TY - GEN A1 - Ślęzak, Jakub A1 - Metzler, Ralf A1 - Magdziarz, Marcin T1 - Superstatistical generalised Langevin equation BT - non-Gaussian viscoelastic anomalous diffusion N2 - Recent advances in single particle tracking and supercomputing techniques demonstrate the emergence of normal or anomalous, viscoelastic diffusion in conjunction with non-Gaussian distributions in soft, biological, and active matter systems. We here formulate a stochastic model based on a generalised Langevin equation in which non-Gaussian shapes of the probability density function and normal or anomalous diffusion have a common origin, namely a random parametrisation of the stochastic force. We perform a detailed analysis demonstrating how various types of parameter distributions for the memory kernel result in exponential, power law, or power-log law tails of the memory functions. The studied system is also shown to exhibit a further unusual property: the velocity has a Gaussian one point probability density but non-Gaussian joint distributions. This behaviour is reflected in the relaxation from a Gaussian to a non-Gaussian distribution observed for the position variable. We show that our theoretical results are in excellent agreement with stochastic simulations. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 413 KW - anomalous diffusion KