memory effects KW - friction Y1 - 2021 U6 - https://doi.org/10.1088/1742-5468/abdead SN - 1742-5468 IS - 3 PB - IOP Publ. Ltd. CY - Bristol ER - TY - THES A1 - Kniepert, Juliane T1 - Correlation between dynamic parameters and device performance of organic solar cells T1 - Zusammenhang zwischen den dynamischen Größen und der Effizienz Organischer Solarzellen N2 - Organic bulk heterojunction (BHJ) solar cells based on polymer:fullerene blends are a promising alternative for a low-cost solar energy conversion. Despite significant improvements of the power conversion efficiency in recent years, the fundamental working principles of these devices are yet not fully understood. In general, the current output of organic solar cells is determined by the generation of free charge carriers upon light absorption and their transport to the electrodes in competition to the loss of charge carriers due to recombination. The object of this thesis is to provide a comprehensive understanding of the dynamic processes and physical parameters determining the performance. A new approach for analyzing the characteristic current-voltage output was developed comprising the experimental determination of the efficiencies of charge carrier generation, recombination and transport, combined with numerical device simulations. Central issues at the beginning of this work were the influence of an electric field on the free carrier generation process and the contribution of generation, recombination and transport to the current-voltage characteristics. An elegant way to directly measure the field dependence of the free carrier generation is the Time Delayed Collection Field (TDCF) method. In TDCF charge carriers are generated by a short laser pulse and subsequently extracted by a defined rectangular voltage pulse. A new setup was established with an improved time resolution compared to former reports in literature. It was found that charge generation is in general independent of the electric field, in contrast to the current view in literature and opposed to the expectations of the Braun-Onsager model that was commonly used to describe the charge generation process. Even in cases where the charge generation was found to be field-dependend, numerical modelling showed that this field-dependence is in general not capable to account for the voltage dependence of the photocurrent. This highlights the importance of efficient charge extraction in competition to non-geminate recombination, which is the second objective of the thesis. Therefore, two different techniques were combined to characterize the dynamics and efficiency of non-geminate recombination under device-relevant conditions. One new approach is to perform TDCF measurements with increasing delay between generation and extraction of charges. Thus, TDCF was used for the first time to measure charge carrier generation, recombination and transport with the same experimental setup. This excludes experimental errors due to different measurement and preparation conditions and demonstrates the strength of this technique. An analytic model for the description of TDCF transients was developed and revealed the experimental conditions for which reliable results can be obtained. In particular, it turned out that the $RC$ time of the setup which is mainly given by the sample geometry has a significant influence on the shape of the transients which has to be considered for correct data analysis. Secondly, a complementary method was applied to characterize charge carrier recombination under steady state bias and illumination, i.e. under realistic operating conditions. This approach relies on the precise determination of the steady state carrier densities established in the active layer. It turned out that current techniques were not sufficient to measure carrier densities with the necessary accuracy. Therefore, a new technique {Bias Assisted Charge Extraction} (BACE) was developed. Here, the charge carriers photogenerated under steady state illumination are extracted by applying a high reverse bias. The accelerated extraction compared to conventional charge extraction minimizes losses through non-geminate recombination and trapping during extraction. By performing numerical device simulations under steady state, conditions were established under which quantitative information on the dynamics can be retrieved from BACE measurements. The applied experimental techniques allowed to sensitively analyse and quantify geminate and non-geminate recombination losses along with charge transport in organic solar cells. A full analysis was exemplarily demonstrated for two prominent polymer-fullerene blends. The model system P3HT:PCBM spincast from chloroform (as prepared) exhibits poor power conversion efficiencies (PCE) on the order of 0.5%, mainly caused by low fill factors (FF) and currents. It could be shown that the performance of these devices is limited by the hole transport and large bimolecular recombination (BMR) losses, while geminate recombination losses are insignificant. The low polymer crystallinity and poor interconnection between the polymer and fullerene domains leads to a hole mobility of the order of 10^-7 cm^2/Vs which is several orders of magnitude lower than the electron mobility in these devices. The concomitant build up of space charge hinders extraction of both electrons and holes and promotes bimolecular recombination losses. Thermal annealing of P3HT:PCBM blends directly after spin coating improves crystallinity and interconnection of the polymer and the fullerene phase and results in comparatively high electron and hole mobilities in the order of 10^-3 cm^2/Vs and 10^-4 cm^2/Vs, respectively. In addition, a coarsening of the domain sizes leads to a reduction of the BMR by one order of magnitude. High charge carrier mobilities and low recombination losses result in comparatively high FF (>65%) and short circuit current (J_SC ≈ 10 mA/cm^2). The overall device performance (PCE ≈ 4%) is only limited by a rather low spectral overlap of absorption and solar emission and a small V_OC, given by the energetics of the P3HT. From this point of view the combination of the low bandgap polymer PTB7 with PCBM is a promising approach. In BHJ solar cells, this polymer leads to a higher V_OC due to optimized energetics with PCBM. However, the J_SC in these (unoptimized) devices is similar to the J_SC in the optimized blend with P3HT and the FF is rather low (≈ 50%). It turned out that the unoptimized PTB7:PCBM blends suffer from high BMR, a low electron mobility of the order of 10^-5 cm^2/Vs and geminate recombination losses due to field dependent charge carrier generation. The use of the solvent additive DIO optimizes the blend morphology, mainly by suppressing the formation of very large fullerene domains and by forming a more uniform structure of well interconnected donor and acceptor domains of the order of a few nanometers. Our analysis shows that this results in an increase of the electron mobility by about one order of magnitude (3 x 10^-4 cm^2/Vs), while BMR and geminate recombination losses are significantly reduced. In total these effects improve the J_SC (≈ 17 mA/cm^2) and the FF (> 70%). In 2012 this polymer/fullerene combination resulted in a record PCE for a single junction OSC of 9.2%. Remarkably, the numerical device simulations revealed that the specific shape of the J-V characteristics depends very sensitively to the variation of not only one, but all dynamic parameters. On the one hand this proves that the experimentally determined parameters, if leading to a good match between simulated and measured J-V curves, are realistic and reliable. On the other hand it also emphasizes the importance to consider all involved dynamic quantities, namely charge carrier generation, geminate and non-geminate recombination as well as electron and hole mobilities. The measurement or investigation of only a subset of these parameters as frequently found in literature will lead to an incomplete picture and possibly to misleading conclusions. Importantly, the comparison of the numerical device simulation employing the measured parameters and the experimental $J-V$ characteristics allows to identify loss channels and limitations of OSC. For example, it turned out that inefficient extraction of charge carriers is a criticical limitation factor that is often disobeyed. However, efficient and fast transport of charges becomes more and more important with the development of new low bandgap materials with very high internal quantum efficiencies. Likewise, due to moderate charge carrier mobilities, the active layer thicknesses of current high-performance devices are usually limited to around 100 nm. However, larger layer thicknesses would be more favourable with respect to higher current output and robustness of production. Newly designed donor materials should therefore at best show a high tendency to form crystalline structures, as observed in P3HT, combined with the optimized energetics and quantum efficiency of, for example, PTB7. N2 - Organische Solarzellen aus Polymer-Fulleren Mischschichten sind eine vielversprechende Alternative für eine kostengünstige Quelle erneuerbarer Energien. Jedoch sind trotz intensiver Forschung und bedeutenden Verbesserungen der Effizienzen während der letzten Jahre, die fundamentalen Prozesse in diesen Solarzellen noch immer nicht vollständig verstanden. Im Allgemeinen wird der Photostrom in organischen Solarzellen durch die Generation freier Ladungsträger nach Absorption von Licht und dem anschliessenden Transport dieser Ladungsträger zu den Elektroden bestimmt, wobei ein Teil durch Rekombination verloren geht. Das Ziel dieser Dissertation ist es, zu einem umfassenden Verständnis aller dynamischen Prozesse und relevanten physikalischen Parameter und deren Zusammenhang mit der Solarzelleneffizienz beizutragen. Dabei wurde eine neue Herangehensweise zur Analyse der Strom-Spannungskennlinien entwickelt, die auf der experimentellen Bestimmung von Ladungsträgergeneration, Rekombination und Transport im Vergleich mit numerischen Simulationen beruht. Die zentralen Fragestellungen zu Beginn der Arbeit waren, in welchem Masse die Ladungsträgergeneration von der Stärke des elektrischen Feldes abhängt und in wie weit die einzelnen Prozesse Generation, Rekombination und Transport zum charakteristischen Verlauf der Strom-Spannungskennlinie beitragen. Eine elegante Methode um direkt die Feldabhängigkeit der Ladungsträgergeneration zu bestimmen, ist die Time Delayed Collection Field (TDCF) Methode. Dabei werden in der Solarzelle Ladungsträger mit einem kurzen Laserpuls erzeugt und anschliessend mit einem wohldefinierten rechteckigen Spannungspuls extrahiert. Im Rahmen dieser Arbeit wurde ein neuer Versuchsaufbau mit stark verbesserter Zeitauflösung im Vergleich zu vorangegangenen Arbeiten aufgebaut. Es hat sich gezeigt, dass die Generation im Allgemeinen nicht vom elektrischen Feld abhängt. Dies steht im Widerspruch zu dem in der Literatur häufig verwendeten Braun-Onsager Modell zur Beschreibung der Ladungsgeneration. Selbst in Fällen, in denen eine feldabhängige Generation gemessen wurde, reichte diese im Allgemeinen nicht aus, um die Spannungsabhängigkeit des Photostroms in diesen Zellen zu erklären. Dies führt zu der zunehmenden Bedeutung einer effizienten Ladungsextraktion in Konkurrenz zur nicht-geminalen Rekombination, welche die zweite wichtige Fragestellung in dieser Arbeit war. Dazu wurden zwei neue Herangehensweisen zur Messung der Rekombinationsdynamik unter Solarzellen-relevanten Messbedingungen erarbeitet. Zum Einen wurde zum ersten Mal TDCF zur Messung von Rekombination und Transport verwendet. Der Vorteil ist, dass die Messung aller drei relevanten Prozesse in einem Versuchsaufbau zu einer höheren Genauigkeit führt, da experimentelle Fehler durch unterschiedliche Probenpräparation und Messbedingungen ausgeschlossen werden können. Es wurde ein analytisches Modell zur Beschreibung der TDCF Transienten entwickelt, welches direkt Rückschlüsse auf die notwendigen Messbedingungen erlaubt, unter denen zuverlässige Ergebnisse erzielt werden können. Insbesondere hat sich dabei gezeigt, dass die RC-Zeit des Aufbaus, die hauptsächlich durch die Probengeometrie bestimmt wird, einen wesentlichen Einfluss auf die Transienten hat, welcher in der Datenauswertung nicht vernachlässigt werden darf. Zum Anderen wurde eine komplementäre Methode zur Messung der Rekombination im Gleichgewicht benutzt, die auf der genauen Bestimmung der statischen Ladungsträgerdichte unter Beleuchtung basiert. Es zeigte sich, dass die in der Literatur etablierten Methoden zur Ladungsträgerbestimmung nicht ausreichend genau waren. Daher wurde eine neue, sensitive Technik entwickelt, die Bias Assisted Charge Extraction (BACE) Methode. Dabei werden die unter kontinuierlicher Beleuchtung generierten Ladungsträger mit einem hohen angelegten elektrischen Feld extrahiert. Dies führt zu einer beschleunigten Extraktion im Vergleich zur konventionellen Charge Extraction Methode, welche die Verluste durch Rekombination und Trapping deutlich reduziert. Mit Hilfe von numerischen Simulationen wurden Bedingungen festgelegt, unter denen zuverlässige Messungen mit BACE möglich sind. Mit den vorgestellten Methoden können alle relevanten Parameter für Generation, Rekombination und Transport experimentell sehr genau bestimmt und analysiert werden. Dies wurde exemplarisch für zwei in der Literatur bedeutende Materialsysteme durchgeführt. Das Modellsystem P3HT:PCBM, hergestellt aus dem Lösungsmittel Chloroform, weist direkt nach der Präparation nur sehr niedrige Effizienzen im Bereich von 0.5% auf, die vor allem durch niedrige Ströme und Füllfaktoren (FF) gekennzeichnet sind. Es konnte gezeigt werden, dass die Effizienz dieser Zellen durch den sehr langsamen Lochtransport und die starke bimolekulare Rekombination begrenzt wird, während Verluste durch geminale Rekombination eher unbedeutend sind. Eine geringe Kristallinität des Polymers und schlechte Verbindungen zwischen den Polymerdomänen führt zu einer Lochmobilität von nur 10^7 cm^2/Vs. Diese ist mehrere Grössenordnungen kleiner als die Elektronenmobilität in diesen Zellen, was den Aufbau von Raumladungen zur Folge hat und damit die Extraktion von beiden Ladungsträgern behindert und bimolekulare Rekombination begünstigt. Die thermische Behandlung der P3HT:PCBM Proben direkt nach dem Spin Coating verbessert sowohl die Kristallinität als auch die Vernetzung der Polymer- und Fullerenphasen. Dies führt zu vergleichsweise hohen Mobilitäten für Elektronen (10^3 cm^2/Vs) und Löcher (10^4 cm^2/Vs). Zusätzlich reduziert eine Vergrösserung der Domänenstruktur die bimolekulare Rekombination um eine Grössenordnung. Die hohen Mobilitäten und niedrigen Rekombinationverluste resultieren in vergleichsweise hohen FF (> 65%) und Strömen (J_SC ≈ 10 mA/cm^2). Die Effizienz (≈ 4%) wird dabei nur durch den eher geringen spektralen Überlapp zwischen Absorptions- und Sonnenemissionsspektrum begrenzt und einer geringen Leerlaufspannung (V_OC), verursacht durch die ungünstige Energetik des P3HT. Aus dieser Perspektive ist die Kombination des Low-Bandgap Polymers PTB7 mit dem Fullerenderivat PCBM ein vielversprechender Ansatz. Aufgrund der angepassten Energetik führt diese Kombination in Mischschichtsolarzellen zu einer höheren V_OC. Jedoch ist der Kurzschlussstrom (J_SC) in den unoptimierten Zellen gegenüber dem der optimierten P3HT:PCBM Zellen nicht erhöht und die FF sind niedrig (≈ 50%). Die Untersuchungen ergaben, dass die Dynamik in den unoptimierten PTB7:PCBM Zellen durch hohe bimolekulare Rekombination (BMR), eine niedrige Elektronenmobilität im Bereich von 10^5 cm^2/Vs und nicht-geminalen Rekombinationsverslusten aufgrund einer feldabhängigen Generation geprägt ist. Die Zugabe des Lösungsmitteladditivs DIO bei der Probenpräparation optimiert die Morphologie in der Schicht. Dabei wird hauptsächlich die Formation von sehr grossen Fullerendomänen unterdrückt und eine gleichmässige Struktur von gut vernetzten, nanometergrossen Donor- und Akzeptordomänen ausgebildet. Unsere Analyse zeigt, dass dadurch die Elektronenmobilität um ca eine Grössenordnung erhöht wird (auf 3 x 10^4 cm^2/Vs), während BMR und geminale Rekombination deutlich unterdrückt werden. Zusammen genommen verbessern diese Effekte sowohl den J_SC (≈ 17 mA/cm^2) als auch den FF (> 70%). Im Jahr 2012 führten die optimierten PTB7:PCBM Solarzellen damit zu einer Rekordeffizienz für organische Solarzellen von 9.2%. Von grösserer Bedeutung ist jedoch, dass der Vergleich zwischen den gemessenen und simulierten Strom-Spannungskennlinien, mit experimentell bestimmten Eingangsparametern, Rückschlüsse auf die entsprechenden Verlustkanäle in den jeweiligen Solarzellen erlaubt. Hierbei zeigte sich, dass die effiziente Extraktion von Ladungsträgern ein kritischer Faktor ist, der in der Literatur oft unterschätzt wird. Der effiziente und schnelle Transport von Ladungen wird aber gerade im Hinblick auf neue Low-Bandgap Materialien mit sehr hohen internen Quanteneffizienzen immer wichtiger. Moderate Mobilitäten limitieren die möglichen Schichtdicken auf ca 100 nm. Grössere Schichtdicken wären jedoch vorteilhafter im Hinblick auf höhere Ausgangsströme und der Robustheit des Produktionsprozesses. Neu entwickelte Donormaterialien sollten daher sowohl möglichst kristalline Strukturen ausbilden mit einhergehenden hohen Mobilitäten, wie beim P3HT, als auch optimierte Energieniveaus und Quanteneffizienzen aufweisen, wie beispielsweise PTB7. KW - organic solar cells KW - optoelectronic measurements KW - transient methods KW - numeric device simulations KW - organische Solarzellen KW - optoelektronische Messungen KW - transiente Messmethoden KW - numerische Bauteilsimulationen Y1 - 2015 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-90087 ER - TY - GEN A1 - Wolff, Christian Michael A1 - Canil, Laura A1 - Rehermann, Carolin A1 - Nguyen, Ngoc Linh A1 - Zu, Fengshuo A1 - Ralaiarisoa, Maryline A1 - Caprioglio, Pietro A1 - Fiedler, Lukas A1 - Stolterfoht, Martin A1 - Kogikoski, Junior, Sergio A1 - Bald, Ilko A1 - Koch, Norbert A1 - Unger, Eva L. A1 - Dittrich, Thomas A1 - Abate, Antonio A1 - Neher, Dieter T1 - Correction to 'Perfluorinated self-assembled monolayers enhance the stability and efficiency of inverted perovskite solar cells' (2020, 14 (2), 1445−1456) T2 - ACS nano Y1 - 2020 U6 - https://doi.org/10.1021/acsnano.0c08081 SN - 1936-0851 SN - 1936-086X VL - 14 IS - 11 SP - 16156 EP - 16156 PB - American Chemical Society CY - Washington, DC ER - TY - JOUR A1 - Vinke, Kira A1 - Gabrysch, Sabine A1 - Paoletti, Emanuela A1 - Rockström, Johan A1 - Schellnhuber, Hans Joachim T1 - Corona and the climate BT - a comparison of two emergencies JF - Global sustainability N2 - Lessons from the corona crisis can help manage the even more daunting challenge of anthropogenic global warming. KW - adaptation and mitigation KW - ecology and biodiversity KW - human behaviour KW - natural resources (biological and non-biological) KW - policies KW - politics KW - and governance Y1 - 2020 U6 - https://doi.org/10.1017/sus.2020.20 SN - 2059-4798 VL - 3 PB - Cambridge Univ. Press CY - Cambridge ER - TY - GEN A1 - Lever, Fabiano A1 - Mayer, Dennis A1 - Metje, Jan A1 - Alisauskas, Skirmantas A1 - Calegari, Francesca A1 - Düsterer, Stefan A1 - Feifel, Raimund A1 - Niebuhr, Mario A1 - Manschwetus, Bastian A1 - Kuhlmann, Marion A1 - Mazza, Tommaso A1 - Robinson, Matthew Scott A1 - Squibb, Richard J. A1 - Trabattoni, Andrea A1 - Wallner, Måns A1 - Wolf, Thomas J. A. A1 - Gühr, Markus T1 - Core-level spectroscopy of 2-thiouracil at the sulfur L1 and L2,3 edges utilizing a SASE free-electron-laser T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - In this paper, we report X-ray absorption and core-level electron spectra of the nucleobase derivative 2-thiouracil at the sulfur L1- and L2,3-edges. We used soft X-rays from the free-electron laser FLASH2 for the excitation of isolated molecules and dispersed the outgoing electrons with a magnetic bottle spectrometer. We identified photoelectrons from the 2p core orbital, accompanied by an electron correlation satellite, as well as resonant and non-resonant Coster–Kronig and Auger–Meitner emission at the L1- and L2,3-edges, respectively. We used the electron yield to construct X-ray absorption spectra at the two edges. The experimental data obtained are put in the context of the literature currently available on sulfur core-level and 2-thiouracil spectroscopy. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1180 KW - X-ray KW - photoelectron KW - sulfur KW - thiouracil KW - nucleobases KW - Coster–Kronig KW - Auger–Meitner KW - NEXAFS KW - FLASH Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-524091 SN - 1866-8372 IS - 21 ER - TY - JOUR A1 - Lever, Fabiano A1 - Mayer, Dennis A1 - Metje, Jan A1 - Alisauskas, Skirmantas A1 - Calegari, Francesca A1 - Düsterer, Stefan A1 - Feifel, Raimund A1 - Niebuhr, Mario A1 - Manschwetus, Bastian A1 - Kuhlmann, Marion A1 - Mazza, Tommaso A1 - Robinson, Matthew Scott A1 - Squibb, Richard J. A1 - Trabattoni, Andrea A1 - Wallner, Måns A1 - Wolf, Thomas J. A. A1 - Gühr, Markus T1 - Core-level spectroscopy of 2-thiouracil at the sulfur L1 and L2,3 edges utilizing a SASE free-electron-laser JF - Molecules N2 - In this paper, we report X-ray absorption and core-level electron spectra of the nucleobase derivative 2-thiouracil at the sulfur L1- and L2,3-edges. We used soft X-rays from the free-electron laser FLASH2 for the excitation of isolated molecules and dispersed the outgoing electrons with a magnetic bottle spectrometer. We identified photoelectrons from the 2p core orbital, accompanied by an electron correlation satellite, as well as resonant and non-resonant Coster–Kronig and Auger–Meitner emission at the L1- and L2,3-edges, respectively. We used the electron yield to construct X-ray absorption spectra at the two edges. The experimental data obtained are put in the context of the literature currently available on sulfur core-level and 2-thiouracil spectroscopy. KW - X-ray KW - photoelectron KW - sulfur KW - thiouracil KW - nucleobases KW - Coster–Kronig KW - Auger–Meitner KW - NEXAFS KW - FLASH Y1 - 2021 U6 - https://doi.org/10.3390/molecules26216469 SN - 1420-3049 VL - 26 IS - 21 PB - MDPI CY - Basel ER - TY - JOUR A1 - Massa, Derck A1 - Oskinova, Lidia M. A1 - Prinja, Raman A1 - Ignace, Richard T1 - Coordinated UV and X-Ray Spectroscopic Observations of the O-type Giant xi Per BT - the Connection between X-Rays and Large-scale Wind Structure JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We present new, contemporaneous Hubble Space Telescope STIS and XMM-Newton observations of the O7. III(n) ((f)) star xi Per. We supplement the new data with archival IUE spectra, to analyze the variability of the wind lines and X-ray flux of xi Per. The variable wind of this star is known to have a 2.086-day periodicity. We use a simple, heuristic spot model that fits the low-velocity (near-surface) IUE wind line variability very well, to demonstrate that the low-velocity absorption in the new STIS spectra of N IV lambda 1718 and Si IV lambda 1402 vary with the same 2.086-day period. It is remarkable that the period and amplitude of the STIS data agree with those of the IUE spectra obtained 22 yr earlier. We also show that the time variability of the new XMM-Newton fluxes is also consistent with the 2.086-day period. Thus, our new, multiwavelength coordinated observations demonstrate that the mechanism that causes the UV wind line variability is also responsible for a significant fraction of the X-rays in single O stars. The sequence of events for the multiwavelength light-curve minima is Si IV lambda 1402, N IV lambda 1718, and X-ray flux, each separated by a phase of about 0.06 relative to the 2.086-day period. Analysis of the X-ray fluxes shows that they become softer as they weaken. This is contrary to expectations if the variability is caused by periodic excess absorption. Furthermore, the high-resolution X-ray spectra suggest that the individual emission lines at maximum are more strongly blueshifted. If we interpret the low-velocity wind line light curves in terms of our model, it implies that there are two bright regions, i.e., regions with less absorption, separated by 180 degrees, on the surface of the star. We note that the presence and persistence of two spots separated by 180 degrees suggest that a weak dipole magnetic field is responsible for the variability of the UV wind line absorption and X-ray flux in xi Per. KW - stars: activity KW - stars: early-type KW - stars: winds, outflows KW - ultraviolet: stars KW - X-rays: stars Y1 - 2019 U6 - https://doi.org/10.3847/1538-4357/ab0283 SN - 0004-637X SN - 1538-4357 VL - 873 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - GEN A1 - Bouchoule, Isabelle A1 - Schemmer, Max A1 - Henkel, Carsten T1 - Cooling phonon modes of a Bose condensate with uniform few body losses T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - We present a general analysis of the cooling produced by losses on condensates or quasi-condensates. We study how the occupations of the collective phonon modes evolve in time, assuming that the loss process is slow enough so that each mode adiabatically follows the decrease of the mean density. The theory is valid for any loss process whose rate is proportional to the jth power of the density, but otherwise spatially uniform. We cover both homogeneous gases and systems confined in a smooth potential. For a low-dimensional gas, we can take into account the modified equation of state due to the broadening of the cloud width along the tightly confined directions, which occurs for large interactions. We find that at large times, the temperature decreases proportionally to the energy scale mc2, where m is the mass of the particles and c the sound velocity. We compute the asymptotic ratio of these two quantities for different limiting cases: a homogeneous gas in any dimension and a one-dimensional gas in a harmonic trap. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1029 KW - 3 body recombination KW - gas KW - scattering KW - Bose-Einstein condensates (BECs) KW - harmonic traps KW - one-dimensional Bose gas KW - phonon modes Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-468811 SN - 1866-8372 IS - 1029 ER - TY - JOUR A1 - Bouchoule, Isabelle A1 - Schemmer, Max A1 - Henkel, Carsten T1 - Cooling phonon modes of a Bose condensate with uniform few body losses JF - Scipost Physics N2 - We present a general analysis of the cooling produced by losses on condensates or quasi-condensates. We study how the occupations of the collective phonon modes evolve in time, assuming that the loss process is slow enough so that each mode adiabatically follows the decrease of the mean density. The theory is valid for any loss process whose rate is proportional to the jth power of the density, but otherwise spatially uniform. We cover both homogeneous gases and systems confined in a smooth potential. For a low-dimensional gas, we can take into account the modified equation of state due to the broadening of the cloud width along the tightly confined directions, which occurs for large interactions. We find that at large times, the temperature decreases proportionally to the energy scale mc(2), where m is the mass of the particles and c the sound velocity. We compute the asymptotic ratio of these two quantities for different limiting cases: a homogeneous gas in any dimension and a one-dimensional gas in a harmonic trap. Y1 - 2018 U6 - https://doi.org/10.21468/SciPostPhys.5.5.043 SN - 2542-4653 VL - 5 IS - 5 PB - Scipost foundation CY - Amsterdam ER - TY - THES A1 - Mandal, Partha Sarathi T1 - Controlling the surface band gap in topological states of matter N2 - In the present study, we employ the angle-resolved photoemission spectroscopy (ARPES) technique to study the electronic structure of topological states of matter. In particular, the so-called topological crystalline insulators (TCIs) Pb1-xSnxSe and Pb1-xSnxTe, and the Mn-doped Z2 topological insulators (TIs) Bi2Te3 and Bi2Se3. The Z2 class of strong topological insulators is protected by time-reversal symmetry and is characterized by an odd number of metallic Dirac type surface states in the surface Brillouin zone. The topological crystalline insulators on the other hand are protected by the individual crystal symmetries and exhibit an even number of Dirac cones. The topological properties of the lead tin chalcogenides topological crystalline insulators can be tuned by temperature and composition. Here, we demonstrate that Bi-doping of the Pb1-xSnxSe(111) epilayers induces a quantum phase transition from a topological crystalline insulator to a Z2 topological insulator. This occurs because Bi-doping lifts the fourfold valley degeneracy in the bulk. As a consequence a gap appears at ⌈¯, while the three Dirac cones at the M̅ points of the surface Brillouin zone remain intact. We interpret this new phase transition is caused by lattice distortion. Our findings extend the topological phase diagram enormously and make strong topological insulators switchable by distortions or electric field. In contrast, the bulk Bi doping of epitaxial Pb1-xSnxTe(111) films induces a giant Rashba splitting at the surface that can be tuned by the doping level. Tight binding calculations identify their origin as Fermi level pinning by trap states at the surface. Magnetically doped topological insulators enable the quantum anomalous Hall effect (QAHE) which provide quantized edge states for lossless charge transport applications. The edge states are hosted by a magnetic energy gap at the Dirac point which has not been experimentally observed to date. Our low temperature ARPES studies unambiguously reveal the magnetic gap of Mn-doped Bi2Te3. Our analysis shows a five times larger gap size below the Tc than theoretically predicted. We assign this enhancement to a remarkable structure modification induced by Mn doping. Instead of a disordered impurity system, a self-organized alternating sequence of MnBi2Te4 septuple and Bi2Te3quintuple layers is formed. This enhances the wave-function overlap and gives rise to a large magnetic gap. Mn-doped Bi2Se3 forms similar heterostructure, but only a nonmagnetic gap is observed in this system. This correlates with the difference in magnetic anisotropy due to the much larger spin-orbit interaction in Bi2Te3 compared to Bi2Se3. These findings provide crucial insights for pushing lossless transport in topological insulators towards room-temperature applications. N2 - In der vorliegenden Studie verwenden wir die Methode der winkelaufgelösten Photoemissionsspektroskopie (ARPES) zur Untersuchung der elektronischen Struktur von topologischen Zuständen der Materie. Insbesondere die sogenannten topologischen kristallinen Isolatoren (TCI) Pb1-xSnxSe und Pb1-xSnxTe sowie die Mn-dotierten Z2 topologischen Isolatoren (TI) Bi2Te3 und Bi2Se3. Die Z2-Klasse der starken topologischen Isolatoren ist durch Zeitumkehrsymmetrie geschützt und durch eine ungerade Anzahl metallischer Dirac-Oberflächenzustände in der Oberflächenbrillouinzone gekennzeichnet. Die topologischen kristallinen Isolatoren hingegen sind durch einzelne Kristallsymmetrien geschützt und weisen eine gerade Anzahl von Dirac-Kegeln auf. Die topologischen Eigenschaften von Blei-Zinn-Chalkogenid-TCI lassen sich durch Temperatur sowie chemische Zusammensetzung einstellen. Hier wird gezeigt, dass Bi-Dotierung von eptiaktischen Pb1-xSnxSe(111)-Schichten einen Quantenphasenübergang von einem topologischen kristallinen Isolator zu einem Z2-topologischen Isolator hervorruft. Dies geschieht, weil die Dotierung mit Bi die vierfache Valley-Entartung im Volumen aufhebt. Als Konsequenz entsteht eine Lücke bei ⌈¯, während die drei Dirac-Kegel an den M̅-Punkten der Oberflächenbrillouinzone intakt bleiben. Wir interpretieren diesen neuen Phasenübergang als durch eine Gitterverzerrung verursacht. Unsere Ergebnisse erweitern das topologische Phasendiagramm enorm und machen starke topologische Isolatoren durch Verzerrungen oder elektrische Felder schaltbar. Im Gegensatz dazu induziert eine Bi-Dotierung im Volumen von epitaktischen Pb1-xSnxTe(111)-Schichten eine riesige Rashba-Aufspaltung an der Oberfläche, die durch das Ausmaß der Dotierung eingestellt werden kann. Tight-Binding-Berechnungen identifizieren ihren Ursprung in einem Fermi-Niveau-Pinning durch Trap-Zustände an der Oberfläche. Magnetisch dotierte topologische Isolatoren ermöglichen den quantisierten anomalen Hall-Effekt (QAHE), der quantisierte Kantenzustände liefert, die für verlustfreien Ladungstransport eingesetzt werden können. Die Kantenzustände treten in einer magnetischen Energielücke am Dirac-Punkt auf, die bisher noch nicht experimentell beobachtet wurde. Unsere Tieftemperatur-ARPES-Untersuchungen weisen die magnetische Energielücke in Mn dotiertem Bi2Te3 eindeutig nach. Unsere Analyse zeigt unterhalb von Tc eine viermal größere Energielücke als theoretisch vorhergesagt. Wir führen diese Erhöhung auf eine bemerkenswerte Strukturmodifikation durch die Mn-Dotierung zurück. Statt eines Systems mit ungeordneten Mn Verunreinigungen entsteht eine selbstorganisierte alternierende Sequenz von MnBi2Te4-Septupel- und Bi2Te3-Quintupel-Schichten. Das erhöht den Überlapp der Wellenfunktionen und führt zu der großen magnetischen Energielücke. Mn-dotiertes Bi2Se3 bildet ähnliche Heterostrukturen aus, jedoch wird in diesem System nur eine nichtmagnetische Energielücke beobachtet. Dies korreliert mit der unterschiedlichen magnetischen Anisotropie aufgrund der viel größeren Spin-Bahn-Wechselwirkung im Bi2Te3 im Vergleich zu Bi2Se3. Diese Resultate liefern entscheidende Erkenntnisse, um verlustfreien Transport in topologischen Isolatoren für Anwendungen bei Raumtemperatur weiterzuentwickeln. KW - ARPES KW - Topological Insulator KW - Topological Crystalline Insulator KW - Rashba effect KW - Rashba-Effekt KW - Topologischer kristalliner Isolator KW - Topologischer Isolator Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-480459 ER - TY - JOUR A1 - Garakani, Tayebeh Mirzaei A1 - Richter, Marina Juliane A1 - Böker, Alexander T1 - Controlling the bio-inspired synthesis of silica JF - Journal of colloid and interface science N2 - The influence of different parameters on the silicification procedure using lysozyme is reported. When polyethoxysiloxane (PEOS), an internally crosslinked silica reservoir, is used, regular structures with a narrow size distribution could be obtained only via introducing the silica precursor in two steps including initial dropping and subsequent addition of residual oil phase in one portion. We found that mixing sequence of mineralizing agents in the presence of a positively charged surfactant plays a key role in terms of silica precipitation when tetraethoxyorthosilicate (TEOS) is the oil phase. In contrast, well mineralized crumpled features with high specific surface area could be synthesized in the presence of PEOS as a silica precursor polymer, regardless of mixing sequence. Moreover, introducing sodium dodecyl sulfate (SDS) as a negatively charged surfactant resulted in regular silica sphere formation only in combination with hexylene glycol (MPD) as a specific co-solvent. Finally, it is demonstrated that by inclusion of different nanoparticles even more sophisticated hybrid materials can be generated. KW - Silicification KW - Lysozyme KW - Polyetlioxysiloxane KW - High specific surface area KW - Surfactant KW - Nanoparticles KW - Hybrid materials Y1 - 2016 U6 - https://doi.org/10.1016/j.jcis.2016.10.069 SN - 0021-9797 SN - 1095-7103 VL - 488 SP - 322 EP - 334 PB - Elsevier CY - San Diego ER - TY - JOUR A1 - Deb, Marwan A1 - Popova, Elena A1 - Jaffrès, Henri-Yves A1 - Keller, Niels A1 - Bargheer, Matias T1 - Controlling high-frequency spin-wave dynamics using double-pulse laser excitation JF - Physical review applied N2 - Manipulating spin waves is highly required for the development of innovative data transport and processing technologies. Recently, the possibility of triggering high-frequency standing spin waves in magnetic insulators using femtosecond laser pulses was discovered, raising the question about how one can manipulate their dynamics. Here we explore this question by investigating the ultrafast magnetiza-tion and spin-wave dynamics induced by double-pulse laser excitation. We demonstrate a suppression or enhancement of the amplitudes of the standing spin waves by precisely tuning the time delay between the two pulses. The results can be understood as the constructive or destructive interference of the spin waves induced by the first and second laser pulses. Our findings open exciting perspectives towards generating single-mode standing spin waves that combine high frequency with large amplitude and low magnetic damping. Y1 - 2022 U6 - https://doi.org/10.1103/PhysRevApplied.18.044001 SN - 2331-7019 VL - 18 IS - 4 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Rosenblum, Michael T1 - Controlling collective synchrony in oscillatory ensembles by precisely timed pulses JF - Chaos : an interdisciplinary journal of nonlinear science N2 - We present an efficient technique for control of synchrony in a globally coupled ensemble by pulsatile action. We assume that we can observe the collective oscillation and can stimulate all elements of the ensemble simultaneously. We pay special attention to the minimization of intervention into the system. The key idea is to stimulate only at the most sensitive phase. To find this phase, we implement an adaptive feedback control. Estimating the instantaneous phase of the collective mode on the fly, we achieve efficient suppression using a few pulses per oscillatory cycle. We discuss the possible relevance of the results for neuroscience, namely, for the development of advanced algorithms for deep brain stimulation, a medical technique used to treat Parkinson's disease. Y1 - 2020 U6 - https://doi.org/10.1063/5.0019823 SN - 1054-1500 SN - 1089-7682 VL - 30 IS - 9 PB - American Institute of Physics CY - Melville ER - TY - THES A1 - Voroshnin, Vladimir T1 - Control over spin and electronic structure of MoS₂ monolayer via interactions with substrates T1 - Kontrolle der Spin- und elektronischen Strukturen von MoS₂-Monolagen durch Wechselwirkungen mit Substraten N2 - The molybdenum disulfide (MoS2) monolayer is a semiconductor with a direct bandgap while it is a robust and affordable material. It is a candidate for applications in optoelectronics and field-effect transistors. MoS2 features a strong spin-orbit coupling which makes its spin structure promising for acquiring the Kane-Mele topological concept with corresponding applications in spintronics and valleytronics. From the optical point of view, the MoS2 monolayer features two valleys in the regions of K and K' points. These valleys are differentiated by opposite spins and a related valley-selective circular dichroism. In this study we aim to manipulate the MoS2 monolayer spin structure in the vicinity of the K and K' points to explore the possibility of getting control over the optical and electronic properties. We focus on two different substrates to demonstrate two distinct routes: a gold substrate to introduce a Rashba effect and a graphene/cobalt substrate to introduce a magnetic proximity effect in MoS2. The Rashba effect is proportional to the out-of-plane projection of the electric field gradient. Such a strong change of the electric field occurs at the surfaces of a high atomic number materials and effectively influence conduction electrons as an in-plane magnetic field. A molybdenum and a sulfur are relatively light atoms, thus, similar to many other 2D materials, intrinsic Rashba effect in MoS2 monolayer is vanishing small. However, proximity of a high atomic number substrate may enhance Rashba effect in a 2D material as it was demonstrated for graphene previously. Another way to modify the spin structure is to apply an external magnetic field of high magnitude (several Tesla), and cause a Zeeman splitting, the conduction electrons. However, a similar effect can be reached via magnetic proximity which allows us to reduce external magnetic fields significantly or even to zero. The graphene on cobalt interface is ferromagnetic and stable for MoS2 monolayer synthesis. Cobalt is not the strongest magnet; therefore, stronger magnets may lead to more significant results. Nowadays most experimental studies on the dichalcogenides (MoS2 included) are performed on encapsulated heterostructures that are produced by mechanical exfoliation. While mechanical exfoliation (or scotch-tape method) allows to produce a huge variety of structures, the shape and the size of the samples as well as distance between layers in heterostructures are impossible to control reproducibly. In our study we used molecular beam epitaxy (MBE) methods to synthesise both MoS2/Au(111) and MoS2/graphene/Co systems. We chose to use MBE, as it is a scalable and reproducible approach, so later industry may adapt it and take over. We used graphene/cobalt instead of just a cobalt substrate because direct contact of MoS2\ monolayer and a metallic substrate may lead to photoluminescence (PL) quenching in the metallic substrate. Graphene and hexagonal boron nitride monolayer are considered building blocks of a new generation of electronics also commonly used as encapsulating materials for PL studies. Moreover graphene is proved to be a suitable substrate for the MBE growth of transitional metal dichalcogenides (TMDCs). In chapter 1, we start with an introduction to TMDCs. Then we focus on MoS2 monolayer state of the art research in the fields of application scenario; synthesis approaches; electronic, spin, and optical properties; and interactions with magnetic fields and magnetic materials. We briefly touch the basics of magnetism in solids and move on to discuss various magnetic exchange interactions and magnetic proximity effect. Then we describe MoS2 optical properties in more detail. We start from basic exciton physics and its manifestation in the MoS2 monolayer. We consider optical selection rules in the MoS2 monolayer and such properties as chirality, spin-valley locking, and coexistence of bright and dark excitons. Chapter 2 contains an overview of the employed surface science methods: angle-integrated, angle-resolved, and spin-resolved photoemission; low energy electron diffraction and scanning tunneling microscopy. In chapter 3, we describe MoS2 monolayer synthesis details for two substrates: gold monocrystal with (111) surface and graphene on cobalt thin film with Co(111) surface orientation. The synthesis descriptions are followed by a detailed characterisation of the obtained structures: fingerprints of MoS2 monolayer formation; MoS2 monolayer symmetry and its relation to the substrate below; characterisation of MoS2 monolayer coverage, domain distribution, sizes and shapes, and moire structures. In chapter~4, we start our discussion with MoS2/Au(111) electronic and spin structure. Combining density functional theory computations (DFT) and spin-resolved photoemission studies, we demonstrate that the MoS2 monolayer band structure features an in-plane Rashba spin splitting. This confirms the possibility of MoS2 monolayer spin structure manipulation via a substrate. Then we investigate the influence of a magnetic proximity in the MoS2/graphene/Co system on the MoS2 monolayer spin structure. We focus our investigation on MoS2 high symmetry points: G and K. First, using spin-resolved measurements, we confirm that electronic states are spin-split at the G point via a magnetic proximity effect. Second, combining spin-resolved measurements and DFT computations for MoS2 monolayer in the K point region, we demonstrate the appearance of a small in-plane spin polarisation in the valence band top and predict a full in-plane spin polarisation for the conduction band bottom. We move forward discussing how these findings are related to the MoS2 monolayer optical properties, in particular the possibility of dark exciton observation. Additionally, we speculate on the control of the MoS2 valley energy via magnetic proximity from cobalt. As graphene is spatially buffering the MoS2 monolayer from the Co thin film, we speculate on the role of graphene in the magnetic proximity transfer by replacing graphene with vacuum and other 2D materials in our computations. We finish our discussion by investigating the K-doped MoS2/graphene/Co system and the influence of this doping on the electronic and spin structure as well as on the magnetic proximity effect. In summary, using a scalable MBE approach we synthesised MoS2/Au(111) and MoS2/graphene/Co systems. We found a Rashba effect taking place in MoS2/Au(111) which proves that the MoS2 monolayer in-plane spin structure can be modified. In MoS2/graphene/Co the in-plane magnetic proximity effect indeed takes place which rises the possibility of fine tuning the MoS2 optical properties via manipulation of the the substrate magnetisation. N2 - Die Molybdändisulfid-Monolage MoS2 ist ein mechanisch und chemisch robuster Direktband-Halbleiter. Seine Valley-Eigenschaften versprechen spin- und valleybezogene spintronische, valleytronische und optoelektronische Anwendungen. Hier wurde die Möglichkeit untersucht, die elektronische und Spin-Struktur von MoS2 durch Wechselwirkung mit einem Substrat zu beeinflussen. Eine Kontrolle der elektronischen Struktur und der Spinstruktur ist unerlässlich, wenn man die optischen Eigenschaften kontrollieren will. Wir haben zwei Methoden angewandt, um die Spinentartung aufzuheben. Zum einen haben wir die MoS2-Monolage einem elektrischen Potentialgradienten ausgesetzt, zum anderen einem starken Magnetfeld in der Ebene, das durch einen magnetischen Proximity-Effekt erzeugt wurde. Zu diesem Zweck haben wir zwei Systeme hergestellt: eine MoS2-Monolage auf einem Gold-Einkristall und eine MoS2-Monolage auf Graphen auf einer dünnen Kobalt-Schicht. Wir modifizierten ein zuvor vorgeschlagenes Verfahren zur Synthese der MoS2-Monolage auf der Au(111)-Oberfläche und erzielten mit der MBE-Methode eine Bedeckung von ≈0,4 MoS2-Monolage. Die Bedeckung wies eine 10×10-Überstruktur auf und bestand aus gleichmäßig verteilten und ähnlich orientierten Domänen von ≈30 nm Größe. Die Kombination aus DFT-Simulationen und spinaufgelöster Photoemissionsspektroskopie bestätigte das Vorhandensein einer Rashba-ähnlichen Spinaufspaltung in der Ebene der MoS2-Valenzbandzustände in Bereichen mit k ⃗≠0, aus Symmetriegründen mit Ausnahme des K ̅-Punktes. Wir untersuchten die Möglichkeit, die Spinstruktur der MoS2-Monolage durch den magnetischen Proximity-Effekt zu manipulieren, einschließlich der Bereiche um Hochsymmetriepunkte. Die Regionen der K ̅ und (K') ̅ Punkte sind am interessantesten, da sich dort die direkte Bandlücke der MoS2-Monolage befindet. Eine energetische Verschiebung der Valenz- und Leitungsbandzustände relativ zueinander in diesen Bereichen sollte die Kontrolle über die Valley-Aufspaltung der MoS2-Monolage ermöglichen. Andererseits sollte eine Neigung der Spinrichtung dieser Zustände in Richtung der Spinrichtung in der Ebene die optischen Auswahlregeln drastisch ändern und ein dunkles Exziton aufhellen. Wir haben ein Kobalt-Dünnschichtsubstrat aufgrund seiner in der Ebene liegenden magnetischen Anisotropie als Testobjekt für den magnetischen Proximity-Effekt gewählt. Wir benutzten die CVD-Methode, um es zunächst mit Graphen zu beschichten, und dann die MBE-Methode, um eine ≈0,4 MoS2-Monolage auf dem Substrat herzustellen. Wir platzierten das Graphen zwischen der MoS2-Monolage und der Kobalt-Dünnschicht, um zu verhindern, dass die Exzitonen von MoS2 durch das metallische Substrat ausgelöscht werden. Für die Herstellung der MoS2-Monolage auf dem Graphen/Co-Substrat haben wir ein Verfahren modifiziert, das für die Herstellung von MoS2 auf einem Graphen/Ir-Substrat vorgeschlagen wurde. Die einschichtige MoS2-Bedeckung von ≈0,4 Atomlagen zeigt eine große Fläche kristalliner Domänen von mehr als 200 nm Größe. Vier MoS2-Einheitszellen entsprechen fünf Graphen-Einheitszellen. Dies lässt auf eine 5×5-Überstruktur schließen, was durch die LEED-Bilder bestätigt wird. Andererseits zeigt die STM-Abbildung mit atomarer Auflösung eine quasi-periodische Struktur mit mehreren Arten von Moiré-Mustern. Da das epitaktische Wachstum von MoS2 entlang der Graphen-Untergitter A und B gleich wahrscheinlich ist, weist die MoS2-Monolage eine gleiche Anzahl von um 180° gedrehten Domänen (Spiegeldomänen) auf. Wir bestätigten zunächst das Vorhandensein des magnetischen Proximity-Effekts, indem wir ≈20 meV Zeeman in-plane Spinaufspaltung von MoS2-Valenzbandzuständen im Bereich von Γ ̅ sowohl in DFT-Berechnungen als auch in spinaufgelösten Photoemissionsmessungen beobachteten. Im Bereich von K ̅ stimmen Berechnungen und Messungen in Bezug auf die Spinpolarisation der Valenzbandzustände von 15% überein. Im Gegensatz dazu deuten die DFT-Berechnungen für die Leitungsbandzustände auf eine 100%ige Spinpolarisierung der Zustände hin. Dieses Ergebnis impliziert die Aufhellung der dunklen Exzitonen der MoS2-Monolage. Im Falle der Magnetisierung von Kobalt senkrecht zur Ebene sagen DFT-Berechnungen eine Spinaufspaltung von ≈16 meV bei Γ ̅ und ≈8 meV bei K ̅ und (K') ̅ voraus. Um die Leitungsbandzustände von MoS2 zu untersuchen, haben wir MoS2/Graphen/Co mit Kaliumatomen dotiert. Das resultierende System zeichnet sich dadurch aus, dass Kaliumatome unter MoS2 interkaliert sind, was den Abstand zwischen der MoS2-Monolage und dem Kobaltfilm vergrößert. Spinaufgelöste Messungen zeigten keine in-plane Spinaufspaltung der MoS2-Valenzbandzustände im Bereich von Γ ̅ und der MoS2-Leitungsbandzustände im Bereich von K ̅. Da Graphen eine Pufferschicht zwischen der MoS2-Monolage und der Kobaltschicht bildet, um ein mögliches Exzitonen-Löschen zu verhindern, untersuchten wir die Rolle von Graphen für den magnetischen Proximity-Effekt. In unseren Berechnungen ersetzten wir Graphen durch eine Graphen-Doppelschicht, h-BN, Vakuum und eine Kupfer-Monolage und kamen zu dem Schluss, dass der Effekt stark vom Orbitalüberlapp abhängt. MoS2/h-BN/Co zeigen ähnliche Ergebnisse, während MoS2-Graphen-Doppelschicht/Co und MoS2/Vakuum/Co fast keinen magnetischen Proximity-Effekt aufweisen. Zusammenfassend haben wir mittels spektroskopischer Methoden und DFT-Berechnungen bewiesen, dass die Spinstruktur einer MoS2-Monolage durch Wechselwirkung mit einem Substrat manipuliert werden kann. Wir blicken daher mit Zuversicht auf zukünftige optische Untersuchungen zur optischen Kontrolle von MoS2 und Dichalcogeniden der Gruppe VI durch den magnetischen Proximity-Effekt. KW - MoS₂ KW - Au(111) KW - gold substrate KW - magnetic proximity effect KW - dark exciton KW - Molybdenum sulfide monolayer KW - graphene KW - spin structure KW - valence band structure KW - Cobalt thin film KW - ARPES KW - Angle- and spin-resolved photoemission spectroscopy KW - MBE KW - Molecular Beam Epitaxy KW - CVD KW - Chemical Vapour Deposition KW - synthesis KW - XPS KW - X-rays Photoemission Spectroscopy KW - LEED KW - Low Energy Electron Diffraction KW - MoS₂ KW - Molybdänsulfid Monolagen KW - Graphene KW - Au(111) KW - Goldsubstrat KW - magnetischer Näherungseffekt KW - dunkles Exziton KW - Spinstruktur KW - Leitungsbandstruktur KW - Kobalt-Dünnfilm KW - Photoelektronenspektroskopie KW - Molekularstrahlepitaxie KW - chemische Gasphasenabscheidung KW - Synthese KW - Beugung niederenergetischer Elektronen Y1 - 2023 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-590709 ER - TY - JOUR A1 - Totz, Sonja Juliana A1 - Löber, Jakob A1 - Totz, Jan Frederik A1 - Engel, Harald T1 - Control of transversal instabilities in reaction-diffusion systems JF - New journal of physics : the open-access journal for physics N2 - In two-dimensional reaction-diffusion systems, local curvature perturbations on traveling waves are typically damped out and vanish. However, if the inhibitor diffuses much faster than the activator, transversal instabilities can arise, leading from flat to folded, spatio-temporally modulated waves and to spreading spiral turbulence. Here, we propose a scheme to induce or inhibit these instabilities via a spatio-temporal feedback loop. In a piecewise-linear version of the FitzHugh-Nagumo model, transversal instabilities and spiral turbulence in the uncontrolled system are shown to be suppressed in the presence of control, thereby stabilizing plane wave propagation. Conversely, in numerical simulations with the modified Oregonator model for the photosensitive Belousov-Zhabotinsky reaction, which does not exhibit transversal instabilities on its own, we demonstrate the feasibility of inducing transversal instabilities and study the emerging wave patterns in a well-controlled manner. KW - traveling waves KW - control KW - transversal instabilities Y1 - 2018 U6 - https://doi.org/10.1088/1367-2630/aabce5 SN - 1367-2630 VL - 20 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Heinrich, Benjamin W. A1 - Ehlert, Christopher A1 - Hatter, Nino A1 - Braun, Lukas A1 - Lotze, Christian A1 - Saalfrank, Peter A1 - Franke, Katharina J. T1 - Control of oxidation and spin state in a single-molecule junction JF - ACS nano N2 - The oxidation and spin state of a metal-organic molecule determine its chemical reactivity and magnetic properties. Here, we demonstrate the reversible control of the oxidation and spin state in a single Fe porphyrin molecule in the force field of the tip of a scanning electron tunneling microscope. Within the regimes of half-integer and integer spin state, we can further track the evolution of the magnetocrystalline anisotropy. Our experimental results are corroborated by density functional theory and wave function theory. This combined analysis allows us to draw a complete picture of the molecular states over a large range of intramolecular deformations. KW - porphyrin KW - oxidation state KW - spin state KW - scanning tunneling microscopy KW - scanning tunneling spectroscopy KW - density functional theory Y1 - 2018 U6 - https://doi.org/10.1021/acsnano.8b00312 SN - 1936-0851 SN - 1936-086X VL - 12 IS - 4 SP - 3172 EP - 3177 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Pawlik, Andreas H. A1 - Pikovskij, Arkadij T1 - Control of oscillators coherence by multiple delayed feedback JF - Modern physics letters : A, Particles and fields, gravitation, cosmology, nuclear physics N2 - We demonstrate that a multiple delayed feedback is a powerful tool to control coherence properties of autonomous self-sustained oscillators. We derive the equation for the phase dynamics in presence of noise and delay, and analyze it analytically. In Gaussian approximation a closed set of equations for the frequency and the diffusion constant is obtained. Solutions of these equations are in good agreement with direct numerical simulations. KW - phase diffusion KW - delayed feedback KW - control Y1 - 2006 U6 - https://doi.org/10.1016/j.physleta.2006.05.013 SN - 0375-9601 VL - 358 IS - 3 SP - 181 EP - 185 PB - American Institute of Physics CY - Amsterdam ER - TY - JOUR A1 - Dahlke, Sandro A1 - Maturilli, Marion T1 - Contribution of atmospheric advection to the amplified winter warming in the arctic north atlantic region JF - Advances in meteorology N2 - Arctic Amplification of climate warming is caused by various feedback processes in the atmosphere-ocean-ice system and yields the strongest temperature increase during winter in the Arctic North Atlantic region. In our study, we attempt to quantify the advective contribution to the observed atmospheric warming in the Svalbard area. Based on radiosonde measurements from Ny-Ålesund, a strong dependence of the tropospheric temperature on the synoptic flow direction is revealed. Using FLEXTRA backward trajectories, an increase of advection from the lower latitude Atlantic region towards Ny-Ålesund is found that is attributed to a change in atmospheric circulation patterns. We find that about one-quarter (0.45 K per decade) of the observed atmospheric winter near surface warming trend in the North Atlantic region of the Arctic (2 K per decade) is due to increased advection of warm and moist air from the lower latitude Atlantic region, affecting the entire troposphere. Y1 - 2017 U6 - https://doi.org/10.1155/2017/4928620 SN - 1687-9309 SN - 1687-9317 PB - Hindawi Publ. Corp. CY - New York ER - TY - JOUR A1 - Mendez, Vicenc A1 - Maso-Puigdellosas, Axel A1 - Sandev, Trifce A1 - Campos, Daniel T1 - Continuous time random walks under Markovian resetting JF - Physical review : E, Statistical, nonlinear and soft matter physics N2 - We investigate the effects of Markovian resetting events on continuous time random walks where the waiting times and the jump lengths are random variables distributed according to power-law probability density functions. We prove the existence of a nonequilibrium stationary state and finite mean first arrival time. However, the existence of an optimum reset rate is conditioned to a specific relationship between the exponents of both power-law tails. We also investigate the search efficiency by finding the optimal random walk which minimizes the mean first arrival time in terms of the reset rate, the distance of the initial position to the target, and the characteristic transport exponents. Y1 - 2021 U6 - https://doi.org/10.1103/PhysRevE.103.022103 SN - 2470-0045 SN - 2470-0053 VL - 103 IS - 2 PB - American Physical Society CY - College Park ER - TY - GEN A1 - Granado, Felipe Le Vot A1 - Abad, Enrique A1 - Metzler, Ralf A1 - Yuste, Santos B. T1 - Continuous time random walk in a velocity field BT - role of domain growth, Galilei-invariant advection-diffusion, and kinetics of particle mixing T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - We consider the emerging dynamics of a separable continuous time random walk (CTRW) in the case when the random walker is biased by a velocity field in a uniformly growing domain. Concrete examples for such domains include growing biological cells or lipid vesicles, biofilms and tissues, but also macroscopic systems such as expanding aquifers during rainy periods, or the expanding Universe. The CTRW in this study can be subdiffusive, normal diffusive or superdiffusive, including the particular case of a Lévy flight. We first consider the case when the velocity field is absent. In the subdiffusive case, we reveal an interesting time dependence of the kurtosis of the particle probability density function. In particular, for a suitable parameter choice, we find that the propagator, which is fat tailed at short times, may cross over to a Gaussian-like propagator. We subsequently incorporate the effect of the velocity field and derive a bi-fractional diffusion-advection equation encoding the time evolution of the particle distribution. We apply this equation to study the mixing kinetics of two diffusing pulses, whose peaks move towards each other under the action of velocity fields acting in opposite directions. This deterministic motion of the peaks, together with the diffusive spreading of each pulse, tends to increase particle mixing, thereby counteracting the peak separation induced by the domain growth. As a result of this competition, different regimes of mixing arise. In the case of Lévy flights, apart from the non-mixing regime, one has two different mixing regimes in the long-time limit, depending on the exact parameter choice: in one of these regimes, mixing is mainly driven by diffusive spreading, while in the other mixing is controlled by the velocity fields acting on each pulse. Possible implications for encounter–controlled reactions in real systems are discussed. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1005 KW - diffusion KW - expanding medium KW - continuous time random walk Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-479997 SN - 1866-8372 IS - 1005 SP - 28 ER - TY - JOUR A1 - Granado, Felipe Le Vot A1 - Abad, Enrique A1 - Metzler, Ralf A1 - Yuste, Santos B. T1 - Continuous time random walk in a velocity field BT - role of domain growth, Galilei-invariant advection-diffusion, and kinetics of particle mixing JF - New Journal of Physics N2 - We consider the emerging dynamics of a separable continuous time random walk (CTRW) in the case when the random walker is biased by a velocity field in a uniformly growing domain. Concrete examples for such domains include growing biological cells or lipid vesicles, biofilms and tissues, but also macroscopic systems such as expanding aquifers during rainy periods, or the expanding Universe. The CTRW in this study can be subdiffusive, normal diffusive or superdiffusive, including the particular case of a Lévy flight. We first consider the case when the velocity field is absent. In the subdiffusive case, we reveal an interesting time dependence of the kurtosis of the particle probability density function. In particular, for a suitable parameter choice, we find that the propagator, which is fat tailed at short times, may cross over to a Gaussian-like propagator. We subsequently incorporate the effect of the velocity field and derive a bi-fractional diffusion-advection equation encoding the time evolution of the particle distribution. We apply this equation to study the mixing kinetics of two diffusing pulses, whose peaks move towards each other under the action of velocity fields acting in opposite directions. This deterministic motion of the peaks, together with the diffusive spreading of each pulse, tends to increase particle mixing, thereby counteracting the peak separation induced by the domain growth. As a result of this competition, different regimes of mixing arise. In the case of Lévy flights, apart from the non-mixing regime, one has two different mixing regimes in the long-time limit, depending on the exact parameter choice: in one of these regimes, mixing is mainly driven by diffusive spreading, while in the other mixing is controlled by the velocity fields acting on each pulse. Possible implications for encounter–controlled reactions in real systems are discussed. KW - diffusion KW - expanding medium KW - continuous time random walk Y1 - 2020 U6 - https://doi.org/10.1088/1367-2630/ab9ae2 SN - 1367-2630 VL - 22 PB - Dt. Physikalische Ges. CY - Bad Honnef ER - TY - JOUR A1 - Kegeles, Alexander A1 - Oriti, Daniele T1 - Continuous point symmetries in group field theories JF - Journal of physics : A, Mathematical and theoretical N2 - We discuss the notion of symmetries in non-local field theories characterized by integro-differential equations of motion, from a geometric perspective. We then focus on group field theory (GFT) models of quantum gravity and provide a general analysis of their continuous point symmetry transformations, including the generalized conservation laws following from them. KW - group field theory KW - quantum field theory KW - conservation laws KW - continuous symmetries Y1 - 2017 U6 - https://doi.org/10.1088/1751-8121/aa5c14 SN - 1751-8113 SN - 1751-8121 VL - 50 IS - 12 PB - IOP Publishing Ltd CY - Bristol ER - TY - JOUR A1 - Koopman, Wouter-Willem Adriaan A1 - Natali, Marco A1 - Bettini, Cristian A1 - Melucci, Manuela A1 - Muccini, Michele A1 - Toffanin, Stefano T1 - Contact Resistance in Ambipolar Organic Field-Effect Transistors Measured by Confocal Photoluminescence Electro-Modulation Microscopy JF - ACS applied materials & interfaces N2 - Although it is theoretically expected that all organic semiconductors support ambipolar charge transport, most organic transistors either transport holes or electrons effectively. Single-layer ambipolar organic field-effect transistors enable the investigation of different mechanisms in hole and electron transport in a single device since the device architecture provides a controllable planar pn-junction within the transistor channel. However, a direct comparison of the injection barriers and of the channel conductivities between electrons and holes within the same device cannot be measured by standard electrical characterization. This article introduces a novel approach for determining threshold gate voltages for the onset of the ambipolar regime from the position of the pn-junction observed by photoluminescence electromodulation (PLEM) microscopy. Indeed, the threshold gate voltage in the ambipolar bias regime considers a vanishing channel length, thus correlating the contact resistance. PLEM microscopy is a valuable tool to directly compare the contact and channel resistances for both carrier types in the same device. The reported results demonstrate that designing the metal/organic semiconductor interfaces by aligning the bulk metal Fermi levels to the highest occupied molecular orbital or lowest unoccupied molecular orbital levels of the organic semiconductors is a too simplistic approach for optimizing the charge injection process in organic field-effect devices. KW - electro-modulation microscopy KW - organic field-effect transistors KW - threshold voltages KW - contact resistance KW - photoluminescence Y1 - 2018 U6 - https://doi.org/10.1021/acsami.8b05518 SN - 1944-8244 VL - 10 IS - 41 SP - 35411 EP - 35419 PB - American Chemical Society CY - Washington ER - TY - THES A1 - Steinhaus, Sebastian Peter T1 - Constructing quantum spacetime T1 - Konstruktion der Quantenraumzeit BT - relation to classical gravity BT - Relation zur klassischen Gravitation N2 - Despite remarkable progress made in the past century, which has revolutionized our understanding of the universe, there are numerous open questions left in theoretical physics. Particularly important is the fact that the theories describing the fundamental interactions of nature are incompatible. Einstein's theory of general relative describes gravity as a dynamical spacetime, which is curved by matter and whose curvature determines the motion of matter. On the other hand we have quantum field theory, in form of the standard model of particle physics, where particles interact via the remaining interactions - electromagnetic, weak and strong interaction - on a flat, static spacetime without gravity. A theory of quantum gravity is hoped to cure this incompatibility by heuristically replacing classical spacetime by quantum spacetime'. Several approaches exist attempting to define such a theory with differing underlying premises and ideas, where it is not clear which is to be preferred. Yet a minimal requirement is the compatibility with the classical theory, they attempt to generalize. Interestingly many of these models rely on discrete structures in their definition or postulate discreteness of spacetime to be fundamental. Besides the direct advantages discretisations provide, e.g. permitting numerical simulations, they come with serious caveats requiring thorough investigation: In general discretisations break fundamental diffeomorphism symmetry of gravity and are generically not unique. Both complicates establishing the connection to the classical continuum theory. The main focus of this thesis lies in the investigation of this relation for spin foam models. This is done on different levels of the discretisation / triangulation, ranging from few simplices up to the continuum limit. In the regime of very few simplices we confirm and deepen the connection of spin foam models to discrete gravity. Moreover, we discuss dynamical, e.g. diffeomorphism invariance in the discrete, to fix the ambiguities of the models. In order to satisfy these conditions, the discrete models have to be improved in a renormalisation procedure, which also allows us to study their continuum dynamics. Applied to simplified spin foam models, we uncover a rich, non--trivial fixed point structure, which we summarize in a phase diagram. Inspired by these methods, we propose a method to consistently construct the continuum theory, which comes with a unique vacuum state. N2 - Trotz bemerkenswerter Fortschritte im vergangenen Jahrhundert, die unser Verständnis des Universums revolutioniert haben, gibt es noch zahlreiche ungeklärte Fragen in der theoretischen Physik. Besondere Bedeutung kommt der Tatsache zu, dass die Theorien, welche die fundamentalen Wechselwirkungen der Natur beschreiben, inkompatibel sind. Nach Einsteins allgemeiner Relativitätstheorie wird die Gravitation durch eine dynamische Raumzeit dargestellt, die von Materie gekrümmt wird und ihrerseits durch die Krümmung die Bewegung der Materie bestimmt. Dem gegenüber steht die Quantenfeldtheorie, die die verbliebenen Wechselwirkungen - elektromagnetische, schwache und starke Wechselwirkung - im Standardmodell der Teilchenphysik beschreibt, in dem Teilchen auf einer statischen Raumzeit -- ohne Gravitation -- miteinander interagieren. Die Hoffnung ist, dass eine Theorie der Quantengravitation diese Inkompatibilität beheben kann, indem, heuristisch, die klassische Raumzeit durch eine 'Quantenraumzeit' ersetzt wird. Es gibt zahlreiche Ansätze eine solche Theorie zu definieren, die auf unterschiedlichen Prämissen und Ideen beruhen, wobei a priori nicht klar ist, welche zu bevorzugen sind. Eine Minimalanforderung an diese Theorien ist Kompatibilität mit der klassischen Theorie, die sie verallgemeinern sollen. Interessanterweise basieren zahlreiche Modelle in ihrer Definition auf Diskretisierungen oder postulieren eine fundamentale Diskretheit der Raumzeit. Neben den unmittelbaren Vorteilen, die Diskretisierungen bieten, z.B. das Ermöglichen numerischer Simulationen, gibt es auch gravierende Nachteile, die einer ausführlichen Untersuchung bedürfen: Im Allgemeinen brechen Diskretisierungen die fundamentale Diffeomorphismensymmetrie der Gravitation und sind in der Regel nicht eindeutig definiert. Beides erschwert die Wiederherstellung der Verbindung zur klassischen, kontinuierlichen Theorie. Das Hauptaugenmerk dieser Doktorarbeit liegt darin diese Verbindung insbesondere für Spin-Schaum-Modelle (spin foam models) zu untersuchen. Dies geschieht auf sehr verschiedenen Ebenen der Diskretisierung / Triangulierung, angefangen bei wenigen Simplizes bis hin zum Kontinuumslimes. Im Regime weniger Simplizes wird die bekannte Verbindung von Spin--Schaum--Modellen zu diskreter Gravitation bestätigt und vertieft. Außerdem diskutieren wir dynamische Prinzipien, z.B. Diffeomorphismeninvarianz im Diskreten, um die Ambiguitäten der Modelle zu fixieren. Um diese Bedingungen zu erfüllen, müssen die diskreten Modelle durch Renormierungsverfahren verbessert werden, wodurch wir auch ihre Kontinuumsdynamik untersuchen können. Angewandt auf vereinfachte Spin-Schaum-Modelle finden wir eine reichhaltige, nicht-triviale Fixpunkt-Struktur, die wir in einem Phasendiagramm zusammenfassen. Inspiriert von diesen Methoden schlagen wir zu guter Letzt eine konsistente Konstruktionsmethode für die Kontinuumstheorie vor, die einen eindeutigen Vakuumszustand definiert. KW - Quantengravitation KW - Spin-Schaum-Modelle KW - Regge Kalkül KW - Renormierung KW - Verfeinerungslimes KW - quantum gravity KW - spin foams KW - regge calculus KW - renormalization and refinement limit Y1 - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-72558 ER - TY - JOUR A1 - Samajdar, Anuradha A1 - Dietrich, Tim T1 - Constructing Love-Q relations with gravitational wave detections JF - Physical review : D, Particles, fields, gravitation, and cosmology N2 - Quasiuniversal relations between the tidal deformability and the quadrupole moment of neutron stars are predicted by theoretical computations, but have not been measured experimentally. We simulate 120 binary neutron star sources and find that Advanced LIGO and Advanced Virgo at design sensitivity could find possible deviations from predicted relations if the neutron stars are highly spinning. A network of envisaged third generation detectors will even allow extracting such relations, providing new tests of general relativity and nuclear physics predictions. Y1 - 2020 U6 - https://doi.org/10.1103/PhysRevD.101.124014 SN - 1550-7998 SN - 1550-2368 VL - 101 IS - 12 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Abdalla, Hassan E. A1 - Adam, R. A1 - Aharonian, Felix A. A1 - Benkhali, F. Ait A1 - Angüner, Ekrem Oǧuzhan A1 - Arakawa, M. A1 - Arcaro, C. A1 - Armand, C. A1 - Ashkar, H. A1 - Backes, M. A1 - Martins, V. Barbosa A1 - Barnard, M. A1 - Becherini, Y. A1 - Berge, D. A1 - Bernloehr, K. A1 - Blackwell, R. A1 - Böttcher, M. A1 - Boisson, C. A1 - Bolmont, J. A1 - Bonnefoy, S. A1 - Bregeon, J. A1 - Breuhaus, M. A1 - Brun, F. A1 - Brun, P. A1 - Bryan, M. A1 - Büchele, M. A1 - Bulik, T. A1 - Bylund, T. A1 - Capasso, M. A1 - Caroff, S. A1 - Carosi, A. A1 - Casanova, Sabrina A1 - Cerruti, M. A1 - Chand, T. A1 - Chandra, S. A1 - Chen, A. A1 - Colafrancesco, S. A1 - Curylo, M. A1 - Davids, I. D. A1 - Deil, C. A1 - Devin, J. A1 - DeWilt, P. A1 - Dirson, L. A1 - Djannati-Ata, A. A1 - Dmytriiev, A. A1 - Donath, A. A1 - Doroshenko, V A1 - Dyks, J. A1 - Egberts, Kathrin A1 - Emery, G. A1 - Ernenwein, J-P A1 - Eschbach, S. A1 - Feijen, K. A1 - Fegan, S. A1 - Fiasson, A. A1 - Fontaine, G. A1 - Funk, S. A1 - Füßling, Matthias A1 - Gabici, S. A1 - Gallant, Y. A. A1 - Gate, F. A1 - Giavitto, G. A1 - Glawion, D. A1 - Glicenstein, J. F. A1 - Gottschall, D. A1 - Grondin, M-H A1 - Hahn, J. A1 - Haupt, M. A1 - Heinzelmann, G. A1 - Henri, G. A1 - Hermann, G. A1 - Hinton, James Anthony A1 - Hofmann, W. A1 - Hoischen, Clemens A1 - Holch, Tim Lukas A1 - Holler, M. A1 - Horns, D. A1 - Huber, D. A1 - Iwasaki, H. A1 - Jamrozy, M. A1 - Jankowsky, D. A1 - Jankowsky, F. A1 - Jardin-Blicq, A. A1 - Jung-Richardt, I A1 - Kastendieck, M. A. A1 - Katarzynski, K. A1 - Katsuragawa, M. A1 - Katz, U. A1 - Khangulyan, D. A1 - Khelifi, B. A1 - King, J. A1 - Klepser, S. A1 - Kluzniak, W. A1 - Komin, Nu A1 - Kosack, K. A1 - Kostunin, D. A1 - Kraus, M. A1 - Lamanna, G. A1 - Lau, J. A1 - Lemiere, A. A1 - Lemoine-Goumard, M. A1 - Lenain, J-P A1 - Leser, Eva A1 - Levy, C. A1 - Lohse, T. A1 - Lypova, I A1 - Mackey, J. A1 - Majumdar, J. A1 - Malyshev, D. A1 - Marandon, V A1 - Marcowith, Alexandre A1 - Mares, A. A1 - Mariaud, C. A1 - Marti-Devesa, G. A1 - Marx, R. A1 - Maurin, G. A1 - Meintjes, P. J. A1 - Mitchell, A. M. W. A1 - Moderski, R. A1 - Mohamed, M. A1 - Mohrmann, L. A1 - Moore, C. A1 - Moulin, Emmanuel A1 - Muller, J. A1 - Murach, T. A1 - Nakashima, S. A1 - de Naurois, M. A1 - Ndiyavala, H. A1 - Niederwanger, F. A1 - Niemiec, J. A1 - Oakes, L. A1 - Odaka, H. A1 - Ohm, S. A1 - Wilhelmi, E. de Ona A1 - Ostrowski, M. A1 - Oya, I A1 - Panter, M. A1 - Parsons, R. D. A1 - Perennes, C. A1 - Petrucci, P-O A1 - Peyaud, B. A1 - Piel, Q. A1 - Pita, S. A1 - Poireau, V A1 - Priyana Noel, A. A1 - Prokhorov, D. A. A1 - Prokoph, H. A1 - Pühlhofer, G. A1 - Punch, M. A1 - Quirrenbach, A. A1 - Raab, S. A1 - Rauth, R. A1 - Reimer, A. A1 - Reimer, O. A1 - Remy, Q. A1 - Renaud, M. A1 - Rieger, F. A1 - Rinchiuso, L. A1 - Romoli, C. A1 - Rowell, G. A1 - Rudak, B. A1 - Ruiz-Velasco, E. A1 - Sahakian, V A1 - Saito, S. A1 - Sanchez, David M. A1 - Santangelo, Andrea A1 - Sasaki, M. A1 - Schlickeiser, R. A1 - Schüssler, F. A1 - Schulz, A. A1 - Schutte, H. A1 - Schwanke, U. A1 - Schwemmer, S. A1 - Seglar-Arroyo, M. A1 - Senniappan, M. A1 - Seyffert, A. S. A1 - Shafi, N. A1 - Shiningayamwe, K. A1 - Simoni, R. A1 - Sinha, A. A1 - Sol, H. A1 - Specovius, A. A1 - Spir-Jacob, M. A1 - Stawarz, L. A1 - Steenkamp, R. A1 - Stegmann, Christian A1 - Steppa, Constantin Beverly A1 - Takahashi, T. A1 - Tavernier, T. A1 - Taylor, A. M. A1 - Terrier, R. A1 - Tiziani, D. A1 - Tluczykont, M. A1 - Trichard, C. A1 - Tsirou, M. A1 - Tsuji, N. A1 - Tuffs, R. A1 - Uchiyama, Y. A1 - van Der Walt, D. J. A1 - van Eldik, C. A1 - van Rensburg, C. A1 - van Soelen, B. A1 - Vasileiadis, G. A1 - Veh, J. A1 - Venter, C. A1 - Vincent, P. A1 - Vink, J. A1 - Voisin, F. A1 - Voelk, H. J. A1 - Vuillaume, T. A1 - Wadiasingh, Z. A1 - Wagner, S. J. A1 - White, R. A1 - Wierzcholska, A. A1 - Yang, R. A1 - Yoneda, H. A1 - Zacharias, Michael A1 - Zanin, R. A1 - Zdziarski, A. A. A1 - Zech, Alraune A1 - Ziegler, A. A1 - Zorn, J. A1 - Zywucka, N. A1 - Meyer, M. T1 - Constraints on the emission region of 3C 279 during strong flares in 2014 and 2015 through VHE gamma-ray observations with HESS JF - Astronomy and astrophysics : an international weekly journal N2 - The flat spectrum radio quasar 3C 279 is known to exhibit pronounced variability in the high-energy (100MeV < E < 100 GeV) gamma-ray band, which is continuously monitored with Fermi-LAT. During two periods of high activity in April 2014 and June 2015 target-of-opportunity observations were undertaken with the High Energy Stereoscopic System (H.E.S.S.) in the very-high-energy (VHE, E > 100 GeV) gamma-ray domain. While the observation in 2014 provides an upper limit, the observation in 2015 results in a signal with 8 : 7 sigma significance above an energy threshold of 66 GeV. No VHE variability was detected during the 2015 observations. The VHE photon spectrum is soft and described by a power-law index of 4.2 +/- 0.3. The H.E.S.S. data along with a detailed and contemporaneous multiwavelength data set provide constraints on the physical parameters of the emission region. The minimum distance of the emission region from the central black hole was estimated using two plausible geometries of the broad-line region and three potential intrinsic spectra. The emission region is confidently placed at r greater than or similar to 1 : 7 X 1017 cm from the black hole, that is beyond the assumed distance of the broad-line region. Time-dependent leptonic and lepto-hadronic one-zone models were used to describe the evolution of the 2015 flare. Neither model can fully reproduce the observations, despite testing various parameter sets. Furthermore, the H.E.S.S. data were used to derive constraints on Lorentz invariance violation given the large redshift of 3C 279. KW - radiation mechanisms: non-thermal KW - quasars: individual: 3C 279 KW - galaxies: active KW - relativistic processes Y1 - 2019 U6 - https://doi.org/10.1051/0004-6361/201935704 SN - 1432-0746 VL - 627 PB - EDP Sciences CY - Les Ulis ER - TY - THES A1 - Schneider von Deimling, Thomas T1 - Constraining uncertainty in climate sensitivity : an ensemble simulation approach based on glacial climate T1 - Einschränkung der Unsicherheit in der Klimasensitivität : ein Ensemble Simulationsansatz basierend auf dem glazialen Klima N2 - Uncertainty about the sensitivity of the climate system to changes in the Earth’s radiative balance constitutes a primary source of uncertainty for climate projections. Given the continuous increase in atmospheric greenhouse gas concentrations, constraining the uncertainty range in such type of sensitivity is of vital importance. A common measure for expressing this key characteristic for climate models is the climate sensitivity, defined as the simulated change in global-mean equilibrium temperature resulting from a doubling of atmospheric CO2 concentration. The broad range of climate sensitivity estimates (1.5-4.5°C as given in the last Assessment Report of the Intergovernmental Panel on Climate Change, 2001), inferred from comprehensive climate models, illustrates that the strength of simulated feedback mechanisms varies strongly among different models. The central goal of this thesis is to constrain uncertainty in climate sensitivity. For this objective we first generate a large ensemble of model simulations, covering different feedback strengths, and then request their consistency with present-day observational data and proxy-data from the Last Glacial Maximum (LGM). Our analyses are based on an ensemble of fully-coupled simulations, that were realized with a climate model of intermediate complexity (CLIMBER-2). These model versions cover a broad range of different climate sensitivities, ranging from 1.3 to 5.5°C, and have been generated by simultaneously perturbing a set of 11 model parameters. The analysis of the simulated model feedbacks reveals that the spread in climate sensitivity results from different realizations of the feedback strengths in water vapour, clouds, lapse rate and albedo. The calculated spread in the sum of all feedbacks spans almost the entire plausible range inferred from a sampling of more complex models. We show that the requirement for consistency between simulated pre-industrial climate and a set of seven global-mean data constraints represents a comparatively weak test for model sensitivity (the data constrain climate sensitivity to 1.3-4.9°C). Analyses of the simulated latitudinal profile and of the seasonal cycle suggest that additional present-day data constraints, based on these characteristics, do not further constrain uncertainty in climate sensitivity. The novel approach presented in this thesis consists in systematically combining a large set of LGM simulations with data information from reconstructed regional glacial cooling. Irrespective of uncertainties in model parameters and feedback strengths, the set of our model versions reveals a close link between the simulated warming due to a doubling of CO2, and the cooling obtained for the LGM. Based on this close relationship between past and future temperature evolution, we define a method (based on linear regression) that allows us to estimate robust 5-95% quantiles for climate sensitivity. We thus constrain the range of climate sensitivity to 1.3-3.5°C using proxy-data from the LGM at low and high latitudes. Uncertainties in glacial radiative forcing enlarge this estimate to 1.2-4.3°C, whereas the assumption of large structural uncertainties may increase the upper limit by an additional degree. Using proxy-based data constraints for tropical and Antarctic cooling we show that very different absolute temperature changes in high and low latitudes all yield very similar estimates of climate sensitivity. On the whole, this thesis highlights that LGM proxy-data information can offer an effective means of constraining the uncertainty range in climate sensitivity and thus underlines the potential of paleo-climatic data to reduce uncertainty in future climate projections. N2 - Eine der entscheidenden Hauptquellen für Unsicherheiten von Klimaprojektionen ist, wie sensitiv das Klimasystem auf Änderungen der Strahlungsbilanz der Erde reagiert. Angesichts des kontinuierlichen Anstiegs der atmosphärischen Treibhausgaskonzentrationen ist die Einschränkung des Unsicherheitsbereichs dieser Sensitivität von entscheidender Bedeutung. Ein häufig verwendetes Maß zur Beschreibung dieser charakteristischen Kenngröße von Klimamodellen ist die sogenannte Klimasensitivität, definiert als die Gleichgewichtsänderung der simulierten globalen Mitteltemperatur, welche sich aus einer Verdoppelung des atmosphärischen CO2-Gehalts ergibt. Die breite Spanne der geschätzten Klimasensitivität (1.5-4.5°C), welche ein Vergleich verschiedener komplexer Klimamodelle nahe legt (IPCC, 2001), verdeutlicht, wie groß die Unsicherheit in der Klimasensitivität ist. Diese Unsicherheit resultiert in erster Linie aus Unterschieden in der Simulation der entscheidenden Rückkopplungs-mechanismen in den verschiedenen Modellen. Das zentrale Ziel dieser Dissertation ist die Einschränkung des breiten Unsicherheitsbereichs der Klimasensitivität. Zunächst wird hierzu ein großes Ensemble an Modellsimulationen erzeugt, in welchem gezielt spezifische Modellparameter variiert, und somit unterschiedliche Rückkopplungsstärken der einzelnen Modellversionen realisiert werden. Diese Simulationen werden dann auf ihre Konsistenz mit sowohl heutigen Beobachtungsdaten, als auch Proxy-Daten des Letzten Glazialen Maximums (LGM) überprüft. Unsere Analysen basieren dabei auf einem Ensemble voll gekoppelter Modellläufe, welche mit einem Klimamodell intermediärer Komplexität (CLIMBER-2) realisiert wurden. Die betrachteten Modellversionen decken eine breite Spanne verschiedener Klimasensitivitäten (1.3-5.5°C) ab und wurden durch gleichzeitiges Variieren von 11 Modellparametern erzeugt. Die Analyse der simulierten Rückkopplungs-mechanismen offenbart, dass unterschiedliche Werte der Klimasensitivität in unserem Modellensemble durch verschiedene Realisierungen der Rückkopplungsstärken von Wasserdampf, Wolken, Temperatur-Vertikalprofil und Albedo zu erklären sind. Die berechneten Gesamt-Rückkopplungsstärken unser Modellversionen decken hierbei fast den gesamten möglichen Bereich von komplexeren Modellen ab. Wir zeigen, dass sich die Forderung nach Konsistenz zwischen simuliertem vorindustriellem Klima und Messdaten, die auf einer Wahl von sieben global gemittelten Datensätzen basieren, als vergleichsweise schwacher Test der Modellsensitivität erweist: Die Daten schränken den plausiblen Bereich der Klimasensitivität lediglich auf 1.3-4.9°C ein. Zieht man neben den genannten global gemittelten Messdaten außerdem klimatische Informationen aus Jahreszeit und geografischer Breite hinzu, lässt sich die Unsicherheit in der Klimasensitivität nicht weiter einschränken. Der neue Ansatz dieser Dissertation besteht darin, in systematischer Weise einen großen Satz an LGM-Simulationen mit Dateninformationen über die rekonstruierte glaziale Abkühlung bestimmter Regionen zu kombinieren. Unabhängig von den Unsicherheiten in Modellparametern und Rückkopplungsstärken offenbaren unsere Modellversionen eine ausgeprägte Beziehung zwischen der simulierten Erwärmung aufgrund der CO2-Verdoppelung und der Abkühlung im LGM. Basierend auf dieser engen Beziehung zwischen vergangener und zukünftiger Temperaturentwicklung definieren wir eine Methode (basierend auf linearer Regression), welche es uns erlaubt, robuste 5-95%-Quantile der Klimasensitivität abzuschätzen. Indem wir Proxy-Daten des LGM von niederen und hohen Breiten heranziehen, können wir die Unsicherheitsspanne der Klimasensitivität auf 1.3-3.5°C beschränken. Unsicherheiten im glazialen Strahlungsantrieb vergrößern diese Abschätzung auf 1.2-4.3°C, wobei die Annahme von großen strukturellen Unsicherheiten die obere Grenze um ein weiteres Grad erhöhen kann. Indem wir Proxy-Daten über tropische und antarktische Abkühlung betrachten, können wir zeigen, dass sehr unterschiedliche absolute Temperatur-Änderungen in hohen und niederen Breiten zu sehr ähnlichen Abschätzungen der Klimasensitivität führen. Vor dem Hintergrund unserer Ergebnisse zeigt diese Dissertation, dass LGM-Proxy-Daten ein effektives Mittel zur Einschränkung des Unsicherheitsbereichs der Klimasensitivität sein können und betont somit das Potenzial von Paläoklimadaten, den großen Unsicherheitsbereich von Klimaprojektionen zu reduzieren. KW - Dynamische Modellierung KW - Potsdam / Potsdam-Institut für Klimafolgenforschung KW - Entscheidung bei Unsicherheit KW - Klimasensitivität KW - Klimaprognose KW - Unsicherheitsanalyse KW - Ensemble-Simulation KW - Letztes Glaziales Maximum KW - climate sensitivity KW - climate projection KW - uncertainty analysis KW - ensemble simulation KW - Last Glacial Maximum Y1 - 2006 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-7778 ER - TY - GEN A1 - Waldrip, Steven H. A1 - Niven, Robert K. A1 - Abel, Markus A1 - Schlegel, Michael T1 - Consistent maximum entropy representations of pipe flow networks T2 - AIP conference proceedings N2 - The maximum entropy method is used to predict flows on water distribution networks. This analysis extends the water distribution network formulation of Waldrip et al. (2016) Journal of Hydraulic Engineering (ASCE), by the use of a continuous relative entropy defined on a reduced parameter set. This reduction in the parameters that the entropy is defined over ensures consistency between different representations of the same network. The performance of the proposed reduced parameter method is demonstrated with a one-loop network case study. Y1 - 2017 SN - 978-0-7354-1527-0 U6 - https://doi.org/10.1063/1.4985365 SN - 0094-243X VL - 1853 IS - 1 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Dybiec, Bartlomiej A1 - Capala, Karol A1 - Chechkin, Aleksei V. A1 - Metzler, Ralf T1 - Conservative random walks in confining potentials JF - Journal of physics : A, Mathematical and theoretical N2 - Levy walks are continuous time random walks with spatio-temporal coupling of jump lengths and waiting times, often used to model superdiffusive spreading processes such as animals searching for food, tracer motion in weakly chaotic systems, or even the dynamics in quantum systems such as cold atoms. In the simplest version Levy walks move with a finite speed. Here, we present an extension of the Levy walk scenario for the case when external force fields influence the motion. The resulting motion is a combination of the response to the deterministic force acting on the particle, changing its velocity according to the principle of total energy conservation, and random velocity reversals governed by the distribution of waiting times. For the fact that the motion stays conservative, that is, on a constant energy surface, our scenario is fundamentally different from thermal motion in the same external potentials. In particular, we present results for the velocity and position distributions for single well potentials of different steepness. The observed dynamics with its continuous velocity changes enriches the theory of Levy walk processes and will be of use in a variety of systems, for which the particles are externally confined. KW - Levy walk KW - conservative random walks KW - Levy flight Y1 - 2018 U6 - https://doi.org/10.1088/1751-8121/aaefc2 SN - 1751-8113 SN - 1751-8121 VL - 52 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - THES A1 - Gutjahr, Petra T1 - Conformations of semiflexible polymers and filaments T1 - Konformationen von semiflexiblen Polymeren und Filamenten N2 - The biological function and the technological applications of semiflexible polymers, such as DNA, actin filaments and carbon nanotubes, strongly depend on their rigidity. Semiflexible polymers are characterized by their persistence length, the definition of which is the subject of the first part of this thesis. Attractive interactions, that arise e.g.~in the adsorption, the condensation and the bundling of filaments, can change the conformation of a semiflexible polymer. The conformation depends on the relative magnitude of the material parameters and can be influenced by them in a systematic manner. In particular, the morphologies of semiflexible polymer rings, such as circular nanotubes or DNA, which are adsorbed onto substrates with three types of structures, are studied: (i) A topographical channel, (ii) a chemically modified stripe and (iii) a periodic pattern of topographical steps. The results are compared with the condensation of rings by attractive interactions. Furthermore, the bundling of two individual actin filaments, whose ends are anchored, is analyzed. This system geometry is shown to provide a systematic and quantitative method to extract the magnitude of the attraction between the filaments from experimentally observable conformations of the filaments. N2 - Die biologische Funktion und die technologischen Anwendungen semiflexibler Polymere, wie DNA, Aktinfilamente und Nanoröhren aus Kohlenstoff, werden wesentlich von deren Biegesteifigkeit bestimmt. Semiflexible Polymere werden charakterisiert durch ihre Persistenzlänge, mit deren Definition sich der erste Teil dieser Arbeit befasst. Anziehende Wechselwirkungen, wie sie z.B. bei der Adsorption, der Kondensation und der Bündelung von Filamenten auftreten, können die Konformation eines semiflexiblen Polymers verändern. Die Konformation ist dabei abhängig von der relativen Größe der Materialparameter und kann durch diese gezielt beeinflusst werden. Im Einzelnen werden hier die Morphologien semiflexibler Polymerringe, wie z.B. DNA oder ringförmiger Nanoröhren, untersucht, die auf drei verschieden strukturierten Substraten adsorbieren: (i) Ein topographischer Kanal, (ii) ein chemisch modifizierter Streifen und (iii) ein periodisches Muster topographischer Oberflächenstufen. Die Ergebnisse werden mit der Kondensation von Ringen durch anziehende Wechselwirkungen verglichen. Des Weiteren wird die Bündelung zweier Aktinfilamente, deren Enden verankert sind, untersucht. Diese Systemgeometrie liefert eine systematische Methode, um die Stärke der Anziehung zwischen den Filamenten aus experimentell beobachtbaren Konformationen zu berechnen. KW - Polymere KW - Filamente KW - Persistenzlänge KW - Adsorption KW - Filament-Bündel KW - polymers KW - filaments KW - persistence length KW - adsorption KW - filament bundles Y1 - 2007 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-15918 ER - TY - JOUR A1 - Aranson, Igor S. A1 - Pikovskij, Arkadij T1 - Confinement and collective escape of active particles JF - Physical review letters N2 - Active matter broadly covers the dynamics of self-propelled particles. While the onset of collective behavior in homogenous active systems is relatively well understood, the effect of inhomogeneities such as obstacles and traps lacks overall clarity. Here, we study how interacting, self-propelled particles become trapped and released from a trap. We have found that captured particles aggregate into an orbiting condensate with a crystalline structure. As more particles are added, the trapped condensates escape as a whole. Our results shed light on the effects of confinement and quenched disorder in active matter. Y1 - 2022 U6 - https://doi.org/10.1103/PhysRevLett.128.108001 SN - 0031-9007 SN - 1079-7114 SN - 1092-0145 VL - 128 IS - 10 PB - American Physical Society CY - College Park, Md. ER - TY - THES A1 - Timpanaro, Salvatore T1 - Conductive properties and morphology of conjugated molecular materials studied by local probe techniques N2 - Die vorgelegte Arbeit befasst sich mit der Untersuchung von Zusammenhängen zwischen der Struktur dünner organischer Schichten und stellt einen Bezug zur Leitfähigkeit der Schichten her. Sie liefert einen Beitrag zum vertieften Verständnis der Transporteigenschaf-ten organischer Halbleiter und soll so zur Verbesserung organischer elektronischer Bauele-mente beitragen. Es ist bekannt, dass die Effizienz von Organischen Leuchtdioden (OLEDs) stark von der Qualität der eingesetzten dünnen Filme abhängt. Es ist deshalb interessant, die Strukturen technologisch interessanter organischer Filme mittels Scanning Probe Mikrosko-pie (SPM) zu charakterisieren, um ein besseres Verständnis sowohl der Oberflächen-Morphologie als auch der molekularen/atomaren Packungen zu erhalten. Die Untersuchung von Quaterthiophen (4T), welches vielfach in Feldeffekt-Transistoren eingesetzt wird, bildet einen ersten Schwerpunkt der Arbeit. Es konnte eine neue Kristall-struktur von 4T gefunden werden, die bisher nicht bekannt war. Dafür wurden Quaterthi-ophen-Filme untersucht, welche auf Kaliumphthalat (KAP) aufwuchsen. Die Aufklärung der neuen Struktur gelang durch Kombinierten Einsatz von optischen und Röntgen-Beugungsmessungen. Zur Bestätigung der triklinen Kristall-Einheitszelle mit den Parame-tern a = 0,721 nm, b = 0,632 nm, c = 0,956 nm und α = 91o, β = 91,4o, γ = 91o wurde außer-dem die Atomkraft-Mikroskopie (AFM) angewendet. In Fortführung der rastermikroskopischen Messungen sind die Morphologien von Filmen vier weiterer organischer Materialien technologischer Relevanz untersucht worden: He-xanthiol-Monoschichten auf Gold, Azobenzenthiol-Monoschichten auf Gold, Para-Phenylenvinylen-Oligomer-Filme auf Gold und Phenyl-Oxadiazol-Filme auf Gold. Dafür kam zusätzlich die Ultrahochvakuum-Scanning-Tunneling-Mikroskopie (UHV-STM) zum Einsatz. Es zeigte sich eine Vielzahl morphologischer Eigenheiten, deren Besonderheiten sowohl von den gewählten Substraten als auch von der chemischen Struktur der untersuchten organischen Materialien abhängen. So zeigen Para-Phenylenvinylen-Oligomer-Filme eine Stäbchen- und Oxadiazol-Filme eine Körnchen-Struktur auf Gold. Auf Basis dieser Kenntnisse gelang es, das optisch induzierte Schalten von Azobenzen durch STM-Untersuchungen und durch Scanning Tunneling Spektroskopie (STS) auf molekularer Skale nachzuweisen. Die Topographie einer Reihe von Poly(3,4-ethylendioxythiophen)-(PEDOT)-Filmen und de-ren Bezug zu den Ladungstransporteigenschaften dieser Filme war ein weiterer Arbeits-schwerpunkt. PEDOT-Filme auf Indium-Zinn-Oxid (ITO) kommen in organischen elektro-nischen Bauelementen als Lochinjektionsschichten zum Einsatz. Für die Schichtherstellung kamen Dispersionen des Polymers unterschiedlicher Konzentration zur Anwendung: Mittels AFM und STM konnte gezeigt werden, dass unterschiedliche Konzentrationen zu unter-schiedlichen Topographien führen. Besonders die Oberfläche der Filme mit hoher Konzent-ration von PEDOT, d.h. die der leitfähigsten Filme, wird durch eine körnchenartige Struktur eingebetteter Teilchen charakterisiert. Durch STM Strom-Abstands-(I-d)-Untersuchungen wurde gefunden, dass diese Teilchen die „Spitze eines Eisbergs“ leitfähiger Domänen dar-stellen. Ausgehend von dieser Erkenntnis wird ein Strukturmodell für die Filme vorgeschla-gen, in dem die leitfähigen Domänen / Partikel in eine weniger leitfähige Matrix eingebettet sind. Durch Zugabe von Polyolen wie Sorbitol zur PEDOT-Dispersion ließen sich Filme mit höherer Leitfähigkeit herstellen. Eine klare Abhängigkeit zwischen der Leitfähigkeit und der bisher nicht beschrieben nano-Morphologie wurde gefunden. N2 - It is known that the efficiency of organic light-emitting devices (OLEDs) is strongly influenced by the ’quality′ of the thin films [1]. On the basis of this conviction, the work presented in this thesis aimed to obtain a better understanding of the structure of organic thin films of general interest in the field of organic light emitting devices by using scanning probe microscopies (SPMs). A not yet reported crystal structure of quaterthiophene film grown on potassium hydrogen (KHP) is determined by optical measurements, a simulation program, diffraction at both normal incidence and grazing angle and AFM. The crystal cell is triclinic with parameters a = 0.721 nm, b = 0.632 nm, c = 0.956 nm and a = 91°, b = 91.4°, g = 91° [2]. The morphologies of four organic thin films deposited on gold are characterized by ultra high vacuum scanning tunneling microscopy (UHV-STM). Terraces in an hexanethiol monolayer, lamellar structures in an azobenzenethiol monolayer, rods in a a poly(paraphenylenevinylene) oligomer film and a granular morphology in an oxadiazole film are shown. The topographies of a series of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) films deposited on indium-tin oxide (ITO) and gold obtained from dispersions with PEDOT:PSS weight ratios of 1:20, 1:6 and 1:1 are investigated by AFM. It is demonstrated that the films show the same topography on gold and on ITO. It is shown that the PEDOT films eliminate the spike features of ITO. It is reported that PEDOT 1:20 and 1:6 appear indistinguishable between each other but different from PEDOT 1:1 (the most conductive). Coupling STM and I-d measurements, a not yet reported structural model of PEDOT 1:1 on gold is obtained [3]. In this model the surface presents grains and the bulk particles/domains rich in PEDOT embedded in a PEDOT-poor matrix. The equation of conductivity is derived. A STM investigation of four PEDOT films deposited on ITO obtained from dispersions with the same PEDOT:PSS weight ratio of 1:1 is carried out [4]. The films differ either for the presence of sorbitol or for a different synthetic route (and they present different conductivities). For the first time a quantitative and qualitative correlation between the nanometer-scale morphology of PEDOT films with and without sorbitol and their conductivity is established. T2 - Conductive properties and morphology of conjugated molecular materials studied by local probe techniques KW - SPMs KW - PEDOT KW - 4T KW - PPV KW - OLEDs KW - SPMs KW - PEDOT KW - 4T KW - PPV KW - OLEDs Y1 - 2004 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-0001929 ER - TY - THES A1 - Valencia Sanmiguel, Antonio T1 - Condensation and crystallization on patterned surfaces N2 - Condensation and crystallization are omnipresent phenomena in nature. The formation of droplets or crystals on a solid surface are familiar processes which, beyond their scientific interest, are required in many technological applications. In recent years, experimental techniques have been developed which allow patterning a substrate with surface domains of molecular thickness, surface area in the mesoscopic scale, and different wettabilities (i.e., different degrees of preference for a substance that is in contact with the substrate). The existence of new patterned surfaces has led to increased theoretical efforts to understand wetting phenomena in such systems. In this thesis, we deal with some problems related to the equilibrium of phases (e.g., liquid-vapor coexistence) and the kinetics of phase separation in the presence of chemically patterned surfaces. Two different cases are considered: (i) patterned surfaces in contact with liquid and vapor, and (ii) patterned surfaces in contact with a crystalline phase. One of the problems that we have studied is the following: It is widely believed that if air containing water vapor is cooled to its dew point, droplets of water are immediately formed. Although common experience seems to support this view, it is not correct. It is only when air is cooled well below its dew point that the phase transition occurs immediately. A vapor cooled slightly below its dew point is in a metastable state, meaning that the liquid phase is more stable than the vapor, but the formation of droplets requires some time to occur, which can be very long. It was first pointed out by J. W. Gibbs that the metastability of a vapor depends on the energy necessary to form a nucleus (a droplet of a critical size). Droplets smaller than the critical size will tend to disappear, while droplets larger than the critical size will tend to grow. This is consistent with an energy barrier that has its maximum at the critical size, as is the case for droplets formed directly in the vapor or in contact with a chemically uniform planar wall. Classical nucleation theory describes the time evolution of the condensation in terms of the random process of droplet growth through this energy barrier. This process is activated by thermal fluctuations, which eventually will form a droplet of the critical size. We consider nucleation of droplets from a vapor on a substrate patterned with easily wettable (lyophilic) circular domains. Under certain conditions of pressure and temperature, the condensation of a droplet on a lyophilic circular domain proceeds through a barrier with two maxima (a double barrier). We have extended classical nucleation theory to account for the kinetics of nucleation through a double barrier, and applied this extension to nucleation on lyophilic circular domains. N2 - Kondensation und Kristallisation sind allgegenwärtige Phänomene in der Natur. Die Bildung von Tröpfchen oder Kristallen auf einer festen Oberfläche sind vertraute Prozesse, die über ihr wissenschaftliches Interesse hinaus in vielen technologischen Anwendungen erforderlich sind. In den letzten Jahren wurden experimentelle Techniken entwickelt, die es ermöglichen, ein Substrat mit Oberflächendomänen von molekularer Dicke, mesoskopischer Oberflächengröße und unterschiedlichen Benetzbarkeiten (d.h. unterschiedlichen Graden der Präferenz für eine Substanz, die mit dem Substrat in Kontakt steht) zu strukturieren. Die Existenz neuer strukturierter Oberflächen hat zu verstärkten theoretischen Bemühungen geführt, Benetzungsphänomene in solchen Systemen zu verstehen. In dieser Arbeit befassen wir uns mit einigen Problemen im Zusammenhang mit dem Gleichgewicht von Phasen (z.B. Flüssig-Dampf-Koexistenz) und der Kinetik der Phasentrennung in Gegenwart von chemisch strukturierten Oberflächen. Zwei verschiedene Fälle werden betrachtet: (i) strukturierte Oberflächen in Kontakt mit Flüssigkeit und Dampf und (ii) strukturierte Oberflächen in Kontakt mit einer kristallinen Phase. Eines der Probleme, die wir untersucht haben, ist das folgende: Es wird allgemein angenommen, dass sich bei Abkühlung von wasserdampfhaltiger Luft auf ihren Taupunkt sofort Wassertröpfchen bilden. Obwohl die allgemeine Erfahrung diese Ansicht zu unterstützen scheint, ist sie nicht korrekt. Erst wenn die Luft deutlich unter ihren Taupunkt gekühlt wird, tritt der Phasenübergang sofort ein. Ein leicht unter seinen Taupunkt gekühlter Dampf befindet sich in einem metastabilen Zustand, was bedeutet, dass die flüssige Phase stabiler als der Dampf ist, aber die Bildung von Tröpfchen einige Zeit benötigt, die sehr lang sein kann. J. W. Gibbs wies erstmals darauf hin, dass die Metastabilität eines Dampfes von der Energie abhängt, die zur Bildung eines Keims (eines Tröpfchens kritischer Größe) notwendig ist. Tröpfchen, die kleiner als die kritische Größe sind, neigen dazu zu verschwinden, während Tröpfchen, die größer als die kritische Größe sind, dazu neigen zu wachsen. Dies stimmt mit einer Energiebarriere überein, die ihr Maximum bei der kritischen Größe hat, wie es bei Tröpfchen der Fall ist, die sich direkt im Dampf oder in Kontakt mit einer chemisch einheitlichen ebenen Wand bilden. Die klassische Nukleationstheorie beschreibt die zeitliche Entwicklung der Kondensation als zufälligen Prozess des Tröpfchenwachstums durch diese Energiebarriere. Dieser Prozess wird durch thermische Fluktuationen aktiviert, die schließlich ein Tröpfchen kritischer Größe bilden. Wir betrachten die Nukleation von Tröpfchen aus einem Dampf auf einem Substrat, das mit leicht benetzbaren (lyophilen) kreisförmigen Domänen strukturiert ist. Unter bestimmten Druck- und Temperaturbedingungen verläuft die Kondensation eines Tröpfchens auf einer lyophilen kreisförmigen Domäne durch eine Barriere mit zwei Maxima (eine Doppelbarriere). Wir haben die klassische Nukleationstheorie erweitert, um die Kinetik der Nukleation durch eine Doppelbarriere zu berücksichtigen, und haben diese Erweiterung auf die Nukleation auf lyophilen kreisförmigen Domänen angewendet. KW - Chemical physics KW - Condensation KW - Nucleation KW - Crystallization KW - Computer simulation KW - Soft condensed matter KW - Phase transitions KW - Chemically patterned surfaces KW - Monte Carlo simulation KW - Kondensation KW - Kristallisation KW - Oberfläche KW - Keimbildung KW - Mathematisches Modell Y1 - 2003 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-651950 ER - TY - JOUR A1 - Kar, M. A1 - Körzdörfer, Thomas T1 - Computational screening of methylammonium based halide perovskites with bandgaps suitable for perovskite-perovskite tandem solar cells JF - The journal of chemical physics : bridges a gap between journals of physics and journals of chemistr N2 - We aim to find homovalent alternatives for lead and iodine in CH3NH3PbI3 perovskites that show bandgaps suitable for building novel perovskite-perovskite tandem solar cells. To this end, we employ a computational screening for materials with a bandgap between 1.0 eV and 1.9 eV, using density functional theory calculations at the Perdew-Burke-Ernzerhof and Heyd-Scuseria-Ernzerhof levels of theory. The room-temperature stability of the materials and their phases that satisfy the bandgap criteria is evaluated based on the empirical Goldschmidt tolerance factor. In total, our screening procedure covers 30 different perovskite structures in three phases (orthorhombic, cubic, tetragonal) each. We find 9 materials that are predicted to be stable at room temperature and to have bandgaps in an energy range suitable for application in tandem solar cells. Published by AIP Publishing. Y1 - 2018 U6 - https://doi.org/10.1063/1.5037535 SN - 0021-9606 SN - 1089-7690 VL - 149 IS - 21 PB - American Institute of Physics CY - Melville ER - TY - THES A1 - Knebe, Alexander T1 - Computational cosmology T1 - Numerische Kosmologie N2 - “Computational Cosmology” is the modeling of structure formation in the Universe by means of numerical simulations. These simulations can be considered as the only “experiment” to verify theories of the origin and evolution of the Universe. Over the last 30 years great progress has been made in the development of computer codes that model the evolution of dark matter (as well as gas physics) on cosmic scales and new research discipline has established itself. After a brief summary of cosmology we will introduce the concepts behind such simulations. We further present a novel computer code for numerical simulations of cosmic structure formation that utilizes adaptive grids to efficiently distribute the work and focus the computing power to regions of interests, respectively. In that regards we also investigate various (numerical) effects that influence the credibility of these simulations and elaborate on the procedure of how to setup their initial conditions. And as running a simulation is only the first step to modelling cosmological structure formation we additionally developed an object finder that maps the density field onto galaxies and galaxy clusters and hence provides the link to observations. Despite the generally accepted success of the cold dark matter cosmology the model still inhibits a number of deviations from observations. Moreover, none of the putative dark matter particle candidates have yet been detected. Utilizing both the novel simulation code and the halo finder we perform and analyse various simulations of cosmic structure formation investigating alternative cosmologies. These include warm (rather than cold) dark matter, features in the power spectrum of the primordial density perturbations caused by non-standard inflation theories, and even modified Newtonian dynamics. We compare these alternatives to the currently accepted standard model and highlight the limitations on both sides; while those alternatives may cure some of the woes of the standard model they also inhibit difficulties on their own. During the past decade simulation codes and computer hardware have advanced to such a stage where it became possible to resolve in detail the sub-halo populations of dark matter halos in a cosmological context. These results, coupled with the simultaneous increase in observational data have opened up a whole new window on the concordance cosmogony in the field that is now known as “Near-Field Cosmology”. We will present an in-depth study of the dynamics of subhaloes and the development of debris of tidally disrupted satellite galaxies.1 Here we postulate a new population of subhaloes that once passed close to the centre of their host and now reside in the outer regions of it. We further show that interactions between satellites inside the radius of their hosts may not be negliable. And the recovery of host properties from the distribution and properties of tidally induced debris material is not as straightforward as expected from simulations of individual satellites in (semi-)analytical host potentials. N2 - Die Kosmologie ist heutzutage eines der spannendsten Arbeitsgebiete in der Astronomie und Astrophysik. Das vorherrschende (Urknall-)Modell in Verbindung mit den neuesten und präzisesten Beobachtungsdaten deutet darauf hin, daß wir in einem Universum leben, welches zu knapp 24% aus Dunkler Materie und zu 72% aus Dunkler Energie besteht; die sichtbare Materie macht gerade einmal 4% aus. Und auch wenn uns derzeit eindeutige bzw. direkte Beweise für die Existenz dieser beiden exotischen Bestandteile des Universums fehlen, so ist es uns dennoch möglich, die Entstehung von Galaxien, Galaxienhaufen und der großräumigen Struktur in solch einem Universum zu modellieren. Dabei bedienen sich Wissenschaftler Computersimulationen, welche die Strukturbildung in einem expandierenden Universum mittels Großrechner nachstellen; dieses Arbeitsgebiet wird Numerische Kosmologie bzw. “Computational Cosmology” bezeichnet und ist Inhalt der vorliegenden Habilitationsschrift. Nach einer kurzen Einleitung in das Themengebiet werden die Techniken zur Durchführung solcher numerischen Simulationen vorgestellt. Die Techniken zur Lösung der relevanten (Differential-)Gleichungen zur Modellierung des “Universums im Computer” unterscheiden sich dabei teilweise drastisch voneinander (Teilchen- vs. Gitterverfahren), und es werden die verfahrenstechnischen Unterschiede herausgearbeitet. Und obwohl unterschiedliche Programme auf unterschiedlichen Methoden basieren, so sind die Unterschiede in den Endergebnissen doch (glücklicherweise) vernachlässigbar gering. Wir stellen desweiteren einen komplett neuen Code – basierend auf dem Gitterverfahren – vor, welcher einen Hauptbestandteil der vorliegenden Habilitation darstellt. Im weiteren Verlauf der Arbeit werden diverse kosmologische Simulationen vorgestellt und ausgewertet. Dabei werden zum einen die Entstehung und Entwicklung von Satellitengalaxien – den (kleinen) Begleitern von Galaxien wie unserer Milchstraße und der Andromedagalaxie – als auch Alternativen zum oben eingeführten “Standardmodell” der Kosmologie untersucht. Es stellt sich dabei heraus, daß keine der (hier vorgeschlagenen) Alternativen eine bedrohliche Konkurenz zu dem Standardmodell darstellt. Aber nichtsdestoweniger zeigen die Rechnungen, daß selbst so extreme Abänderungen wie z.B. modifizierte Newton’sche Dynamik (MOND) zu einem Universum führen können, welches dem beobachteten sehr nahe kommt. Die Ergebnisse in Bezug auf die Dynamik der Satellitengalaxien zeigen auf, daß die Untersuchung der Trümmerfelder von durch Gezeitenkräfte zerriebenen Satellitengalaxien Rückschlüsse auf Eigenschaften des ursprünglichen Satelliten zulassen. Diese Tatsache wird bei der Aufschlüsselung der Entstehungsgeschichte unserer eigenen Milchstraße von erheblichem Nutzen sein. Trotzdem deuten die hier vorgestellten Ergebnisse auch darauf hin, daß dieser Zusammenhang nicht so eindeutig ist, wie er zuvor mit Hilfe kontrollierter Einzelsimulationen von Satellitengalaxien in analytischen “Mutterpotentialen” vorhergesagt wurde: Das Zusammenspiel zwischen den Satelliten und der Muttergalaxie sowie die Einbettung der Rechnungen in einen kosmologischen Rahmen sind von entscheidender Bedeutung. Y1 - 2008 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-41147 ER - TY - THES A1 - Shenar, Tomer T1 - Comprehensive analyses of massive binaries and implications on stellar evolution T1 - Umfassende Analysen massereicher Doppelsterne und Implikationen für die Sternentwicklung N2 - Via their powerful radiation, stellar winds, and supernova explosions, massive stars (Mini & 8 M☉) bear a tremendous impact on galactic evolution. It became clear in recent decades that the majority of massive stars reside in binary systems. This thesis sets as a goal to quantify the impact of binarity (i.e., the presence of a companion star) on massive stars. For this purpose, massive binary systems in the Local Group, including OB-type binaries, high mass X-ray binaries (HMXBs), and Wolf-Rayet (WR) binaries, were investigated by means of spectral, orbital, and evolutionary analyses. The spectral analyses were performed with the non-local thermodynamic equillibrium (non-LTE) Potsdam Wolf-Rayet (PoWR) model atmosphere code. Thanks to critical updates in the calculation of the hydrostatic layers, the code became a state-of-the-art tool applicable for all types of hot massive stars (Chapter 2). The eclipsing OB-type triple system δ Ori served as an intriguing test-case for the new version of the PoWR code, and provided key insights regarding the formation of X-rays in massive stars (Chapter 3). We further analyzed two prototypical HMXBs, Vela X-1 and IGR J17544-2619, and obtained fundamental conclusions regarding the dichotomy of two basic classes of HMXBs (Chapter 4). We performed an exhaustive analysis of the binary R 145 in the Large Magellanic Cloud (LMC), which was claimed to host the most massive stars known. We were able to disentangle the spectrum of the system, and performed an orbital, polarimetric, and spectral analysis, as well as an analysis of the wind-wind collision region. The true masses of the binary components turned out to be significantly lower than suggested, impacting our understanding of the initial mass function and stellar evolution at low metallicity (Chapter 5). Finally, all known WR binaries in the Small Magellanic Cloud (SMC) were analyzed. Although it was theoretical predicted that virtually all WR stars in the SMC should be formed via mass-transfer in binaries, we find that binarity was not important for the formation of the known WR stars in the SMC, implying a strong discrepancy between theory and observations (Chapter 6). N2 - Durch ihre intensive Strahlung, Sternwinde und Supernovaexplosionen tragen massereiche Sterne (Minitial & 8 M☉) erheblich zur Entwicklung von Galaxien bei. In den letzten Jahren wurde es immer klarer, dass sich die Mehrheit der massereichen Sterne in Doppelsternsystemen befindet. Die vorliegende Doktorarbeit hat das Ziel, den Einfluss dieser Tatsache auf die Entwicklung massereicher Sterne quantitativ zu untersuchen. Um dies zu erreichen, haben wir eine Analyse der Umläufe, Spektren und Entwicklung verschiedener Doppelsternsysteme in der lokalen Gruppe durchgeführt, die OB-Sterne, massereicher Röntgendoppelsterne (HMXBs) und Wolf-Rayet-(WR)-Doppelsterne einschließt. Die Spektralanalyse wurde mithilfe des Potsdam-Wolf-Rayet-(PoWR)-Modellatmosphären-Programms für Strahlungstransport in Abwesenheit von lokalem thermodynamischen Gleichgewicht (non-LTE) durchgeführt. Das PoWR-Programm wurde im Laufe der Arbeit aktualisiert, sodass die Berechnung der hydrostatischen Schichten des Sternes nun wesentlich genauer erfolgt, was die Verwendung dieses Programms für alle Typen heißer massereicher Sterne erlaubte (Kapitel 2). Das bedeckungsveränderliche Dreifachsternsystem δ Ori diente als Test für diese neue Version von PoWR, und lieferte wesentliche Informationen bezüglich der Entstehung von Röntgenstrahlung in massereichen Sternen (Kapitel 3). Die Analyse zweier prototypischer massereicher Röntgendoppelsterne, Vela X-1 und IGR J17544-2619, machte den Ursprung der Dichotomie der zwei Hauptklassen der Röntgendoppelsterne deutlich (Kapitel 4). Eine umfassende Analyse des Doppelsterns R145 in der Großen Magellanschen Wolke (LMC), der angeblich aus den massereichsten uns bekannten Sternen besteht, wurde durchgeführt. Mithilfe einer Dekomposition des Spektrums, einer Orbital- und Spektralanalyse, sowie einer Analyse der Kollision von Sternwinden in diesem System konnten wir zeigen, dass die Massen der Komponenten wesentlich kleiner sind als bisher angenommen. Dies ist ein wichtiger Beitrag zu unserem Verständnis der Anfangsmassenfunktion und der Entwicklung massereicher Sterne in Umgebungen geringer Metalizität (Kapitel 5). Schließlich wurde die Gesamtpopulation der WR-Doppelsterne in der Kleinen Magellanschen Wolke (SMC) analysiert. Im Widerspruch zur theoretischen Vorhersage, alle WR-Doppelsterne in der SMC seien dank Massentransfer in Doppelsternsystemen entstanden, finden wir, dass Massentransfer unerheblich für die Entstehung der uns bekannten WR-Sterne in der SMC war (Kapitel 6). KW - massive stars KW - Wolf-Rayet KW - stellar evolution KW - stellar winds KW - binary stars KW - massereiche Sterne KW - Wolf-Rayet KW - Sternentwicklung KW - Sternwinde KW - Doppelsterne Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-104857 ER - TY - JOUR A1 - Wiesner, Karoline A1 - Ladyman, James T1 - Complex systems are always correlated but rarely information processing JF - Journal of physics. Complexity N2 - 'Complex systems are information processors' is a statement that is frequently made. Here we argue for the distinction between information processing-in the sense of encoding and transmitting a symbolic representation-and the formation of correlations (pattern formation/self-organisation). The study of both uses tools from information theory, but the purpose is very different in each case: explaining the mechanisms and understanding the purpose or function in the first case, versus data analysis and correlation extraction in the latter. We give examples of both and discuss some open questions. The distinction helps focus research efforts on the relevant questions in each case. KW - correlations KW - information theory KW - complex systems KW - information KW - processing KW - self-organisation Y1 - 2021 U6 - https://doi.org/10.1088/2632-072X/ac371c SN - 2632-072X VL - 2 IS - 4 PB - IOP Publ. Ltd. CY - Bristol ER - TY - THES A1 - Yeldesbay, Azamat T1 - Complex regimes of synchronization T1 - Komplexe Synchronisationszustände BT - modeling and analysis N2 - Synchronization is a fundamental phenomenon in nature. It can be considered as a general property of self-sustained oscillators to adjust their rhythm in the presence of an interaction. In this work we investigate complex regimes of synchronization phenomena by means of theoretical analysis, numerical modeling, as well as practical analysis of experimental data. As a subject of our investigation we consider chimera state, where due to spontaneous symmetry-breaking of an initially homogeneous oscillators lattice split the system into two parts with different dynamics. Chimera state as a new synchronization phenomenon was first found in non-locally coupled oscillators system, and has attracted a lot of attention in the last decade. However, the recent studies indicate that this state is also possible in globally coupled systems. In the first part of this work, we show under which conditions the chimera-like state appears in a system of globally coupled identical oscillators with intrinsic delayed feedback. The results of the research explain how initially monostable oscillators became effectivly bistable in the presence of the coupling and create a mean field that sustain the coexistence of synchronized and desynchronized states. Also we discuss other examples, where chimera-like state appears due to frequency dependence of the phase shift in the bistable system. In the second part, we make further investigation of this topic by modeling influence of an external periodic force to an oscillator with intrinsic delayed feedback. We made stability analysis of the synchronized state and constructed Arnold tongues. The results explain formation of the chimera-like state and hysteric behavior of the synchronization area. Also, we consider two sets of parameters of the oscillator with symmetric and asymmetric Arnold tongues, that correspond to mono- and bi-stable regimes of the oscillator. In the third part, we demonstrate the results of the work, which was done in collaboration with our colleagues from Psychology Department of University of Potsdam. The project aimed to study the effect of the cardiac rhythm on human perception of time using synchronization analysis. From our part, we made a statistical analysis of the data obtained from the conducted experiment on free time interval reproduction task. We examined how ones heartbeat influences the time perception and searched for possible phase synchronization between heartbeat cycles and time reproduction responses. The findings support the prediction that cardiac cycles can serve as input signals, and is used for reproduction of time intervals in the range of several seconds. N2 - Synchronisation ist ein fundamentales Naturphänomen. Es ist die grundlegende Eigenschaft sich selbsterhaltender Oszillatoren, in Gegenwart einer Wechselwirkung, danach zu streben, ihre Rhythmen anzupassen. In dieser Arbeit betrachten wir komplexe Synchronisationszustände sowohl mit Hilfe analytischer Methoden als auch durch numerische Simulation und in experimentellen Daten. Unser Untersuchungsobjekt sind die sogenannten Chimera Zustände, in welchen sich Ensemble von gekoppelten, identischen Oszillatoren auf Grund eines Symmetriebruches spontan in Gruppen mit unterschiedlicher Dynamik aufteilen. Die Entdeckung von Chimeras in zunächst nichtlokal gekoppelten Systemen hat in den letzten zehn Jahren ein großes Interesse an neuartigen Synchronisationsphänomenen geweckt. Neueste Forschungsergebnisse belegen, dass diese Zustände unter bestimmten Bedingungen auch in global gekoppelten Systemen existieren können. Solche Bedingungen werden im ersten Teil der Arbeit in Ensemblen global gekoppelter Oszillatoren mit zusätzlicher, zeitverzögerter Selbstkopplung untersucht. Wir zeigen, wie zunächst monostabile Oszillatoren in Gegenwart von dem Treiben der globalen Kopplung effektiv bistabil werden, und sich so in zwei Gruppen organisieren. Das mittlere Feld, welches durch diese Gruppen aufgebaut wird, ist quasiperiodisch wodurch der Chimera Zustand sich selbst stabilisiert. In einem anderen Beispiel zeigen wir, dass der Chimera Zustand auch durch einen frequenzabhängigen Phasenunterschied in der globalen Kopplung erreicht werden kann. Zur genaueren Untersuchung der Mechanismen, die zur effektiven Bistabilität führen, betrachten wir im zweiten Teil der Arbeit den Einfluss einer externen periodischen Kraft auf einzelne Oszillatoren mit zeitverzögerter Selbstkopplung. Wir führen die Stabilitätanalyse des synchronen Zustands durch, und stellen die Arnoldzunge dar. Im dritten Teil der Arbeit stellen wir die Ergebnisse einer Synchronisationsanalyse vor, welche in Kooperation mit Wissenschaftlern der Psychologischen Fakultät der Universität Potsdam durchgeführt wurde. In dem Projekt wurde die Auswirkung des Herzrhythmus auf die menschliche Zeitwahrnehmung erforscht. Unsere Aufgabe war es, die experimentellen Daten statistisch zu analysieren. Im Experiment sollten Probanden ein gegebenes Zeitintervall reproduzieren während gleichzeitig ihr Herzschlag aufgezeichnet wurde. Durch eine Phasenanalyse haben wir den Zusammenhang zwischen dem Herzschlag und der Start- bzw. Stoppzeit der zu reproduzierenden Zeitintervalle untersucht. Es stellt sich heraus, dass Herzschläge bei Zeitintervallen über einige Sekunden als Taktgeber dienen können. KW - synchronization KW - phase oscillators KW - chimera state KW - time perception KW - Synchronisation KW - Phasen Oszillatoren KW - chimera Zustände KW - zeitverzögerte Selbstkopplung KW - Synchronisationsanalyse KW - Zeitwahrnehmung Y1 - 2014 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-73348 ER - TY - THES A1 - Donges, Jonathan T1 - Complex networks in the climate system N2 - Complex network theory provides an elegant and powerful framework to statistically investigate the topology of local and long range dynamical interrelationships, i.e., teleconnections, in the climate system. Employing a refined methodology relying on linear and nonlinear measures of time series analysis, the intricate correlation structure within a multivariate climatological data set is cast into network form. Within this graph theoretical framework, vertices are identified with grid points taken from the data set representing a region on the the Earth's surface, and edges correspond to strong statistical interrelationships between the dynamics on pairs of grid points. The resulting climate networks are neither perfectly regular nor completely random, but display the intriguing and nontrivial characteristics of complexity commonly found in real world networks such as the internet, citation and acquaintance networks, food webs and cortical networks in the mammalian brain. Among other interesting properties, climate networks exhibit the "small-world" effect and possess a broad degree distribution with dominating super-nodes as well as a pronounced community structure. We have performed an extensive and detailed graph theoretical analysis of climate networks on the global topological scale focussing on the flow and centrality measure betweenness which is locally defined at each vertex, but includes global topological information by relying on the distribution of shortest paths between all pairs of vertices in the network. The betweenness centrality field reveals a rich internal structure in complex climate networks constructed from reanalysis and atmosphere-ocean coupled general circulation model (AOGCM) surface air temperature data. Our novel approach uncovers an elaborately woven meta-network of highly localized channels of strong dynamical information flow, that we relate to global surface ocean currents and dub the backbone of the climate network in analogy to the homonymous data highways of the internet. This finding points to a major role of the oceanic surface circulation in coupling and stabilizing the global temperature field in the long term mean (140 years for the model run and 60 years for reanalysis data). Carefully comparing the backbone structures detected in climate networks constructed using linear Pearson correlation and nonlinear mutual information, we argue that the high sensitivity of betweenness with respect to small changes in network structure may allow to detect the footprints of strongly nonlinear physical interactions in the climate system. The results presented in this thesis are thoroughly founded and substantiated using a hierarchy of statistical significance tests on the level of time series and networks, i.e., by tests based on time series surrogates as well as network surrogates. This is particularly relevant when working with real world data. Specifically, we developed new types of network surrogates to include the additional constraints imposed by the spatial embedding of vertices in a climate network. Our methodology is of potential interest for a broad audience within the physics community and various applied fields, because it is universal in the sense of being valid for any spatially extended dynamical system. It can help to understand the localized flow of dynamical information in any such system by combining multivariate time series analysis, a complex network approach and the information flow measure betweenness centrality. Possible fields of application include fluid dynamics (turbulence), plasma physics and biological physics (population models, neural networks, cell models). Furthermore, the climate network approach is equally relevant for experimental data as well as model simulations and hence introduces a novel perspective on model evaluation and data driven model building. Our work is timely in the context of the current debate on climate change within the scientific community, since it allows to assess from a new perspective the regional vulnerability and stability of the climate system while relying on global and not only on regional knowledge. The methodology developed in this thesis hence has the potential to substantially contribute to the understanding of the local effect of extreme events and tipping points in the earth system within a holistic global framework. N2 - Die Theorie komplexer Netzwerke bietet einen eleganten Rahmen zur statistischen Untersuchung der Topologie lokaler und langreichweitiger dynamischer Zusammenhänge (Telekonnektionen) im Klimasystem. Unter Verwendung einer verfeinerten, auf linearen und nichtlinearen Korrelationsmaßen der Zeitreihenanalyse beruhenden Netzwerkkonstruktionsmethode, bilden wir die komplexe Korrelationsstruktur eines multivariaten klimatologischen Datensatzes auf ein Netzwerk ab. Dabei identifizieren wir die Knoten des Netzwerkes mit den Gitterpunkten des zugrundeliegenden Datensatzes, während wir Paare von besonders stark korrelierten Knoten als Kanten auffassen. Die resultierenden Klimanetzwerke zeigen weder die perfekte Regularität eines Kristallgitters, noch eine vollkommen zufällige Topologie. Vielmehr weisen sie faszinierende und nichttriviale Eigenschaften auf, die charakteristisch für natürlich gewachsene Netzwerke wie z.B. das Internet, Zitations- und Bekanntschaftsnetzwerke, Nahrungsnetze und kortikale Netzwerke im Säugetiergehirn sind. Besonders erwähnenswert ist, dass in Klimanetzwerken das Kleine-Welt-Phänomen auftritt. Desweiteren besitzen sie eine breite Gradverteilung, werden von Superknoten mit sehr vielen Nachbarn dominiert, und bilden schließlich regional wohldefinierte Untergruppen von intern dicht vernetzten Knoten aus. Im Rahmen dieser Arbeit wurde eine detaillierte, graphentheoretische Analyse von Klimanetzwerken auf der globalen topologischen Skala durchgeführt, wobei wir uns auf das Netzwerkfluss- und Zentralitätsmaß Betweenness konzentrierten. Betweenness ist zwar lokal an jedem Knoten definiert, enthält aber trotzdem Informationen über die globale Netzwerktopologie. Dies beruht darauf, dass die Verteilung kürzester Pfade zwischen allen möglichen Paaren von Knoten in die Berechnung des Maßes eingeht. Das Betweennessfeld zeigt reichhaltige und zuvor verborgene Strukturen in aus Reanalyse- und Modelldaten der erdoberflächennahen Lufttemperatur gewonnenen Klimanetzen. Das durch unseren neuartigen Ansatz enthüllte Metanetzwerk, bestehend aus hochlokalisierten Kanälen stark gebündelten Informationsflusses, bringen wir mit der Oberflächenzirkulation des Weltozeans in Verbindung. In Analogie mit den gleichnamigen Datenautobahnen des Internets nennen wir dieses Metanetzwerk den Backbone des Klimanetzwerks. Unsere Ergebnisse deuten insgesamt darauf hin, dass Meeresoberflächenströmungen einen wichtigen Beitrag zur Kopplung und Stabilisierung des globalen Oberflächenlufttemperaturfeldes leisten. Wir zeigen weiterhin, dass die hohe Sensitivität des Betweennessmaßes hinsichtlich kleiner Änderungen der Netzwerktopologie die Detektion stark nichtlinearer physikalischer Wechselwirkungen im Klimasystem ermöglichen könnte. Die in dieser Arbeit vorgestellten Ergebnisse wurden mithilfe statistischer Signifikanztests auf der Zeitreihen- und Netzwerkebene gründlich auf ihre Robustheit geprüft. In Anbetracht fehlerbehafteter Daten und komplexer statistischer Zusammenhänge zwischen verschiedenen Netzwerkmaßen ist diese Vorgehensweise besonders wichtig. Weiterhin ist die Entwicklung neuer, allgemein anwendbarer Surrogate für räumlich eingebettete Netzwerke hervorzuheben, die die Berücksichtigung spezieller Klimanetzwerkeigenschaften wie z.B. der Wahrscheinlichkeitsverteilung der Kantenlängen erlauben. Unsere Methode ist universell, weil sie zum Verständnis des lokalisierten Informationsflusses in allen räumlich ausgedehnten, dynamischen Systemen beitragen kann. Deshalb ist sie innerhalb der Physik und anderer angewandter Wissenschaften von potentiell breitem Interesse. Mögliche Anwendungen könnten sich z.B. in der Fluiddynamik (Turbulenz), der Plasmaphysik und der Biophysik (Populationsmodelle, neuronale Netzwerke und Zellmodelle) finden. Darüber hinaus ist der Netzwerkansatz für experimentelle Daten sowie Modellsimulationen gültig, und eröffnet folglich neue Perspektiven für Modellevaluation und datengetriebene Modellierung. Im Rahmen der aktuellen Klimawandeldebatte stellen Klimanetzwerke einen neuartigen Satz von Analysemethoden zur Verfügung, der die Evaluation der lokalen Vulnerabilität und Stabilität des Klimasystems unter Berücksichtigung globaler Randbedingungen ermöglicht. Die in dieser Arbeit entwickelten und untersuchten Methoden könnten folglich in der Zukunft, innerhalb eines holistisch-globalen Ansatzes, zum Verständnis der lokalen Auswirkungen von Extremereignissen und Kipppunkten im Erdsystem beitragen. KW - Komplexe Netzwerke KW - Klimanetzwerke KW - Datenanalyse KW - Graphentheorie KW - Klimadaten KW - Complex networks KW - climate networks KW - data analysis KW - graph theory KW - climate data Y1 - 2009 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-49775 ER - TY - JOUR A1 - Ozturk, Ugur A1 - Marwan, Norbert A1 - Korup, Oliver A1 - Saito, H. A1 - Agarwa, Ankit A1 - Grossman, M. J. A1 - Zaiki, M. A1 - Kurths, Jürgen T1 - Complex networks for tracking extreme rainfall during typhoons JF - Chaos : an interdisciplinary journal of nonlinear science N2 - Reconciling the paths of extreme rainfall with those of typhoons remains difficult despite advanced forecasting techniques. We use complex networks defined by a nonlinear synchronization measure termed event synchronization to track extreme rainfall over the Japanese islands. Directed networks objectively record patterns of heavy rain brought by frontal storms and typhoons but mask out contributions of local convective storms. We propose a radial rank method to show that paths of extreme rainfall in the typhoon season (August-November, ASON) follow the overall southwest-northeast motion of typhoons and mean rainfall gradient of Japan. The associated eye-of-the-typhoon tracks deviate notably and may thus distort estimates of heavy typhoon rainfall. We mainly found that the lower spread of rainfall tracks in ASON may enable better hindcasting than for westerly-fed frontal storms in June and July. Y1 - 2018 U6 - https://doi.org/10.1063/1.5004480 SN - 1054-1500 SN - 1089-7682 VL - 28 IS - 7 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Kurilovich, Aleksandr A. A1 - Mantsevich, Vladimir A1 - Stevenson, Keith J. A1 - Chechkin, Aleksei V. A1 - Palyulin, V. V. T1 - Complex diffusion-based kinetics of photoluminescence in semiconductor nanoplatelets JF - Physical chemistry, chemical physics : a journal of European Chemical Societies N2 - We present a diffusion-based simulation and theoretical models for explanation of the photoluminescence (PL) emission intensity in semiconductor nanoplatelets. It is shown that the shape of the PL intensity curves can be reproduced by the interplay of recombination, diffusion and trapping of excitons. The emission intensity at short times is purely exponential and is defined by recombination. At long times, it is governed by the release of excitons from surface traps and is characterized by a power-law tail. We show that the crossover from one limit to another is controlled by diffusion properties. This intermediate region exhibits a rich behaviour depending on the value of diffusivity. The proposed approach reproduces all the features of experimental curves measured for different nanoplatelet systems. Y1 - 2020 U6 - https://doi.org/10.1039/d0cp03744c SN - 1463-9076 SN - 1463-9084 VL - 22 IS - 42 SP - 24686 EP - 24696 PB - Royal Society of Chemistry CY - Cambridge ER - TY - GEN A1 - Bolotov, Maxim A1 - Smirnov, Lev A. A1 - Osipov, Grigory V. A1 - Pikovskij, Arkadij T1 - Complex chimera states in a nonlinearly coupled oscillatory medium T2 - 2018 2nd School on Dynamics of Complex Networks and their Application in Intellectual Robotics (DCNAIR) N2 - We consider chimera states in a one-dimensional medium of nonlinear nonlocally coupled phase oscillators. Stationary inhomogeneous solutions of the Ott-Antonsen equation for a complex order parameter that correspond to fundamental chimeras have been constructed. Stability calculations reveal that only some of these states are stable. The direct numerical simulation has shown that these structures under certain conditions are transformed to breathing chimera regimes because of the development of instability. Further development of instability leads to turbulent chimeras. KW - phase oscillator KW - nonlocal coupling KW - synchronization KW - chimera state KW - partial synchronization KW - phase lag KW - nonlinear dynamics Y1 - 2018 SN - 978-1-5386-5818-5 U6 - https://doi.org/10.1109/DCNAIR.2018.8589210 SP - 17 EP - 20 PB - IEEE CY - New York ER - TY - JOUR A1 - Menzel, Ralf A1 - Puhlmann, Dirk A1 - Heuer, Axel T1 - Complementarity in single photon interference – the role of the mode function and vacuum fields JF - Journal of the European Optical Society-Rapid N2 - Background In earlier experiments the role of the vacuum fields could be demonstrated as the source of complementarity with respect to the temporal properties (Heuer et al., Phys. Rev. Lett. 114:053601, 2015). Methods Single photon first order interferences of spatially separated regions from the cone structure of spontaneous parametric down conversion allow for analyzing the role of the mode function in quantum optics regarding the complementarity principle. Results Here the spatial coherence properties of these vacuum fields are demonstrated as the physical reason for complementarity in these single photon quantum optical experiments. These results are directly connected to the mode picture in classical optics. Conclusion The properties of the involved vacuum fields selected via the measurement process are the physical background of the complementarity principle in quantum optics. KW - Quantum optics KW - Complementarity KW - Mode function KW - Vacuum fields Y1 - 2017 U6 - https://doi.org/10.1186/s41476-017-0036-x SN - 1990-2573 VL - 13 PB - Springer ER - TY - GEN A1 - Menzel, Ralf A1 - Puhlmann, Dirk A1 - Heuer, Axel T1 - Complementarity in single photon interference – the role of the mode function and vacuum fields N2 - Background In earlier experiments the role of the vacuum fields could be demonstrated as the source of complementarity with respect to the temporal properties (Heuer et al., Phys. Rev. Lett. 114:053601, 2015). Methods Single photon first order interferences of spatially separated regions from the cone structure of spontaneous parametric down conversion allow for analyzing the role of the mode function in quantum optics regarding the complementarity principle. Results Here the spatial coherence properties of these vacuum fields are demonstrated as the physical reason for complementarity in these single photon quantum optical experiments. These results are directly connected to the mode picture in classical optics. Conclusion The properties of the involved vacuum fields selected via the measurement process are the physical background of the complementarity principle in quantum optics. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 333 KW - Complementarity KW - Mode function KW - Quantum optics KW - Vacuum fields Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-395210 ER - TY - JOUR A1 - Niskanen, Johannes A1 - Fondell, Mattis A1 - Sahle, Christoph J. A1 - Eckert, Sebastian A1 - Jay, Raphael Martin A1 - Gilmore, Keith A1 - Pietzsch, Annette A1 - Dantz, Marcus A1 - Lu, Xingye A1 - McNally, Daniel E. A1 - Schmitt, Thorsten A1 - Vaz da Cruz, Vinicius A1 - Kimberg, Victor A1 - Föhlisch, Alexander A1 - Gel’mukhanov, Faris T1 - Compatibility of quantitative X-ray spectroscopy with continuous distribution models of water at ambient conditions JF - Proceedings of the National Academy of Sciences of the United States of America N2 - The phase diagram of water harbors controversial views on underlying structural properties of its constituting molecular moieties, its fluctuating hydrogen-bonding network, as well as pair-correlation functions. In this work, long energy-range detection of the X-ray absorption allows us to unambiguously calibrate the spectra for water gas, liquid, and ice by the experimental atomic ionization cross-section. In liquid water, we extract the mean value of 1.74 +/- 2.1% donated and accepted hydrogen bonds per molecule, pointing to a continuous-distribution model. In addition, resonant inelastic X-ray scattering with unprecedented energy resolution also supports continuous distribution of molecular neighborhoods within liquid water, as do X-ray emission spectra once the femtosecond scattering duration and proton dynamics in resonant X-ray-matter interaction are taken into account. Thus, X-ray spectra of liquid water in ambient conditions can be understood without a two-structure model, whereas the occurrence of nanoscale-length correlations within the continuous distribution remains open. KW - structure of water KW - X-ray spectroscopy KW - continuous distribution model Y1 - 2019 U6 - https://doi.org/10.1073/pnas.1815701116 SN - 0027-8424 VL - 116 IS - 10 SP - 4058 EP - 4063 PB - National Acad. of Sciences CY - Washington ER - TY - JOUR A1 - He, Yongli A1 - Huang, Jianping A1 - Li, Dongdong A1 - Xie, Yongkun A1 - Zhang, Guolong A1 - Qi, Yulei A1 - Wang, Shanshan A1 - Totz, Sonja Juliana T1 - Comparison of the effect of land-sea thermal contrast on interdecadal variations in winter and summer blockings JF - Climate dynamics : observational, theoretical and computational research on the climate system N2 - The influence of winter and summer land-sea surface thermal contrast on blocking for 1948-2013 is investigated using observations and the coupled model intercomparison project outputs. The land-sea index (LSI) is defined to measure the changes of zonal asymmetric thermal forcing under global warming. The summer LSI shows a slower increasing trend than winter during this period. For the positive of summer LSI, the EP flux convergence induced by the land-sea thermal forcing in the high latitude becomes weaker than normal, which induces positive anomaly of zonal-mean westerly and double-jet structure. Based on the quasiresonance amplification mechanism, the narrow and reduced westerly tunnel between two jet centers provides a favor environment for more frequent blocking. Composite analysis demonstrates that summer blocking shows an increasing trend of event numbers and a decreasing trend of durations. The numbers of the short-lived blocking persisting for 5-9 days significantly increases and the numbers of the long-lived blocking persisting for longer than 10 days has a weak increase than that in negative phase of summer LSI. The increasing transient wave activities induced by summer LSI is responsible for the decreasing duration of blockings. The increasing blocking due to summer LSI can further strengthen the continent warming and increase the summer LSI, which forms a positive feedback. The opposite dynamical effect of LSI on summer and winter blocking are discussed and found that the LSI-blocking negative feedback partially reduces the influence of the above positive feedback and induce the weak summer warming rate. KW - Land-sea thermal contrast KW - Blocking KW - Asymmetric warming KW - Double-jet Y1 - 2017 U6 - https://doi.org/10.1007/s00382-017-3954-9 SN - 0930-7575 SN - 1432-0894 VL - 51 IS - 4 SP - 1275 EP - 1294 PB - Springer CY - New York ER - TY - JOUR A1 - Eerqing, Narima A1 - Subramanian, Sivaraman A1 - Rubio Jimenez, Jesus A1 - Lutz, Tobias A1 - Wu, Hsin-Yu A1 - Anders, Janet A1 - Soeller, Christian A1 - Vollmer, Frank T1 - Comparing transient oligonucleotide hybridization kinetics using DNA-PAINT and optoplasmonic single-molecule sensing on gold nanorods JF - ACS photonics / American Chemical Society N2 - We report a comparison of two photonic techniques for single-molecule sensing: fluorescence nanoscopy and optoplasmonic sensing. As the test system, oligonucleotides with and without fluorescent labels are transiently hybridized to complementary "docking" strands attached to gold nanorods. Comparing the measured single-molecule kinetics helps to examine the influence of the fluorescent labels as well as factors arising from different sensing geometries. Our results demonstrate that DNA dissociation is not significantly altered by the fluorescent labels and that DNA association is affected by geometric factors in the two techniques. These findings open the door to exploiting plasmonic sensing and fluorescence nanoscopy in a complementary fashion, which will aid in building more powerful sensors and uncovering the intricate effects that influence the behavior of single molecules. KW - single-molecule KW - plasmonics KW - whispering gallery modes KW - optoplasmonic KW - DNA-PAINT KW - fluorescence KW - localization microscopy Y1 - 2021 U6 - https://doi.org/10.1021/acsphotonics.1c01179 SN - 2330-4022 VL - 8 IS - 10 SP - 2882 EP - 2888 PB - American Chemical Society CY - Washington ER - TY - THES A1 - Ahnert, Karsten T1 - Compactons in strongly nonlinear lattices T1 - Kompaktonen in stark nichtlinearen Gittern N2 - In the present work, we study wave phenomena in strongly nonlinear lattices. Such lattices are characterized by the absence of classical linear waves. We demonstrate that compactons – strongly localized solitary waves with tails decaying faster than exponential – exist and that they play a major role in the dynamics of the system under consideration. We investigate compactons in different physical setups. One part deals with lattices of dispersively coupled limit cycle oscillators which find various applications in natural sciences such as Josephson junction arrays or coupled Ginzburg-Landau equations. Another part deals with Hamiltonian lattices. Here, a prominent example in which compactons can be found is the granular chain. In the third part, we study systems which are related to the discrete nonlinear Schrödinger equation describing, for example, coupled optical wave-guides or the dynamics of Bose-Einstein condensates in optical lattices. Our investigations are based on a numerical method to solve the traveling wave equation. This results in a quasi-exact solution (up to numerical errors) which is the compacton. Another ansatz which is employed throughout this work is the quasi-continuous approximation where the lattice is described by a continuous medium. Here, compactons are found analytically, but they are defined on a truly compact support. Remarkably, both ways give similar qualitative and quantitative results. Additionally, we study the dynamical properties of compactons by means of numerical simulation of the lattice equations. Especially, we concentrate on their emergence from physically realizable initial conditions as well as on their stability due to collisions. We show that the collisions are not exactly elastic but that a small part of the energy remains at the location of the collision. In finite lattices, this remaining part will then trigger a multiple scattering process resulting in a chaotic state. N2 - In der hier vorliegenden Arbeit werden Wellenphänomene in stark nichtlinearen Gittern untersucht. Diese Gitter zeichnen sich vor allem durch die Abwesenheit von klassischen linearen Wellen aus. Es wird gezeigt, dass Kompaktonen – stark lokalisierte solitäre Wellen, mit Ausläufern welche schneller als exponentiell abfallen – existieren, und dass sie eine entscheidende Rolle in der Dynamik dieser Gitter spielen. Kompaktonen treten in verschiedenen diskreten physikalischen Systemen auf. Ein Teil der Arbeit behandelt dabei Gitter von dispersiv gekoppelten Oszillatoren, welche beispielsweise Anwendung in gekoppelten Josephsonkontakten oder gekoppelten Ginzburg-Landau-Gleichungen finden. Ein weiterer Teil beschäftigt sich mit Hamiltongittern, wobei die granulare Kette das bekannteste Beispiel ist, in dem Kompaktonen beobachtet werden können. Im dritten Teil werden Systeme, welche im Zusammenhang mit der Diskreten Nichtlinearen Schrödingergleichung stehen, studiert. Diese Gleichung beschreibt beispielsweise Arrays von optischen Wellenleitern oder die Dynamik von Bose-Einstein-Kondensaten in optischen Gittern. Das Studium der Kompaktonen basiert hier hauptsächlich auf dem numerischen Lösen der dazugehörigen Wellengleichung. Dies mündet in einer quasi-exakten Lösung, dem Kompakton, welches bis auf numerische Fehler genau bestimmt werden kann. Ein anderer Ansatz, der in dieser Arbeit mehrfach verwendet wird, ist die Approximation des Gitters durch ein kontinuierliches Medium. Die daraus resultierenden Kompaktonen besitzen einen im mathematischen Sinne kompakten Definitionsbereich. Beide Methoden liefern qualitativ und quantitativ gut übereinstimmende Ergebnisse. Zusätzlich werden die dynamischen Eigenschaften von Kompaktonen mit Hilfe von direkten numerischen Simulationen der Gittergleichungen untersucht. Dabei wird ein Hauptaugenmerk auf die Entstehung von Kompaktonen unter physikalisch realisierbaren Anfangsbedingungen und ihre Kollisionen gelegt. Es wird gezeigt, dass die Wechselwirkung nicht exakt elastisch ist, sondern dass ein Teil ihrer Energie an der Position der Kollision verharrt. In endlichen Gittern führt dies zu einem multiplen Streuprozess, welcher in einem chaotischen Zustand endet. KW - Gitterdynamik KW - Hamilton KW - Compacton KW - Soliton KW - granulare Kette KW - Lattice dynamics KW - Hamiltonian KW - Compacton KW - Soliton KW - Granular chain Y1 - 2010 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-48539 ER - TY - GEN A1 - Goychuk, Igor T1 - Comment on "Anomalous Escape Governed by Thermal 1/f Noise" Reply (R. K. Singh) T2 - Physical review letters Y1 - 2019 U6 - https://doi.org/10.1103/PhysRevLett.123.238902 SN - 0031-9007 SN - 1079-7114 VL - 123 IS - 23 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Marschall, Raphael A1 - Skorov, Yuri A1 - Zakharov, Vladimir A1 - Rezac, Ladislav A1 - Gerig, Selina-Barbara A1 - Christou, Chariton A1 - Dadzie, S. Kokou A1 - Migliorini, Alessandra A1 - Rinaldi, Giovanna A1 - Agarwal, Jessica A1 - Vincent, Jean-Baptiste A1 - Kappel, David T1 - Cometary comae-surface links the physics of gas and dust from the surface to a spacecraft JF - Space science reviews N2 - A comet is a highly dynamic object, undergoing a permanent state of change. These changes have to be carefully classified and considered according to their intrinsic temporal and spatial scales. The Rosetta mission has, through its contiguous in-situ and remote sensing coverage of comet 67P/Churyumov-Gerasimenko (hereafter 67P) over the time span of August 2014 to September 2016, monitored the emergence, culmination, and winding down of the gas and dust comae. This provided an unprecedented data set and has spurred a large effort to connect in-situ and remote sensing measurements to the surface. In this review, we address our current understanding of cometary activity and the challenges involved when linking comae data to the surface. We give the current state of research by describing what we know about the physical processes involved from the surface to a few tens of kilometres above it with respect to the gas and dust emission from cometary nuclei. Further, we describe how complex multidimensional cometary gas and dust models have developed from the Halley encounter of 1986 to today. This includes the study of inhomogeneous outgassing and determination of the gas and dust production rates. Additionally, the different approaches used and results obtained to link coma data to the surface will be discussed. We discuss forward and inversion models and we describe the limitations of the respective approaches. The current literature suggests that there does not seem to be a single uniform process behind cometary activity. Rather, activity seems to be the consequence of a variety of erosion processes, including the sublimation of both water ice and more volatile material, but possibly also more exotic processes such as fracture and cliff erosion under thermal and mechanical stress, sub-surface heat storage, and a complex interplay of these processes. Seasons and the nucleus shape are key factors for the distribution and temporal evolution of activity and imply that the heliocentric evolution of activity can be highly individual for every comet, and generalisations can be misleading. KW - comets KW - coma KW - gas KW - dust KW - dynamics KW - modelling KW - inversion Y1 - 2020 U6 - https://doi.org/10.1007/s11214-020-00744-0 SN - 0038-6308 SN - 1572-9672 VL - 216 IS - 8 PB - Springer CY - Dordrecht ER - TY - JOUR A1 - Lopez, Carlos G. A1 - Manova, Anna A1 - Hoppe, Corinna A1 - Dreja, Michael A1 - Schmiedel, Peter A1 - Job, Mareile A1 - Richtering, Walter A1 - Böker, Alexander A1 - Tsarkova, Larisa A. T1 - Combined UV-Vis-absorbance and reflectance spectroscopy study of dye transfer kinetics in aqueous mixtures of surfactants JF - Colloids and surfaces : an international journal devoted to the principles and applications of colloid and interface science ; A, Physicochemical and engineering aspects N2 - We report an analytical approach to study the competitive processes of solubilisation in micelles and of adsorption onto hydrophobic surfaces of poorly soluble hydrophobic dyes. The method is demonstrated on model systems containing two sources of Disperse Red 60: a bulk powder and a donor red textile, with molecularly dissolved dye stabilised in an aqueous environment by mixed micelles of anionic and non-ionic surfactants. The process of dye transfer between a donor textile (red polyester), surfactant micelles and an acceptor textile (white polyamide) was quantified by a combination of colorimetric analyses. UV-Vis absorbance was used to follow the extraction of the dye and to evaluate the solubilisation capacity of the micellar solution. A calibration curve for textile reflectance versus the adsorbed dye was generated to quantify the mass of dye transferred onto the acceptor textile. A combination of both techniques allowed us to compare the amount of dye desorbed from the donor textile and adsorbed onto the acceptor textile as a function of time for systems undergoing exhaustion-solubilisation mechanisms and only solubilisation mechanism. Up to similar or equal to 10 min of the washing process, the released dye is predominantly solubilised in surfactant micelles. At later times, the adsorption of the dye on the hydrophobic surface is energetically favoured. The shift of the desorption equilibrium in the presence of the acceptor textile results in similar or equal to 30% increase in the release of the dye. The reported methodology provides direct comparative analysis between the solubilisation capacity of amphiphilic stabilisers and the tendency of the dye to adsorb on solid substrates, important for designing novel concepts of disperse dye solubilisation and dye transfer inhibition during textile washing. KW - Dye transfer KW - Disperse dyes KW - Surfactants KW - Detergent KW - Colorimetric analysis KW - Washing fastness Y1 - 2018 U6 - https://doi.org/10.1016/j.colsurfa.2018.04.024 SN - 0927-7757 SN - 1873-4359 VL - 550 SP - 74 EP - 81 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Prol, Fabricio S. A1 - Smirnov, Artem G. A1 - Hoque, M. Mainul A1 - Shprits, Yuri T1 - Combined model of topside ionosphere and plasmasphere derived from radio-occultation and Van Allen Probes data JF - Scientific reports N2 - In the last years, electron density profile functions characterized by a linear dependence on the scale height showed good results when approximating the topside ionosphere. The performance above 800 km, however, is not yet well investigated. This study investigates the capability of the semi-Epstein functions to represent electron density profiles from the peak height up to 20,000 km. Electron density observations recorded by the Van Allen Probes were used to resolve the scale height dependence in the plasmasphere. It was found that the linear dependence of the scale height in the topside ionosphere cannot be directly used to extrapolate profiles above 800 km. We find that the dependence of scale heights on altitude is quadratic in the plasmasphere. A statistical model of the scale heights is therefore proposed. After combining the topside ionosphere and plasmasphere by a unified model, we have obtained good estimations not only in the profile shapes, but also in the Total Electron Content magnitude and distributions when compared to actual measurements from 2013, 2014, 2016 and 2017. Our investigation shows that Van Allen Probes can be merged to radio-occultation data to properly represent the upper ionosphere and plasmasphere by means of a semi-Epstein function. Y1 - 2022 U6 - https://doi.org/10.1038/s41598-022-13302-1 SN - 2045-2322 VL - 12 IS - 1 PB - Macmillan Publishers Limited, part of Springer Nature CY - London ER - TY - JOUR A1 - Petreska, Irina A1 - Sandev, Trifce A1 - Lenzi, Ervin Kaminski T1 - Comb-like geometric constraints leading to emergence of the time-fractional Schrödinger equation JF - Modern physics letters : A, Particles and fields, gravitation, cosmology, nuclear physics N2 - This paper presents an overview over several examples, where the comb-like geometric constraints lead to emergence of the time-fractional Schrodinger equation. Motion of a quantum object on a comb structure is modeled by a suitable modification of the kinetic energy operator, obtained by insertion of the Dirac delta function in the Laplacian. First, we consider motion of a free particle on two- and three-dimensional comb structures, and then we extend the study to the interacting cases. A general form of a nonlocal term, which describes the interactions of the particle with the medium, is included in the Hamiltonian, and later on, the cases of constant and Dirac delta potentials are analyzed. At the end, we discuss the case of non-integer dimensions, considering separately the case of fractal dimension between one and two, and the case of fractal dimension between two and three. All these examples show that even though we are starting with the standard time-dependent Schrodinger equation on a comb, the time-fractional equation for the Green's functions appears, due to these specific geometric constraints. KW - Comb model KW - time-fractional Schrödinger equation KW - Green’ s functions Y1 - 2021 U6 - https://doi.org/10.1142/S0217732321300056 SN - 0217-7323 SN - 1793-6632 VL - 36 IS - 14 PB - World Scientific CY - Singapore ER - TY - JOUR A1 - Omel'chenko, Oleh A1 - Laing, Carlo R. T1 - Collective states in a ring network of theta neurons JF - Proceedings of the Royal Society of London. Series A, Mathematical, physical and engineering sciences N2 - We consider a ring network of theta neurons with non-local homogeneous coupling. We analyse the corresponding continuum evolution equation, analytically describing all possible steady states and their stability. By considering a number of different parameter sets, we determine the typical bifurcation scenarios of the network, and put on a rigorous footing some previously observed numerical results. KW - theta neurons KW - neural networks KW - bumps Y1 - 2022 U6 - https://doi.org/10.1098/rspa.2021.0817 SN - 1364-5021 SN - 1471-2946 VL - 478 IS - 2259 PB - Royal Society CY - London ER - TY - JOUR A1 - Goldobin, Denis S. A1 - Tyulkina, Irina V. A1 - Klimenko, Lyudmila S. A1 - Pikovskij, Arkadij T1 - Collective mode reductions for populations of coupled noisy oscillators JF - Chaos : an interdisciplinary journal of nonlinear science N2 - We analyze the accuracy of different low-dimensional reductions of the collective dynamics in large populations of coupled phase oscillators with intrinsic noise. Three approximations are considered: (i) the Ott-Antonsen ansatz, (ii) the Gaussian ansatz, and (iii) a two-cumulant truncation of the circular cumulant representation of the original system’s dynamics. For the latter, we suggest a closure, which makes the truncation, for small noise, a rigorous first-order correction to the Ott-Antonsen ansatz, and simultaneously is a generalization of the Gaussian ansatz. The Kuramoto model with intrinsic noise and the population of identical noisy active rotators in excitable states with the Kuramoto-type coupling are considered as examples to test the validity of these approximations. For all considered cases, the Gaussian ansatz is found to be more accurate than the Ott-Antonsen one for high-synchrony states only. The two-cumulant approximation is always superior to both other approximations. Synchrony of large ensembles of coupled elements can be characterised by the order parameters—the mean fields. Quite often, the evolution of these collective variables is surprisingly simple, which makes a description with only a few order parameters feasible. Thus, one tries to construct accurate closed low-dimensional mathematical models for the dynamics of the first few order parameters. These models represent useful tools for gaining insight into the underlaying mechanisms of some more sophisticated collective phenomena: for example, one describes coupled populations by virtue of coupled equations for the relevant order parameters. A regular approach to the construction of closed low-dimensional systems is also beneficial for dealing with phenomena, which are beyond the applicability scope of these models; for instance, with such an approach, one can determine constraints on clustering in populations. There are two prominent types of situations, where the low-dimensional models can be constructed: (i) for a certain class of ideal paradigmatic systems of coupled phase oscillators, the Ott-Antonsen ansatz yields an exact equation for the main order parameter and (ii) the Gaussian approximation for the probability density of the phases, also yielding a low-dimensional closure, is frequently quite accurate. In this paper, we compare applications of these two model reductions for situations, where neither of them is perfectly accurate. Furthermore, we construct a new reduction approach which practically works as a first-order correction to the best of the two basic approximations. Y1 - 2018 U6 - https://doi.org/10.1063/1.5053576 SN - 1054-1500 SN - 1089-7682 VL - 28 IS - 10 PB - American Institute of Physics CY - Melville ER - TY - THES A1 - Henkel, Carsten T1 - Coherence theory of atomic de Broglie waves and electromagnetic near fields N2 - Die Arbeit untersucht theoretisch die Wechselwirkung neutraler Teilchen (Atome, Moleküle) mit Oberflächen, soweit sie durch das elektromagnetische Feld vermittelt wird. Spektrale Energiedichten und Kohärenzfunktionen werden hergeleitet und liefern eine umfassende Charakterisierung des Felds auf der sub-Wellenlängen-Skala. Die Ergebnisse finden auf zwei Teilgebieten Anwendung: in der integrierten Atomoptik, wo ultrakalte Atome an thermische Oberflächen koppeln, und in der Nahfeldoptik, wo eine Auflösung unterhalb der Beugungsbegrenzung mit einzelnen Molekülen als Sonden und Detektoren erzielt werden kann. N2 - We theoretically discuss the interaction of neutral particles (atoms, molecules) with surfaces in the regime where it is mediated by the electromagnetic field. A thorough characterization of the field at sub-wavelength distances is worked out, including energy density spectra and coherence functions. The results are applied to typical situations in integrated atom optics, where ultracold atoms are coupled to a thermal surface, and to single molecule probes in near field optics, where sub-wavelength resolution can be achieved. KW - Kohärenztheorie KW - Quantenoptik KW - Quanten-Elektrodynamik (QED) KW - Atomoptik KW - Atomchip KW - Spektroskopie KW - Oberfläche KW - coherence theory KW - quantum optics KW - quantum electrodynamics (QED) KW - atom optics KW - atom chip KW - spectroscopy KW - surface Y1 - 2004 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-0001272 ER - TY - THES A1 - Goldobin, Denis S. T1 - Coherence and synchronization of noisy-driven oscillators T1 - Kohärenz und Synchronisation verrauschter Oszillatoren N2 - In the present dissertation paper we study problems related to synchronization phenomena in the presence of noise which unavoidably appears in real systems. One part of the work is aimed at investigation of utilizing delayed feedback to control properties of diverse chaotic dynamic and stochastic systems, with emphasis on the ones determining predisposition to synchronization. Other part deals with a constructive role of noise, i.e. its ability to synchronize identical self-sustained oscillators. First, we demonstrate that the coherence of a noisy or chaotic self-sustained oscillator can be efficiently controlled by the delayed feedback. We develop the analytical theory of this effect, considering noisy systems in the Gaussian approximation. Possible applications of the effect for the synchronization control are also discussed. Second, we consider synchrony of limit cycle systems (in other words, self-sustained oscillators) driven by identical noise. For weak noise and smooth systems we proof the purely synchronizing effect of noise. For slightly different oscillators and/or slightly nonidentical driving, synchrony becomes imperfect, and this subject is also studied. Then, with numerics we show moderate noise to be able to lead to desynchronization of some systems under certain circumstances. For neurons the last effect means “antireliability” (the “reliability” property of neurons is treated to be important from the viewpoint of information transmission functions), and we extend our investigation to neural oscillators which are not always limit cycle ones. Third, we develop a weakly nonlinear theory of the Kuramoto transition (a transition to collective synchrony) in an ensemble of globally coupled oscillators in presence of additional time-delayed coupling terms. We show that a linear delayed feedback not only controls the transition point, but effectively changes the nonlinear terms near the transition. A purely nonlinear delayed coupling does not affect the transition point, but can reduce or enhance the amplitude of collective oscillations. N2 - In dieser Dissertation werden Synchronisationsphänomene im Vorhandensein von Rauschen studiert. Ein Ziel dieser Arbeit besteht in der Untersuchung der Anwendbarkeit verzögerter Rückkopplung zur Kontrolle von bestimmten Eigenschaften chaotischer oder stochastischer Systeme. Der andere Teil beschäftigt sich mit den konstruktiven Eigenschaften von Rauschen. Insbesondere wird die Möglichkeit, identische selbsterregte Oszillatoren zu synchronisieren untersucht. Als erstes wird gezeigt, dass Kohärenz verrauschter oder chaotischer Oszillatoren durch verzögertes Rückkoppeln kontrolliert werden kann. Es wird eine analytische Beschreibung dieses Phänomens in verrauschten Systemen entwickelt. Außerdem werden mögliche Anwendungen im Zusammenhang mit Synchronisationskontrolle vorgestellt und diskutiert. Als zweites werden Oszillatoren unter dem Einfluss von identischem Rauschen betrachtet. Für schwaches Rauschen und genügend glatte Systeme wird bewiesen, das Rauschen zu Synchronisation führt. Für leicht unterschiedliche Oszillatoren und leicht unterschiedliches Rauschen wird die Synchronisation unvollständig. Dieser Effekt wird auch untersucht. Dann wird mit Hilfe von Numerik gezeigt, dass moderates Rauschen zur Desynchronisierung von bestimmten Systemen führen kann. Dieser Effekt wird auch in neuronalen Oszillatoren untersucht, welche nicht unbedingt Grenzzyklen besitzen müssen. Im dritten Teil wird eine schwache nichtlineare Theorie des Kuramoto-Übergangs, dem Übergang zur kollektiven Synchronisation, in einem Ensemble von global gekoppelten Oszillatoren mit zusätzlichen zeitverzögerten Kopplungstermen entwickelt. Es wird gezeigt, dass lineare Rückkopplung nicht nur den Übergangspunkt bestimmt, sondern auch die nichtlinearen Terme in der Nähe des Übergangs entscheidend verändert. Eine rein nichtlineare Rückkopplung verändert den Übergang nicht, kann aber die Amplitude der kollektiven Oszillationen vergrößern oder verringern. KW - Rauschen KW - Chaos KW - Phasendiffusion KW - Neuronsreliabilität KW - Synchronisation KW - Noise KW - Chaos KW - Phase Diffusion KW - Reliability of Neurons KW - Synchronization Y1 - 2007 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-15047 ER - TY - JOUR A1 - Awad, Emad A1 - Metzler, Ralf T1 - Closed-form multi-dimensional solutions and asymptotic behaviours for subdiffusive processes with crossovers: II. Accelerating case JF - Journal of physics : A, Mathematical and theoretical N2 - Anomalous diffusion with a power-law time dependence vertical bar R vertical bar(2)(t) similar or equal to t(alpha i) of the mean squared displacement occurs quite ubiquitously in numerous complex systems. Often, this anomalous diffusion is characterised by crossovers between regimes with different anomalous diffusion exponents alpha(i). Here we consider the case when such a crossover occurs from a first regime with alpha(1) to a second regime with alpha(2) such that alpha(2) > alpha(1), i.e., accelerating anomalous diffusion. A widely used framework to describe such crossovers in a one-dimensional setting is the bi-fractional diffusion equation of the so-called modified type, involving two time-fractional derivatives defined in the Riemann-Liouville sense. We here generalise this bi-fractional diffusion equation to higher dimensions and derive its multidimensional propagator (Green's function) for the general case when also a space fractional derivative is present, taking into consideration long-ranged jumps (Levy flights). We derive the asymptotic behaviours for this propagator in both the short- and long-time as well the short- and long-distance regimes. Finally, we also calculate the mean squared displacement, skewness and kurtosis in all dimensions, demonstrating that in the general case the non-Gaussian shape of the probability density function changes. KW - multidimensional fractional diffusion equation KW - continuous time random KW - walks KW - crossover anomalous diffusion dynamics KW - non-Gaussian probability KW - density Y1 - 2022 U6 - https://doi.org/10.1088/1751-8121/ac5a90 SN - 1751-8113 SN - 1751-8121 VL - 55 IS - 20 PB - IOP Publ. Ltd. CY - Bristol ER - TY - THES A1 - Wenz, Leonie T1 - Climate change impacts in an increasingly connected world Y1 - 2016 ER - TY - JOUR A1 - Vilk, Ohad A1 - Aghion, Erez A1 - Nathan, Ran A1 - Toledo, Sivan A1 - Metzler, Ralf A1 - Assaf, Michael T1 - Classification of anomalous diffusion in animal movement data using power spectral analysis JF - Journal of physics : A, Mathematical and theoretical N2 - The field of movement ecology has seen a rapid increase in high-resolution data in recent years, leading to the development of numerous statistical and numerical methods to analyse relocation trajectories. Data are often collected at the level of the individual and for long periods that may encompass a range of behaviours. Here, we use the power spectral density (PSD) to characterise the random movement patterns of a black-winged kite (Elanus caeruleus) and a white stork (Ciconia ciconia). The tracks are first segmented and clustered into different behaviours (movement modes), and for each mode we measure the PSD and the ageing properties of the process. For the foraging kite we find 1/f noise, previously reported in ecological systems mainly in the context of population dynamics, but not for movement data. We further suggest plausible models for each of the behavioural modes by comparing both the measured PSD exponents and the distribution of the single-trajectory PSD to known theoretical results and simulations. KW - diffusion KW - anomalous diffusion KW - power spectral analysis KW - ecological KW - movement data Y1 - 2022 U6 - https://doi.org/10.1088/1751-8121/ac7e8f SN - 1751-8113 SN - 1751-8121 VL - 55 IS - 33 PB - IOP Publishing CY - Bristol ER - TY - JOUR A1 - Silanteva, Irina A. A1 - Komolkin, Andrei A1 - Mamontova, Veronika V. A1 - Gabrusenok, Pavel A1 - Vorontsov-Velyaminov, Pavel N. A1 - Santer, Svetlana A1 - Kasyanenko, Nina A. T1 - Cis-isomers of photosensitive cationic azobenzene surfactants in DNA solutions at different NaCl concentrations BT - Experiment and modeling JF - The journal of physical chemistry : B, Condensed matter, materials, surfaces, interfaces & biophysical chemistry N2 - The DNA interaction with cis-isomers of photosensitive azobenzene-containing surfactants was studied by both experimental methods and computer simulation. It was shown that before the organization of micelles, such surfactants in the cis-conformation form associates of only a single type with a disordered orientation of molecules. In contrast, for trans-isomers, there exist two types of associates with head-to-head or head-to-tail orientations of molecules in dependence on salt concentration in a solution. The comparison of cis- and trans-isomer binding to DNA and the influence of salt concentration on the formation of their complexes with DNA were studied. It was shown that cis-isomers interact with phosphate groups of DNA and that their molecules were also located along the minor groove of DNA. Y1 - 2021 U6 - https://doi.org/10.1021/acs.jpcb.1c07864 SN - 1520-6106 SN - 1520-5207 VL - 125 IS - 40 SP - 11197 EP - 11207 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Spahn, Frank A1 - Sachse, Manuel A1 - Seiss, Martin A1 - Hsu, Hsiang-Wen A1 - Kempf, Sascha A1 - Horanyi, Mihaly T1 - Circumplanetary Dust Populations JF - Space science reviews N2 - We summarize the current state of observations of circumplanetary dust populations, including both dilute and dense rings and tori around the giant planets, ejecta clouds engulfing airless moons, and rings around smaller planetary bodies throughout the Solar System. We also discuss the theoretical models that enable these observations to be understood in terms of the sources, sinks and transport of various dust populations. The dynamics and resulting transport of the particles can be quite complex, due to the fact that their motion is influenced by neutral and plasma drag, radiation pressure, and electromagnetic forcesall in addition to gravity. The relative importance of these forces depends on the environment, as well as the makeup and size of the particles. Possible dust sources include the generation of ejecta particles by impacts, active volcanoes and geysers, and the capture of exogenous particles. Possible dust sinks include collisions with moons, rings, or the central planet, erosion due to sublimation and sputtering, even ejection and escape from the circumplanetary environment. KW - Circumplanetary dust KW - Planetary rings and tori KW - Dust sources and sinks KW - Dust dynamics Y1 - 2019 U6 - https://doi.org/10.1007/s11214-018-0577-3 SN - 0038-6308 SN - 1572-9672 VL - 215 IS - 1 PB - Springer CY - Dordrecht ER - TY - JOUR A1 - Diercke, Andrea A1 - Denker, Carsten T1 - Chromospheric Synoptic Maps of Polar Crown Filaments JF - Solar physics : a journal for solar and solar-stellar research and the study of solar terrestrial physics N2 - Polar crown filaments form above the polarity inversion line between the old magnetic flux of the previous cycle and the new magnetic flux of the current cycle. Studying their appearance and their properties can lead to a better understanding of the solar cycle. We use full-disk data of the Chromospheric Telescope (ChroTel) at the Observatorio del Teide, Tenerife, Spain, which were taken in three different chromospheric absorption lines (H alpha lambda 6563 angstrom, Ca II K lambda 3933 angstrom, and He I lambda 10830 angstrom), and we create synoptic maps. In addition, the spectroscopic He I data allow us to compute Doppler velocities and to create synoptic Doppler maps. ChroTel data cover the rising and decaying phase of Solar Cycle 24 on about 1000 days between 2012 and 2018. Based on these data, we automatically extract polar crown filaments with image-processing tools and study their properties. We compare contrast maps of polar crown filaments with those of quiet-Sun filaments. Furthermore, we present a super-synoptic map summarizing the entire ChroTel database. In summary, we provide statistical properties, i.e. number and location of filaments, area, and tilt angle for both the maximum and the declining phase of Solar Cycle 24. This demonstrates that ChroTel provides a promising data set to study the solar cycle. KW - Chromosphere, quiet KW - Prominences, quiescent KW - Prominences, magnetic field KW - Solar Cycle, observations KW - Instrumentation and data management Y1 - 0201 U6 - https://doi.org/10.1007/s11207-019-1538-z SN - 0038-0938 SN - 1573-093X VL - 294 IS - 11 PB - Springer CY - Dordrecht ER - TY - JOUR A1 - Zaks, Michael A. A1 - Pikovskij, Arkadij T1 - Chimeras and complex cluster states in arrays of spin-torque oscillators JF - Scientific reports N2 - We consider synchronization properties of arrays of spin-torque nano-oscillators coupled via an RC load. We show that while the fully synchronized state of identical oscillators may be locally stable in some parameter range, this synchrony is not globally attracting. Instead, regimes of different levels of compositional complexity are observed. These include chimera states (a part of the array forms a cluster while other units are desynchronized), clustered chimeras (several clusters plus desynchronized oscillators), cluster state (all oscillators form several clusters), and partial synchronization (no clusters but a nonvanishing mean field). Dynamically, these states are also complex, demonstrating irregular and close to quasiperiodic modulation. Remarkably, when heterogeneity of spin-torque oscillators is taken into account, dynamical complexity even increases: close to the onset of a macroscopic mean field, the dynamics of this field is rather irregular. Y1 - 2017 U6 - https://doi.org/10.1038/s41598-017-04918-9 SN - 2045-2322 VL - 7 PB - Macmillan Publishers Limited CY - London ER - TY - JOUR A1 - Smirnov, Lev A. A1 - Osipov, Grigory V. A1 - Pikovskij, Arkadij T1 - Chimera patterns in the Kuramoto-Battogtokh model JF - Journal of physics : A, Mathematical and theoretical N2 - Kuramoto and Battogtokh (2002 Nonlinear Phenom. Complex Syst. 5 380) discovered chimera states represented by stable coexisting synchrony and asynchrony domains in a lattice of coupled oscillators. After a reformulation in terms of a local order parameter, the problem can be reduced to partial differential equations. We find uniformly rotating, spatially periodic chimera patterns as solutions of a reversible ordinary differential equation, and demonstrate a plethora of such states. In the limit of neutral coupling they reduce to analytical solutions in the form of one-and two-point chimera patterns as well as localized chimera solitons. Patterns at weakly attracting coupling are characterized by virtue of a perturbative approach. Stability analysis reveals that only the simplest chimeras with one synchronous region are stable. KW - nonlocal coupled oscillators KW - chimera state KW - coarse-grained order parameter KW - Ott-Antonsen reduction KW - perturbation approach KW - linear stability analysis Y1 - 2017 U6 - https://doi.org/10.1088/1751-8121/aa55f1 SN - 1751-8113 SN - 1751-8121 VL - 50 IS - 8 PB - IOP Publ. Ltd. CY - Bristol ER - TY - THES A1 - Leonhardt, Helmar T1 - Chemotaxis, shape and adhesion dynamics of amoeboid cells studied by impedance fluctuations in open and confined spaces T1 - Chemotaxis, Formänderung und Adhäsionsdynamik amöboider Zellen gemessen durch Impedanzfluktuation N2 - Die vorliegende Arbeit befasst sich mit elektrischen Impedanzmessungen von ameoboiden Zellen auf Mikroelektroden. Der Modellorganismus Dictyostelium discoideum zeigt unter der Bedingung des Nahrungsentzugs einen Übergang zum kollektiven Verhalten, bei dem sich chemotaktische Zellen zu einem multizellulären Aggregat zusammenschliessen. Wir zeigen wie Impedanzaufnahmen über die Dynamik der Zell-substrat Adhäsion ein präzises Bild der Phasen der Aggregation liefern. Darüberhinaus zeigen wir zum ersten mal systematische Einzelzellmessungen von Wildtyp-Zellen und vier Mutanten, die sich in der Stärke der Substratadh äsion unterscheiden. Wir zeichneten die projizierte Zellfläche durch Zeitverlaufsmikroskopie auf und fanden eine Korrelation zwischen den quasi-periodischen Oszillationen in der Kinetik der projizierten Fläche - der Zellform-Oszillation - und dem Langzeittrend des Impedanzsignals. Amoeboidale Motilität offenbart sich typischerweise durch einen Zyklus von Membranausstülpung, Substratadhäsion, Vorwärtsziehen des Zellkörpers und Einziehen des hinteren Teils der Zelle. Dieser Motilitätszyklus resultiert in quasi-periodischen Oszillationen der projizierten Zellfläche und der Impedanz. In allen gemessenen Zelllinien wurden für diesen Zyklus ähnliche Periodendauern beobachtet trotz der Unterschiede in der Anhaftungsstärke. Wir beobachteten, dass die Stärke der Zell-substrat Anhaftung die Impedanz stark beeinflusst, indem die Abweichungen vom Mittelwert (die Grösse der Fluktuationen) vergrössert sind bei Zellen, die die vom Zytoskelett generierten Kräfte effektiv auf das Substrat übertragen. Zum Beispiel sind bei talA- Zellen, in welchen das Actin verankernde Protein Talin fehlt, die Fluktuationen stark reduziert. Einzelzellkraft-Spektroskopie und Ergebnisse eines Ablösungsassays, bei dem Adhäsionskraft gemessen wird indem Zellen einer Scherspannung ausgesetzt werden, bestätigen, dass die Grösse der Impedanz-fluktuationen ein korrektes Mass für die Stärke der Substratadhäsion ist. Schliesslich haben wir uns auch mit dem Einbau von Zell-substrat-Impedanz-Sensoren in mikro-fluidische Apparaturen befasst. Ein chip-basierter elektrischer Chemotaxis Assay wurde entwickelt, der die Geschwindigkeit chemotaktischer Zellen misst, welche entlang eines chemischen Konzentrationsgradienten über Mikroelektroden wandern. N2 - We present electrical impedance measurements of amoeboid cells on microelectrodes. The model organism Dictyostelium discoideum shows under starvation conditions a transition to collective behavior when chemotactic cells collect in multicellular aggregates. We show how impedance recordings give a precise picture of the stages of aggregation by tracing the dynamics of cell-substrate adhesion. Furthermore, we present for the first time systematic single cell measurements of wild type cells and four mutant strains that differ in their substrate adhesion strength. We recorded the projected cell area by time lapse microscopy and found a correlation between quasi-periodic oscillations in the kinetics of the projected area - the cell shape oscillation - and the long-term trend in the impedance signal. Typically, amoeboid motility advances via a cycle of membrane protrusion, substrate adhesion, traction of the cell body and tail retraction. This motility cycle results in the quasi-periodic oscillations of the projected cell area and the impedance. In all cell lines measured, similar periods were observed for this cycle, despite the differences in attachment strength. We observed that cell-substrate attachment strength strongly affects the impedance in that the deviations from mean (the magnitude of fluctuations) are enhanced in cells that effectively transmit forces, generated by the cytoskeleton, to the substrate. For example, in talA- cells, which lack the actin anchoring protein talin, the fluctuations are strongly reduced. Single cell force spectroscopy and results from a detachment assay, where adhesion is measured by exposing cells to shear stress, confirm that the magnitude of impedance fluctuations is a correct measure for the strength of substrate adhesion. Finally, we also worked on the integration of cell-substrate impedance sensors into microfluidic devices. A chip-based electrical chemotaxis assay is designed which measures the speed of chemotactic cells migrating over microelectrodes along a chemical concentration gradient. KW - ECIS KW - cell-substrate adhesion KW - cell movement KW - electrical chemotaxis assay KW - ECIS KW - Zell-substrat Adhäsion KW - Zell Bewegung KW - elektrischer Chemotaxis Assy Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-405016 ER - TY - GEN A1 - Alirezaeizanjani, Zahra A1 - Großmann, Robert A1 - Pfeifer, Veronika A1 - Hintsche, Marius A1 - Beta, Carsten T1 - Chemotaxis strategies of bacteria with multiple run modes T2 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - Bacterial chemotaxis-a fundamental example of directional navigation in the living world-is key to many biological processes, including the spreading of bacterial infections. Many bacterial species were recently reported to exhibit several distinct swimming modes-the flagella may, for example, push the cell body or wrap around it. How do the different run modes shape the chemotaxis strategy of a multimode swimmer? Here, we investigate chemotactic motion of the soil bacterium Pseudomonas putida as a model organism. By simultaneously tracking the position of the cell body and the configuration of its flagella, we demonstrate that individual run modes show different chemotactic responses in nutrition gradients and, thus, constitute distinct behavioral states. On the basis of an active particle model, we demonstrate that switching between multiple run states that differ in their speed and responsiveness provides the basis for robust and efficient chemotaxis in complex natural habitats. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1418 KW - instability KW - flagellum KW - exploit KW - time Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-519098 SN - 1866-8372 IS - 22 ER - TY - JOUR A1 - Alirezaeizanjani, Zahra A1 - Großmann, Robert A1 - Pfeifer, Veronika A1 - Hintsche, Marius A1 - Beta, Carsten T1 - Chemotaxis strategies of bacteria with multiple run modes JF - Science advances N2 - Bacterial chemotaxis-a fundamental example of directional navigation in the living world-is key to many biological processes, including the spreading of bacterial infections. Many bacterial species were recently reported to exhibit several distinct swimming modes-the flagella may, for example, push the cell body or wrap around it. How do the different run modes shape the chemotaxis strategy of a multimode swimmer? Here, we investigate chemotactic motion of the soil bacterium Pseudomonas putida as a model organism. By simultaneously tracking the position of the cell body and the configuration of its flagella, we demonstrate that individual run modes show different chemotactic responses in nutrition gradients and, thus, constitute distinct behavioral states. On the basis of an active particle model, we demonstrate that switching between multiple run states that differ in their speed and responsiveness provides the basis for robust and efficient chemotaxis in complex natural habitats. KW - exploit KW - flagellum KW - instability KW - time Y1 - 2020 U6 - https://doi.org/10.1126/sciadv.aaz6153 SN - 2375-2548 VL - 6 IS - 22 PB - American Association for the Advancement of Science CY - Washington ER - TY - THES A1 - Bierbaum, Veronika T1 - Chemomechanical coupling and motor cycles of the molecular motor myosin V T1 - Chemomechanische Kopplung und Motorzyklen für den molekularen Motor Myosin V N2 - In the living cell, the organization of the complex internal structure relies to a large extent on molecular motors. Molecular motors are proteins that are able to convert chemical energy from the hydrolysis of adenosine triphosphate (ATP) into mechanical work. Being about 10 to 100 nanometers in size, the molecules act on a length scale, for which thermal collisions have a considerable impact onto their motion. In this way, they constitute paradigmatic examples of thermodynamic machines out of equilibrium. This study develops a theoretical description for the energy conversion by the molecular motor myosin V, using many different aspects of theoretical physics. Myosin V has been studied extensively in both bulk and single molecule experiments. Its stepping velocity has been characterized as a function of external control parameters such as nucleotide concentration and applied forces. In addition, numerous kinetic rates involved in the enzymatic reaction of the molecule have been determined. For forces that exceed the stall force of the motor, myosin V exhibits a 'ratcheting' behaviour: For loads in the direction of forward stepping, the velocity depends on the concentration of ATP, while for backward loads there is no such influence. Based on the chemical states of the motor, we construct a general network theory that incorporates experimental observations about the stepping behaviour of myosin V. The motor's motion is captured through the network description supplemented by a Markov process to describe the motor dynamics. This approach has the advantage of directly addressing the chemical kinetics of the molecule, and treating the mechanical and chemical processes on equal grounds. We utilize constraints arising from nonequilibrium thermodynamics to determine motor parameters and demonstrate that the motor behaviour is governed by several chemomechanical motor cycles. In addition, we investigate the functional dependence of stepping rates on force by deducing the motor's response to external loads via an appropriate Fokker-Planck equation. For substall forces, the dominant pathway of the motor network is profoundly different from the one for superstall forces, which leads to a stepping behaviour that is in agreement with the experimental observations. The extension of our analysis to Markov processes with absorbing boundaries allows for the calculation of the motor's dwell time distributions. These reveal aspects of the coordination of the motor's heads and contain direct information about the backsteps of the motor. Our theory provides a unified description for the myosin V motor as studied in single motor experiments. N2 - Die hier vorgelegte Arbeit entwickelt unter Verwendung vieler verschiedener Aspekte der statistischen Physik eine Theorie der chemomechanischen Kopplung für den Energieumsatz des molekularen Motors Myosin V. Das Myosin V ist sowohl in chemokinetischen wie in Einzelmolekülexperimenten grundlegend untersucht worden. Seine Schrittgeschwindigkeit ist in Abhängigkeit verschiedener externer Parameter, wie der Nukleotidkonzentration und einer äußeren Kraft, experimentell bestimmt. Darüber hinaus ist eine große Anzahl verschiedener chemokinetischer Raten, die an der enzymatischen Reaktion des Moleküls beteiligt sind, quantitativ erfasst. Unter der Wirkung externer Kräfte, die seine Anhaltekraft überschreiten, verhält sich der Motor wie eine Ratsche: Für Kräfte, die entlang der Schrittbewegung des Motors wirken, hängt seine Geschwindigkeit von der ATP-Konzentration ab, für rückwärts angreifende Kräfte jedoch ist die Bewegung des Motors unabhängig von ATP. Auf der Grundlage der chemischen Zustände des Motors wird eine Netzwerktheorie aufgebaut, die die experimentellen Beobachtungen des Schrittverhaltens für Myosin V einschließt. Diese Netzwerkbeschreibung dient als Grundlage für einen Markovprozess, der die Dynamik des Motors beschreibt. Die Verwendung diskreter Zustände bietet den Vorteil der direkten Erfassung der chemischen Kinetik des Moleküls. Darüber hinaus werden chemische und mechanische Eigenschaften des Motors in gleichem Maße im Modell berücksichtigt. Durch die Erfassung der Enzymkinetik mittels eines stochastischen Prozesses lässt sich die Motordynamik mit Hilfe des stationären Zustands der Netzwerkdarstellung beschreiben. Um diesen zu bestimmen, verwenden wir eine graphentheoretische Methode, die auf Kirchhoff zurückgreift. Wir zeigen in Einklang mit den Gesetzen der Thermodynamik für Nichtgleichgewichtssysteme, dass das Schrittverhalten des Motors von mehreren chemomechanischen Zyklen beeinflusst wird. Weiterhin untersuchen wir das funktionale Verhalten mechanischer Schrittraten in Abhängigkeit der äußeren Kraft unter Verwendung einer geeigneten Fokker-Planck-Gleichung. Hierfür wird auf die Theorie einer kontinuierlichen Beschreibung von molekularen Methoden zurückgegriffen. Wir berechnen Größen wie die mittlere Schrittgeschwindigkeit, das Verhältnis von Vorwärts- und Rückwärtsschritten, und die Lauflänge des Motors in Abhängigkeit einer äußeren angreifenden Kraft sowie der Nukleotidkonzentration, und vergleichen diese mit experimentellen Daten. Für Kräfte, die kleiner als die Anhaltekraft des Motors sind, unterscheidet sich der chemomechanische Zyklus grundlegend von demjenigen, der für große Kräfte dominiert. Diese Eigenschaft resultiert in einem Schrittverhalten, das mit den experimentellen Beobachtungen übereinstimmt. Es ermöglicht weiterhin die Zerlegung des Netzwerks in einzelne Zyklen, die die Bewegung des Motors für verschiedene Bereiche externer Kräfte erfassen. Durch die Erweiterung unseres Modells auf Markovprozesse mit absorbierenden Zuständen können so die Wartezeitenverteilungen für einzelne Zyklen des Motors analytisch berechnet werden. Sie erteilen Aufschluss über die Koordination des Motors und enthalten zudem direkte Informationen über seine Rückwärtsschritte, die experimentell nicht erfasst sind. Für das gesamte Netzwerk werden die Wartezeitenverteilungen mit Hilfe eines Gillespie-Algorithmus bestimmt. Unsere Theorie liefert eine einheitliche Beschreibung der Eigenschaften von Myosin V, die in Einzelmolekülexperimenten erfasst werden können. KW - statistische Physik KW - Markov-Prozesse KW - molekulare Motoren KW - statistical physics KW - markov processes KW - molecular motors Y1 - 2011 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-53614 ER - TY - JOUR A1 - Fox, Andrew J. A1 - Barger, Kathleen A. A1 - Wakker, Bart P. A1 - Richter, Philipp A1 - Antwi-Danso, Jacqueline A1 - Casetti-Dinescu, Dana I. A1 - Howk, J. Christopher A1 - Lehner, Nicolas A1 - Crowther, Paul A. A1 - Lockman, Felix J. T1 - Chemical Abundances in the Leading Arm of the Magellanic Stream JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The Leading Arm (LA) of the Magellanic Stream is a vast debris field of H I clouds connecting the Milky Way and the Magellanic Clouds. It represents an example of active gas accretion onto the Galaxy. Previously, only one chemical abundance measurement had been made in the LA. Here we present chemical abundance measurements using Hubble Space Telescope/Cosmic Origins Spectrograph and Green Bank Telescope spectra of four AGN sightlines passing through the LA and three nearby sightlines that may trace outer fragments of the LA. We find low oxygen abundances, ranging from 4.0+(2.0)(2.0)% 12.6(4.1)(6.0)% solar, in the confirmed LA directions, with the lowest values found in the region known as LA III, farthest from the LMC. These abundances are substantially lower than the single previous measurement, S/H = 35 +/- 7% solar, but are in agreement with those reported in the SMC filament of the trailing Stream, supporting a common origin in the SMC (not the LMC) for the majority of the LA and trailing Stream. This provides important constraints for models of the formation of the Magellanic System. Finally, two of the three nearby sightlines show high-velocity clouds with H I columns, kinematics, and oxygen abundances consistent with LA membership. This suggests that the LA is larger than traditionally thought, extending at least 20 degrees further to the Galactic northwest. KW - Galaxy: evolution KW - Galaxy: halo KW - ISM: abundances KW - Magellanic Clouds KW - quasars: absorption lines Y1 - 2018 U6 - https://doi.org/10.3847/1538-4357/aaa9bb SN - 0004-637X SN - 1538-4357 VL - 854 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - THES A1 - Moreira, André Guérin T1 - Charged systems in bulk and at interfaces N2 - Eine der Faustregeln der Kolloid- und Oberflächenphysik ist, dass die meisten Oberflächen geladen sind, wenn sie mit einem Lösungsmittel, normalerweise Wasser, in Kontakt treten. Dies ist zum Beispiel bei ladungsstabilisierten Kolloidalen Suspensionen der Fall, bei denen die Oberfläche der Kolloidteilchen geladen ist (gewöhnlich mit einer Ladung von mehreren Hunderttausend Elementarladungen), oder bei Monoschichten ionischer Tenside, die auf einer Luft-Wasser Grenzfläche sitzen (wobei die wasserliebenden Kopfgruppen durch die Freisetzung von Gegenionen geladen werden), sowie bei Doppelschichten, die geladene phospholipide enthalten (wie Zellmembranen). In dieser Arbeit betrachten wir einige Modellsysteme, die zwar eine vereinfachte Fassung der Realität darstellen, von denen wir aber dennoch erwarten koennen, dass wir mit ihrer Hilfe einige physikalische Eigenschaften realer geladener Systeme (Kolloide und Elektrolyte) einfangen können. N2 - One of the rules-of-thumb of colloid and surface physics is that most surfaces are charged when in contact with a solvent, usually water. This is the case, for instance, in charge-stabilized colloidal suspensions, where the surface of the colloidal particles are charged (usually with a charge of hundreds to thousands of e, the elementary charge), monolayers of ionic surfactants sitting at an air-water interface (where the water-loving head groups become charged by releasing counterions), or bilayers containing charged phospholipids (as cell membranes). In this work, we look at some model-systems that, although being a simplified version of reality, are expected to capture some of the physical properties of real charged systems (colloids and electrolytes). We initially study the simple double layer, composed by a charged wall in the presence of its counterions. The charges at the wall are smeared out and the dielectric constant is the same everywhere. The Poisson-Boltzmann (PB) approach gives asymptotically exact counterion density profiles around charged objects in the weak-coupling limit of systems with low-valent counterions, surfaces with low charge density and high temperature (or small Bjerrum length). Using Monte Carlo simulations, we obtain the profiles around the charged wall and compare it with both Poisson-Boltzmann (in the low coupling limit) and the novel strong coupling (SC) theory in the opposite limit of high couplings. In the latter limit, the simulations show that the SC leads in fact to asymptotically correct density profiles. We also compare the Monte Carlo data with previously calculated corrections to the Poisson-Boltzmann theory. We also discuss in detail the methods used to perform the computer simulations. After studying the simple double layer in detail, we introduce a dielectric jump at the charged wall and investigate its effect on the counterion density distribution. As we will show, the Poisson-Boltzmann description of the double layer remains a good approximation at low coupling values, while the strong coupling theory is shown to lead to the correct density profiles close to the wall (and at all couplings). For very large couplings, only systems where the difference between the dielectric constants of the wall and of the solvent is small are shown to be well described by SC. Another experimentally relevant modification to the simple double layer is to make the charges at the plane discrete. The counterions are still assumed to be point-like, but we constraint the distance of approach between ions in the plane and counterions to a minimum distance D. The ratio between D and the distance between neighboring ions in the plane is, as we will see, one of the important quantities in determining the influence of the discrete nature of the charges at the wall over the density profiles. Another parameter that plays an important role, as in the previous case, is the coupling as we will demonstrate, systems with higher coupling are more subject to discretization effects than systems with low coupling parameter. After studying the isolated double layer, we look at the interaction between two double layers. The system is composed by two equally charged walls at distance d, with the counterions confined between them. The charge at the walls is smeared out and the dielectric constant is the same everywhere. Using Monte-Carlo simulations we obtain the inter-plate pressure in the global parameter space, and the pressure is shown to be negative (attraction) at certain conditions. The simulations also show that the equilibrium plate separation (where the pressure changes from attractive to repulsive) exhibits a novel unbinding transition. We compare the Monte Carlo results with the strong-coupling theory, which is shown to describe well the bound states of systems with moderate and high couplings. The regime where the two walls are very close to each other is also shown to be well described by the SC theory. Finally, Using a field-theoretic approach, we derive the exact low-density ("virial") expansion of a binary mixture of positively and negatively charged hard spheres (two-component hard-core plasma, TCPHC). The free energy obtained is valid for systems where the diameters d_+ and d_- and the charge valences q_+ and q_- of positive and negative ions are unconstrained, i.e., the same expression can be used to treat dilute salt solutions (where typically d_+ ~ d_- and q_+ ~ q_-) as well as colloidal suspensions (where the difference in size and valence between macroions and counterions can be very large). We also discuss some applications of our results. KW - geladene Systeme KW - elektrische Doppelschicht KW - Elektrolyte KW - Theorie KW - Simulation KW - charged systems KW - electric double layer KW - electrolytes KW - theory KW - simulation Y1 - 2001 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-0000677 ER - TY - THES A1 - Boroudjerdi, Hoda T1 - Charged polymer-macroion complexes T1 - Geladene Polymer-Makroionen Komplexe N2 - This work explores the equilibrium structure and thermodynamic phase behavior of complexes formed by charged polymer chains (polyelectrolytes) and oppositely charged spheres (macroions). Polyelectrolyte-macroion complexes form a common pattern in soft-matter physics, chemistry and biology, and enter in numerous technological applications as well. From a fundamental point of view, such complexes are interesting in that they combine the subtle interplay between electrostatic interactions and elastic as well as entropic effects due to conformational changes of the polymer chain, giving rise to a wide range of structural properties. This forms the central theme of theoretical studies presented in this thesis, which concentrate on a number of different problems involving strongly coupled complexes, i.e. complexes that are characterized by a large adsorption energy and small chain fluctuations. In the first part, a global analysis of the structural phase behavior of a single polyelectrolyte-macroion complex is presented based on a dimensionless representation, yielding results that cover a wide range of realistic system parameters. Emphasize is made on the interplay between the effects due to the polyelectrolytes chain length, salt concentration and the macroion charge as well as the mechanical chain persistence length. The results are summarized into generic phase diagrams characterizing the wrapping-dewrapping behavior of a polyelectrolyte chain on a macroion. A fully wrapped chain state is typically obtained at intermediate salt concentrations and chain lengths, where the amount of polyelectrolyte charge adsorbed on the macroion typically exceeds the bare macroion charge leading thus to a highly overcharged complex. Perhaps the most striking features occur when a single long polyelectrolyte chain is complexed with many oppositely charged spheres. In biology, such complexes form between DNA (which carries the cell's genetic information) and small oppositely charged histone proteins serving as an efficient mechanism for packing a huge amount of DNA into the micron-size cell nucleus in eucaryotic cells. The resultant complex fiber, known as the chromatin fiber, appears with a diameter of 30~nm under physiological conditions. Recent experiments indicate a zig-zag spatial arrangement for individual DNA-histone complexes (nucleosome core particles) along the chromatin fiber. A numerical method is introduced in this thesis based on a simple generic chain-sphere cell model that enables one to investigate the mechanism of fiber formation on a systematic level by incorporating electrostatic and elastic contributions. As will be shown, stable complex fibers exhibit an impressive variety of structures including zig-zag, solenoidal and beads-on-a-string patterns, depending on system parameters such as salt concentration, sphere charge as well as the chain contour length (per sphere). The present results predict fibers of compact zig-zag structure within the physiologically relevant regime with a diameter of about 30~nm, when DNA-histone parameters are adopted. In the next part, a numerical method is developed in order to investigate the role of thermal fluctuations on the structure and thermodynamic phase behavior of polyelectrolyte-macroion complexes. This is based on a saddle-point approximation, which allows to describe the experimentally observed reaction (or complexation) equilibrium in a dilute solution of polyelectrolytes and macroions on a systematic level. This equilibrium is determined by the entropy loss a single polyelectrolyte chain suffers as it binds to an oppositely charged macroion. This latter quantity can be calculated from the spectrum of polyelectrolyte fluctuations around a macroion, which is determined by means of a normal-mode analysis. Thereby, a stability phase diagram is obtained, which exhibits qualitative agreement with experimental findings. At elevated complex concentrations, one needs to account for the inter-complex interactions as well. It will be shown that at small separations, complexes undergo structural changes in such a way that positive patches from one complex match up with negative patches on the other. Furthermore, one of the polyelectrolyte chains may bridge between the two complexes. These mechanisms lead to a strong inter-complex attraction. As a result, the second virial coefficient associated with the inter-complex interaction becomes negative at intermediate salt concentrations in qualitative agreement with recent experiments on solutions of nucleosome core particles. N2 - In dieser Arbeit werden Gleichgewichtsstrukturen und die thermodynamischen Phasen von Komplexen aus geladenen Polymeren (Polyelektrolyten) und entgegengesetzt geladenen Kugeln (Makroionen) untersucht. Polyelektrolyt-Makroion-Komplexe bilden ein grundlegendes und wiederkehrendes Prinzip in der Physik weicher Materie sowie in Chemie und Biologie. In zahlreichen technologischen Prozessen finden sich ebenfalls Anwendungsbeispiele für derartige Komplexe. Zusätzlich zu ihrem häufigen Auftreten sind sie aufgrund ihrer Vielfalt von strukturellen Eigenschaften von grundlegendem Interesse. Diese Vielfalt wird durch ein Zusammenspiel von elektrostatischen Wechselwirkungen sowie elastischen und entropischen Effekten aufgrund von Konformationsänderungen in der Polymerkette bedingt und bildet das zentrale Thema der theoretischen Studien, die mit dieser Arbeit vorgelegt werden. Verschiedene Strukturen und Prozesse, die stark gekoppelte Komplexe beinhalten - das sind solche, für die eine hohe Adsorptionsenergie und geringe Fluktuationen in den Polymerketten charakteristisch sind -, bilden das Hauptthema der Arbeit. Basierend auf einer dimensionslosen Darstellung wird im ersten Teil der Arbeit in einer umfassenden Analyse das strukturelle Phasenverhalten einzelner Polyelektrolyt-Makroion-Komplexe behandelt. Der Schwerpunkt wird hier auf das Wechselspiel zwischen Effekten aufgrund der Polyelektrolytkettenlänge, ihrer mechanischen Persistenzlänge, der Salzkonzentration und der Ladung des Makroions gelegt. Die Ergebnisse werden in allgemeinen Phasendiagrammen zusammengestellt, das das Aufwickeln-Abwickeln-Verhalten der Polyelektrolytkette auf einem Makroion beschreibt. Ein Zustand mit komplett aufgewickelter Kette tritt typischerweise bei mittleren Salzkonzentrationen und Kettenlängen auf; häufig ist hier die auf dem Makroion adsorbierte Gesamtladung des Polyelektrolyts größ er als die Ladung des nackten Makroions, d.h. es findet in hohem Grad Ladungsinversion statt. Äußerst bemerkenswerte Eigenschaften treten auf, wenn eine einzelne lange Polyelektrolytkette viele, ihr entgegengesetzt geladene Kugeln komplexiert. In biologischen Systemen findet man solche Komplexe zwischen DNS, die die genetische Information einer Zelle trägt, und kleinen, entgegengesetzt geladenen Histonproteinen. Diese Komplexe dienen als effizienter Mechanismus, die groß e Menge an DNS im Mikrometer-groß en Zellkern eukaryotischer Zellen zu komprimieren. Die dadurch erhaltene komplexe Faser, eine Chromatinfaser, hat unter physiologischen Bedingungen einen Durchmesser von nur etwa 30~nm. Neue Experimente haben gezeigt, dass eine räumliche Zickzack-Anordnung einzelner DNA-Histon-Komplexe entlang der Chromatinfaser vorliegt. In der hier vorgelegten Arbeit wird eine numerische Methode vorgestellt, die auf einem einfachen Ketten-Kugel-Zell-Modell basiert und die die systematische Untersuchung des Mechnismus zur Faserbildung ermöglicht, wobei sowohl elektrostatische als auch elastische Wechselwirkungen berücksichtigt werden. Es wird gezeigt, dass stabile Komplexfasern in Abhängigkeit von der Salzkonzentration, der Kugelladung und der Kettenkonturlänge eine Vielfalt von Strukturen aufweisen, darunter Zickzack-, Solenoid- und Perlenkettenformen. Für physiologisch relevante Bedingungen werden mit dieser Methode für DNA-Histon-Komplexe Fasern kompakter Zickzack-Struktur mit einem Durchmesser von etwa 30~nm erhalten. Im folgenden Teil wird eine numerische Methode entwickelt, um den Einfluss thermischer Fluktuationen auf Struktur und thermodynamisches Phasenverhalten der Polyelektrolyt-Makroion-Komplexe zu untersuchen. Basierend auf der Sattelpunktsnäherung werden die experimentell beobachteten Reaktionsgleichgewichte in verdünnten Lösungen von Polyelektrolyten und Makroionen systematisch beschrieben. Das Gleichgewicht ist durch einen Verlust an Entropie für die einzelne Polyelektrolytkette durch die Bindung an das entgegengesetzt geladene Makroion gekennzeichnet. Diese Größ e wurde aus dem Spektrum der Polyelektrolytfluktuationen um das Makroion erhalten und mittels einer Analyse der Normalmoden berechnet. Hierüber wird ein Phasendiagramm zur Stabilität der Komplexe erhalten, das qualitativ gute Übereinstimmungen mit experimentellen Ergebnissen aufweist. Bei höheren Komplexkonzentrationen müssen auch die Wechselwirkungen zwischen den Komplexen berücksichtigt werden. Es wird gezeigt, dass sich die Struktur der Komplexe bei kleinen Abständen so ändert, dass positiv geladene Bereiche eines Komplexes mit negativ geladenen auf einem Nachbarkomplex räumlich korrelieren. Weiterhin können einzelne Polyelektrolytketten als verbrückendes Element zwischen zwei Komplexen dienen. Dieser Mechanismus führt zu starker effektiver Anziehung zwischen den Komplexen. In Übereinstimmung mit kürzlich durchgeführten Experimenten ist als Folge davon der zweite Virialkoeffizient der Wechselwirkung zwischen Komplexen bei mittleren Salzkonzentrationen negativ. KW - Biopolymere KW - Histon-DNS-Komplex KW - DNS-Bindungsproteine KW - DNS KW - Chromatin KW - Kolloides System KW - Kolloidphysik KW - Polyelektrolyt KW - geladene Systeme KW - Theorie KW - Polyelektrolytkomplexe KW - Chromatin KW - Histone-DNA Complexes KW - Charged Systems KW - Polyelectrolyte Complexes Y1 - 2005 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-6282 ER - TY - THES A1 - Camacho González, Francisco T1 - Charge-Storage mechanisms in polymer electrets T1 - Ladungsspeicherung Mechanismen in Polymerelektreten N2 - In view of the importance of charge storage in polymer electrets for electromechanical transducer applications, the aim of this work is to contribute to the understanding of the charge-retention mechanisms. Furthermore, we will try to explain how the long-term storage of charge carriers in polymeric electrets works and to identify the probable trap sites. Charge trapping and de-trapping processes were investigated in order to obtain evidence of the trap sites in polymeric electrets. The charge de-trapping behavior of two particular polymer electrets was studied by means of thermal and optical techniques. In order to obtain evidence of trapping or de-trapping, charge and dipole profiles in the thickness direction were also monitored. In this work, the study was performed on polyethylene terephthalate (PETP) and on cyclic-olefin copolymers (COCs). PETP is a photo-electret and contains a net dipole moment that is located in the carbonyl group (C = O). The electret behavior of PETP arises from both the dipole orientation and the charge storage. In contrast to PETP, COCs are not photo-electrets and do not exhibit a net dipole moment. The electret behavior of COCs arises from the storage of charges only. COC samples were doped with dyes in order to probe their internal electric field. COCs show shallow charge traps at 0.6 and 0.11 eV, characteristic for thermally activated processes. In addition, deep charge traps are present at 4 eV, characteristic for optically stimulated processes. PETP films exhibit a photo-current transient with a maximum that depends on the temperature with an activation energy of 0.106 eV. The pair thermalization length (rc) calculated from this activation energy for the photo-carrier generation in PETP was estimated to be approx. 4.5 nm. The generated photo-charge carriers can recombine, interact with the trapped charge, escape through the electrodes or occupy an empty trap. PETP possesses a small quasi-static pyroelectric coefficient (QPC): ~0.6 nC/(m²K) for unpoled samples, ~60 nC/(m²K) for poled samples and ~60 nC/(m²K) for unpoled samples under an electric bias (E ~10 V/µm). When stored charges generate an internal electric field of approx. 10 V/µm, they are able to induce a QPC comparable to that of the oriented dipoles. Moreover, we observe charge-dipole interaction. Since the raw data of the QPC-experiments on PETP samples is noisy, a numerical Fourier-filtering procedure was applied. Simulations show that the data analysis is reliable when the noise level is up to 3 times larger than the calculated pyroelectric current for the QPC. PETP films revealed shallow traps at approx. 0.36 eV during thermally-stimulated current measurements. These energy traps are associated with molecular dipole relaxations (C = O). On the other hand, photo-activated measurements yield deep charge traps at 4.1 and 5.2 eV. The observed wavelengths belong to the transitions in PETP that are analogous to the π - π* benzene transitions. The observed charge de-trapping selectivity in the photocharge decay indicates that the charge detrapping is from a direct photon-charge interaction. Additionally, the charge de-trapping can be facilitated by photo-exciton generation and the interaction of the photo-excitons with trapped charge carriers. These results indicate that the benzene rings (C6H4) and the dipolar groups (C = O) can stabilize and share an extra charge carrier in a chemical resonance. In this way, this charge could be de-trapped in connection with the photo-transitions of the benzene ring and with the dipole relaxations. The thermally-activated charge release shows a difference in the trap depth to its optical counterpart. This difference indicates that the trap levels depend on the de-trapping process and on the chemical nature of the trap site. That is, the processes of charge detrapping from shallow traps are related to secondary forces. The processes of charge de-trapping from deep traps are related to primary forces. Furthermore, the presence of deep trap levels causes the stability of the charge for long periods of time. N2 - Angesichts der Bedeutung der Ladungsspeicherung in Polymerelektreten für viele Anwendungen, wie z.B. in elektromechanischen Wandler, ist es das Ziel dieser Arbeit, zum Verständnis der zugrundeliegenden Mechanismen der kurz- und langfristigen Ladungsstabilisierung beizutragen sowie mögliche Haftstellen zu identifizieren. Ladungs- und Entladungsprozesse in Elektreten geben Hinweise auf Ladungshaftstellen. Diese Prozesse wurden mit thermischen und optischen Methoden bei gleichzeitiger Messung von Ladungs- und Polarisationprofilen untersucht. Die experimentellen Untersuchungen der vorliegenden Arbeit wurden an Polyethylenterephthalat (PETP) und an Cyclischen-Olefin Copolymeren (COC) durchgeführt. PETP ist ein Photoelektret und weist in der Carbonylgruppe (C = O) ein Dipolmoment auf. Die Elektreteigenschaften ergeben sich sowohl aus der Orientierungspolarisation als auch aus der Ladungsspeicherung. Im Gegensatz zu PETP ist COC kein Photoelektret und zeigt auch keine Orientierungspolarisation. Deshalb folgen die Elektreteigenschaften des COC ausschließlich aus der Ladungsspeicherung. Die COC-Proben wurden mit Farbstoffen dotiert, um das innere elektrische Feld zu untersuchen. Diese Systeme zeigen flache Ladungshaftstellen bei 0,6 und 0,11 eV, die durch thermisch stimulierte Prozesse entladen werden sowie tiefe Haftstellen bei 4 eV, die optisch stimuliert werden können. PETP-Filme zeigen einen transienten Photostrom mit einem Maximalwert ( jp), der von der Temperatur mit einer Aktivierungsenergie von 0,106 eV abhängt. Der thermische Paarabstand (rc) kann für die Photoladungsgeneration in PETP auf ca. 4,5 nm abgeschätzt werden. Die Photoladungsträger können rekombinieren, mit den gespeicherten Ladungen interagieren, über die Elektroden entkommen oder eine leere Haftstelle einnehmen. PETP zeigt einen kleinen quasi-statischen pyroelektrischen Koeffizienten (QPC) von ca. 0,6 nC/(m²K) für nicht polarisierte Proben, ca. 60 nC/(m²K) für polarisierte Proben und ca. 60 nC/(m²K) für nicht polarisierte Proben mit Vorspannung (E ~10 V/µm). Wenn die gespeicherten Ladungen ein internes elektrisches Feld von ca. 10 V/µm generieren können, sind sie in der Lage, einen QPC herbeizuführen, der vergleichbar mit dem von orientierten Dipolen ist. Es ist außerdem möglich, eine Ladungs-Dipol-Wechselwirkung zu beobachten. Da die QPM-Daten von PETP auf Grund des geringen Signals verrauscht sind, wurde ein numerisches Fourier-Filterverfahren angewandt. Simulationen zeigen, dass eine zuverlässige Datenanalyse noch bei einem Signal möglich ist, dessen Rauschen bis zu 3-mal größer ist als der berechnete pyroelektrische Strom. Messungen der thermisch stimulierten Entladung von PETP-Filmen ergaben flache Haftstellen bei ca. 0,36 eV, welche mit der Dipolrelaxation der Carbonylgruppe (C = O) assoziiert sind. Messungen der photostimulierten Entladung ergaben tiefe Haftstellen bei 4,1 und 5,2 eV. Die beobachteten Wellenlängen entsprechen Übergängen in PETP analog den π - π* Übergängen in Benzol. Die beobachtete Selektivität bei der photostimulierten Entladung lässt auf eine direkte Wechselwirkung von Photonen und Ladungen schließen. Einen zusätzlichen Einfluß auf die Entladung hat die Erzeugung von Photo-Exzitonen und deren Wechselwirkung mit den gespeicherten Ladungsträgern. Diese Ergebnisse deuten darauf hin, dass die Phenylringe (C6H4) und die Dipolgruppen (C = O) eine zusätzliche Ladung in einer chemischen Resonanz stabilisieren und miteinander teilen können. Daher kann die gebundene Ladung auch durch einen Photoübergang im Benzolring oder durch eine Dipolrelaxation freigesetzt werden. Die mittels thermisch stimulierter Entladung bestimmte Tiefe der Haftstellen unterscheidet sich deutlich von den mittels photostimulierter Entladung gemessenen Werten. Flachere Haftstellen werden bei der thermisch stimulierten Entladung gefunden und können sekundären Kräften zugeordnet werden. Die tieferen Haftstellen sind chemischer Natur und können primären Kräften zugeordnet werden. Letztere sind für die Langzeitstabilität der Ladung in Polymerelektreten verantwortlich. KW - Charge-Storage KW - polymer-electret KW - photo-stimulated discharge KW - polyethylene terephthalate KW - cyclic-olefin copolymer KW - charge-dipole interaction KW - thermo-stimulated discharge KW - thermo-luminescence KW - trap-depth KW - charge profiling KW - charge storage KW - charge trap Y1 - 2006 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-8756 ER - TY - JOUR A1 - Koopman, Wouter-Willem Adriaan A1 - Natali, Marco A1 - Donati, Giovanni P. A1 - Muccini, Michele A1 - Toffanin, Stefano T1 - Charge-exciton interaction rate in organic field-effect transistors by means of transient photoluminescence electromodulated spectroscopy JF - ACS photonics N2 - Organic light-emitting transistors (OLETs) offer a huge potential for the design of highly integrated multifunctional optoelectronic systems and of intense nano scale light sources, such as the long-searched-for electrically pumped organic laser. In order to fulfill these promises, the efficiency and brightness of the current state-of-the-art devices have to be increased significantly. The dominating quenching process limiting the external quantum efficiency in OLETs is charge-exciton interaction. A comprehensive understanding of this quenching process is therefore of paramount importance. The present article reports a systematic investigation of charge-exciton interaction in organic transistors employing time resolved photoluminescence electro-modulation (PLEM) spectroscopy on the picosecond time scale. The results show that the injected charges reduce the exciton radiative recombination in two ways: (i) charges may prevent the generation of excitons and (ii) charges activate a further nonradiative channel for the exciton decay. Moreover, the transient PLEM measurements clearly reveal that not only trapped charges, as already reported in literature, but rather the entire injected charge density contributes to the quenching of the exciton population. KW - photoluminescence quenching KW - charge density KW - exciton dynamics KW - organic KW - field-effect transistor KW - light emission KW - optical spectroscopy Y1 - 2017 U6 - https://doi.org/10.1021/acsphotonics.6b00573 SN - 2330-4022 VL - 4 IS - 2 SP - 282 EP - 291 PB - American Chemical Society CY - Washington, DC ER - TY - JOUR A1 - Le Corre, Vincent M. A1 - Stolterfoht, Martin A1 - Perdigón-Toro, Lorena A1 - Feuerstein, Markus A1 - Wolff, Christian Michael A1 - Gil-Escrig, Lidon A1 - Bolink, Henk J. A1 - Neher, Dieter A1 - Koster, L. Jan Anton T1 - Charge Transport Layers Limiting the Efficiency of Perovskite Solar Cells: How To Optimize Conductivity, Doping, and Thickness JF - ACS Applied Energy Materials N2 - Perovskite solar cells (PSCs) are one of the main research topics of the photovoltaic community; with efficiencies now reaching up to 24%, PSCs are on the way to catching up with classical inorganic solar cells. However, PSCs have not yet reached their full potential. In fact, their efficiency is still limited by nonradiative recombination, mainly via trap-states and by losses due to the poor transport properties of the commonly used transport layers (TLs). Indeed, state-of-the-art TLs (especially if organic) suffer from rather low mobilities, typically within 10(-5) and 10(-2) cm(-2) V-1 s(-1), when compared to the high mobilities, 1-10 cm(-2) V-1 s(-1), measured for perovskites. This work presents a comprehensive analysis of the effect of the mobility, thickness, and doping density of the transport layers based on combined experimental and modeling results of two sets of devices made of a solution-processed high-performing triple-cation (PCE approximate to 20%). The results are also cross-checked on vacuum-processed MAPbI(3) devices. From this analysis, general guidelines on how to optimize a TL are introduced and especially a new and simple formula to easily calculate the amount of doping necessary to counterbalance the low mobility of the TLs. KW - perovskite solar cells KW - transport layers KW - conductivity KW - doping KW - charge transport Y1 - 2019 U6 - https://doi.org/10.1021/acsaem.9b00856 SN - 2574-0962 VL - 2 IS - 9 SP - 6280 EP - 6287 PB - American Chemical Society CY - Washington ER - TY - THES A1 - Mellinger, Axel T1 - Charge storage in electret polymers: mechanisms, characterization and applications T1 - Ladungsspeicherung in Elektret-Polymeren: Mechanismen, Charakterisierung und Anwendungen N2 - Electrets are materials capable of storing oriented dipoles or an electric surplus charge for long periods of time. The term "electret" was coined by Oliver Heaviside in analogy to the well-known word "magnet". Initially regarded as a mere scientific curiosity, electrets became increasingly imporant for applications during the second half of the 20th century. The most famous example is the electret condenser microphone, developed in 1962 by Sessler and West. Today, these devices are produced in annual quantities of more than 1 billion, and have become indispensable in modern communications technology. Even though space-charge electrets are widely used in transducer applications, relatively little was known about the microscopic mechanisms of charge storage. It was generally accepted that the surplus charges are stored in some form of physical or chemical traps. However, trap depths of less than 2 eV, obtained via thermally stimulated discharge experiments, conflicted with the observed lifetimes (extrapolations of experimental data yielded more than 100000 years). Using a combination of photostimulated discharge spectroscopy and simultaneous depth-profiling of the space-charge density, the present work shows for the first time that at least part of the space charge in, e.g., polytetrafluoroethylene, polypropylene and polyethylene terephthalate is stored in traps with depths of up to 6 eV, indicating major local structural changes. Based on this information, more efficient charge-storing materials could be developed in the future. The new experimental results could only be obtained after several techniques for characterizing the electrical, electromechanical and electrical properties of electrets had been enhanced with in situ capability. For instance, real-time information on space-charge depth-profiles were obtained by subjecting a polymer film to short laser-induced heat pulses. The high data acquisition speed of this technique also allowed the three-dimensional mapping of polarization and space-charge distributions. A highly active field of research is the development of piezoelectric sensor films from electret polymer foams. These materials store charges on the inner surfaces of the voids after having been subjected to a corona discharge, and exhibit piezoelectric properties far superior to those of traditional ferroelectric polymers. By means of dielectric resonance spectroscopy, polypropylene foams (presently the most widely used ferroelectret) were studied with respect to their thermal and UV stability. Their limited thermal stability renders them unsuitable for applications above 50 °C. Using a solvent-based foaming technique, we found an alternative material based on amorphous Teflon® AF, which exhibits a stable piezoelectric coefficient of 600 pC/N at temperatures up to 120 °C. N2 - Elektrete sind Materialien, welche orientierte elektrische Dipole oder eine elektrische Überschussladung über längere Zeit speichern können. Der Begriff wurde 1885 von Oliver Heaviside in Anlehnung an das Wort "Magnet" eingeführt. Zunächst nur als wissenschaftliche Kuriosität betrachtet, wurden sie seit Mitte des 20. Jahrhunderts in zunehmendem Maße für technische Anwendungen interessant. Als bekanntestes Beispiel sei hier das 1962 von Sessler und West entwickelte Elektret-Kondensator-Mikrofon erwähnt, welches in jährlichen Stückzahlen von mehr als 1 Milliarde hergestellt wird und aus der modernen Kommunikationstechnik nicht mehr wegzudenken ist. Trotz der weit verbreiteten Anwendungen in der Sensorik war bisher nur wenig über die mikroskopischen Mechanismen der Ladungsspeicherung bekannt. Allgemein wird davon ausgegangen, dass die Überschussladungen in physikalischen oder chemischen Haftstellen gespeichert sind. Bisherige Experimente zur thermisch stimulierten Entladung ergaben Bindungsenergien unterhalb von 2 eV, was im Widerspruch zu den beobachteten Lebensdauern (extrapoliert wurden Werte von mehr als 100000 Jahren) steht. Mittels photostimulierter Entladung sowie simultaner Messung des Ladungsprofils konnte nun für eine Reihe wichtiger Elektret-Polymere (darunter das unter dem Handelsnamen Teflon® bekannte Polytetrafluorethylen, Polypropylen und Polyethylenterephthalat) erstmals gezeigt werden, dass zumindest ein Teil der Ladungen in tiefen Haftstellen von bis zu 6 eV gespeichert wird, was auf eine tiefgreifende lokale Strukturänderung hinweist. Ausgehend von dieser Information könnten in Zukunft Materialien mit verbesserter Ladungsspeicherung gezielt entwickelt werden. Die neuen Messungen waren erst möglich, nachdem mehrere Verfahren zur Bestimmung elektrischer, elektromechanischer und mechanischer Eigenschaften von Elektreten für einen In Situ-Einsatz weiterentwickelt wurden. So konnten z. B. durch Anregung von kurzen Wärmepulsen in der Polymerfolie Informationen über das Tiefenprofil der Raumladung in Echtzeit gewonnen werden. Die schnelle Abtastung ermöglichte darüber hinaus die dreidimensionale Kartierung von Polarisationsprofilen und Raumladungen. Ein zur Zeit sehr aktives Forschungsgebiet ist die Entwicklung piezoelektrischer Sensorfolien aus geschäumten Elektret-Polymeren. Nach elektrischer Aufladung in einer Korona-Entladung werden Ladungen an der Innenseite der Gasbläschen gespeichert, wodurch das Material piezoelektrische Eigenschaften erhält, welche deutlich besser sind als die der herkömmlichen ferroelektrischen Polymere. Für die bisher gebräuchlichen Polypropylenschäume wurde neben der Temperaturstabilität mittels dielektrischer Resonanzspektroskopie auch das Verhalten unter UV-Bestrahlung untersucht. Aufgrund ihrer beschränkten thermischen Stabilität sind diese Schäume nicht für Anwendungen oberhalb von 50 °C geeignet. Mittels eines Lösungsmittel-basierten Schäumungsverfahrens wurde ein alternativer Werkstoff auf der Basis von amorphem Teflon® AF entwickelt, welcher einen stabilen piezoelektrischen Koeffizienten von 600 pC/N bei Temperaturen von bis zu 120 °C aufweist. KW - Elektret KW - Elektretfolie KW - Polymere / Physik KW - Fluorpolymere KW - Dielektrikum KW - Ladungsspeicherung KW - Ferroelektret KW - electret KW - charge storage KW - dielectrics KW - ferroelectret Y1 - 2004 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-5689 ER - TY - JOUR A1 - Ran, Niva A. A1 - Love, John A. A1 - Heiber, Michael C. A1 - Jiao, Xuechen A1 - Hughes, Michael P. A1 - Karki, Akchheta A1 - Wang, Ming A1 - Brus, Viktor V. A1 - Wang, Hengbin A1 - Neher, Dieter A1 - Ade, Harald A1 - Bazan, Guillermo C. A1 - Thuc-Quyen Nguyen, T1 - Charge generation and recombination in an organic solar cell with low energetic offsets JF - dvanced energy materials N2 - Organic bulk heterojunction (BHJ) solar cells require energetic offsets between the donor and acceptor to obtain high short-circuit currents (J(SC)) and fill factors (FF). However, it is necessary to reduce the energetic offsets to achieve high open-circuit voltages (V-OC). Recently, reports have highlighted BHJ blends that are pushing at the accepted limits of energetic offsets necessary for high efficiency. Unfortunately, most of these BHJs have modest FF values. How the energetic offset impacts the solar cell characteristics thus remains poorly understood. Here, a comprehensive characterization of the losses in a polymer:fullerene BHJ blend, PIPCP:phenyl-C61-butyric acid methyl ester (PC61BM), that achieves a high V-OC (0.9 V) with very low energy losses (E-loss = 0.52 eV) from the energy of absorbed photons, a respectable J(SC) (13 mA cm(-2)), but a limited FF (54%) is reported. Despite the low energetic offset, the system does not suffer from field-dependent generation and instead it is characterized by very fast nongeminate recombination and the presence of shallow traps. The charge-carrier losses are attributed to suboptimal morphology due to high miscibility between PIPCP and PC61BM. These results hold promise that given the appropriate morphology, the J(SC), V-OC, and FF can all be improved, even with very low energetic offsets. KW - energetic offset KW - fill factor KW - morphology KW - organic solar cells KW - recombination Y1 - 2018 U6 - https://doi.org/10.1002/aenm.201701073 SN - 1614-6832 SN - 1614-6840 VL - 8 IS - 5 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Roland, Steffen A1 - Yan, Liang A1 - Zhang, Qianqian A1 - Jiao, Xuechen A1 - Hunt, Adrian A1 - Ghasemi, Masoud A1 - Ade, Harald A1 - You, Wei A1 - Neher, Dieter T1 - Charge Generation and Mobility-Limited Performance of Bulk Heterojunction Solar Cells with a Higher Adduct Fullerene JF - The journal of physical chemistry : C, Nanomaterials and interfaces N2 - Alternative electron acceptors are being actively explored in order to advance the development of bulk-heterojunction (BHJ) organic solar cells (OSCs). The indene-C-60 bisadduct (ICBA) has been regarded as a promising candidate, as it provides high open-circuit voltage in BHJ solar cells; however, the photovoltaic performance of such ICBA-based devices is often inferior when compared to cells with the omnipresent PCBM electron acceptor. Here, by pairing the high performance polymer (FTAZ) as the donor with either PCBM or ICBA as the acceptor, we explore the physical mechanism behind the reduced performance of the ICBA-based device. Time delayed collection field (TDCF) experiments reveal reduced, yet field-independent free charge generation in the FTAZ:ICBA system, explaining the overall lower photocurrent in its cells. Through the analysis of the photoluminescence, photogeneration, and electroluminescence, we find that the lower generation efficiency is neither caused by inefficient exciton splitting, nor do we find evidence for significant energy back-transfer from the CT state to singlet excitons. In fact, the increase in open circuit voltage when replacing PCBM by ICBA is entirely caused by the increase in the CT energy, related to the shift in the LUMO energy, while changes in the radiative and nonradiative recombination losses are nearly absent. On the other hand, space charge limited current (SCLC) and bias-assisted charge extraction (BACE) measurements consistently reveal a severely lower electron mobilitiy in the FTAZ:ICBA blend. Studies of the blends with resonant soft X-ray scattering (R-SoXS), grazing incident wide-angle X-ray scattering (GIWAXS), and scanning transmission X-ray microscopy (STXM) reveal very little differences in the mesoscopic morphology but significantly less nanoscale molecular ordering of the fullerene domains in the ICBA based blends, which we propose as the main cause for the lower generation efficiency and smaller electron mobility. Calculations of the JV curves with an analytical model, using measured values, show good agreement with the experimentally determined JV characteristics, proving that these devices suffer from slow carrier extraction, resulting in significant bimolecular recombination losses. Therefore, this study highlights the importance of high charge carrier mobility for newly synthesized acceptor materials, in addition to having suitable energy levels. Y1 - 2017 U6 - https://doi.org/10.1021/acs.jpcc.7b02288 SN - 1932-7447 VL - 121 SP - 10305 EP - 10316 PB - American Chemical Society CY - Washington ER - TY - THES A1 - Roland, Steffen T1 - Charge carrier recombination and open circuit voltage in organic solar cells T1 - Ladungsträger Rekombination und Leerlaufspannung in organischen Solarzellen BT - from bilayer-model systems to hybrid multi-junctions BT - von Bilayer Modellsystemen zu hybriden Mehrschichtsolarzellen N2 - Tremendous progress in the development of thin film solar cell techniques has been made over the last decade. The field of organic solar cells is constantly developing, new material classes like Perowskite solar cells are emerging and different types of hybrid organic/inorganic material combinations are being investigated for their physical properties and their applicability in thin film electronics. Besides typical single-junction architectures for solar cells, multi-junction concepts are also being investigated as they enable the overcoming of theoretical limitations of a single-junction. In multi-junction devices each sub-cell operates in different wavelength regimes and should exhibit optimized band-gap energies. It is exactly this tunability of the band-gap energy that renders organic solar cell materials interesting candidates for multi-junction applications. Nevertheless, only few attempts have been made to combine inorganic and organic solar cells in series connected multi-junction architectures. Even though a great diversity of organic solar cells exists nowadays, their open circuit voltage is usually low compared to the band-gap of the active layer. Hence, organic low band-gap solar cells in particular show low open circuit voltages and the key factors that determine the voltage losses are not yet fully understood. Besides open circuit voltage losses the recombination of charges in organic solar cells is also a prevailing research topic, especially with respect to the influence of trap states. The exploratory focus of this work is therefore set, on the one hand, on the development of hybrid organic/inorganic multi-junctions and, on the other hand, on gaining a deeper understanding of the open circuit voltage and the recombination processes of organic solar cells. In the first part of this thesis, the development of a hybrid organic/inorganic triple-junction will be discussed which showed at that time (Jan. 2015) a record power conversion efficiency of 11.7%. The inorganic sub-cells of these devices consist of hydrogenated amorphous silicon and were delivered by the Competence Center Thin-Film and Nanotechnology for Photovoltaics in Berlin. Different recombination contacts and organic sub-cells were tested in conjunction with these inorganic sub-cells on the basis of optical modeling predictions for the optimal layer thicknesses to finally reach record efficiencies for this type of solar cells. In the second part, organic model systems will be investigated to gain a better understanding of the fundamental loss mechanisms that limit the open circuit voltage of organic solar cells. First, bilayer systems with different orientation of the donor and acceptor molecules were investigated to study the influence of the donor/acceptor orientation on non-radiative voltage loss. Secondly, three different bulk heterojunction solar cells all comprising the same amount of fluorination and the same polymer backbone in the donor component were examined to study the influence of long range electrostatics on the open circuit voltage. Thirdly, the device performance of two bulk heterojunction solar cells was compared which consisted of the same donor polymer but used different fullerene acceptor molecules. By this means, the influence of changing the energetics of the acceptor component on the open circuit voltage was investigated and a full analysis of the charge carrier dynamics was presented to unravel the reasons for the worse performance of the solar cell with the higher open circuit voltage. In the third part, a new recombination model for organic solar cells will be introduced and its applicability shown for a typical low band-gap cell. This model sheds new light on the recombination process in organic solar cells in a broader context as it re-evaluates the recombination pathway of charge carriers in devices which show the presence of trap states. Thereby it addresses a current research topic and helps to resolve alleged discrepancies which can arise from the interpretation of data derived by different measurement techniques. N2 - In der Photovoltaikforschung spielen neuartige Dünnschichtsolarzellen eine immer größere Rolle. Neben innovativen Design und Anwendungskonzepten sind Material und Kostenreduzierung in der Herstellung die größten Triebfedern für die Entwicklung neuer Technologien. Hier sind neben den vielversprechenden Perowskitsolarzellen insbesondere organische Solarzellen zu nennen, die sich durch ihre chemische Vielseitigkeit, einfache Verarbeitung und stetige Weiterentwicklung in Bezug auf ihre Effizienz auszeichnen. Diese Vielseitigkeit ermöglicht die Herstellung organischer Solarzellen mit unterschiedlicher spektraler Empfindlichkeit, was wiederum Vorteile für den Einsatz in seriengeschaltete Mehrschichtsolarzellen bietet. Diese erlauben es, fundamentale Limitierungen von Einschichtsolarzellsystemen zu überwinden. Der erste Teil dieser Arbeit befasst sich daher mit der Entwicklung einer neuartigen hybriden Multischichtsolarzelle, die sowohl aus anorganischen als auch organischen Subzellen besteht und zum Zeitpunkt ihrer Veröffentlichung einen neuen Effizienzrekord für diese Klasse von Solarzellen aufzeigte. Der zweite Teil der Arbeit befasst sich mit fundamentalen physikalischen Prozessen in organischen Solarzellen, da viele Funktionsmechanismen noch nicht im Detail geklärt sind. An verschiedenen organischen Modellsolarzellsystemen wurde daher unter anderem der Einfluss molekularer Orientierung von Donor- und Akzeptorkomponenten der Solarzelle oder der Einfluss von Fluorinierung des Donors auf die Leerlaufspannung der Solarzelle untersucht. Auf diese Weise konnten neue wichtige Erkenntnisse über den Einfluss von verschiedenen Verlustkanälen und der Energetik auf die Leerlaufspannung gewonnen werden. Der letzte Teil der Arbeit widmet sich der Entwicklung eines neuen Modells, welches den Rekombinationsprozess von Ladungen in einer bestimmten organischen Solarzelle beschreibt. Dieses neue Modell wurde anhand umfangreicher Experimente validiert und ermöglicht es, insbesondere den Einfluss freier und in sogenannten Fallenzuständen gefangener Ladungen auf die Rekombination zu trennen. Damit hat dieses Modell eine weitreichende Bedeutung, zum einen für die Beurteilung von typischen Rekombinationsexperimenten in organischen Solarzellen und zum anderen für die Bewertung des Einflusses von Fallenzuständen auf den Rekombinationsstrom. KW - organic solar cells KW - charge carrier recombination KW - open circuit voltage KW - hybrid multi-junction solar cell KW - organische Solarzellen KW - Ladungsträgerrekombination KW - Leerlaufspannung KW - hybride Mehrschichtsolarzellen Y1 - 2017 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-397721 ER -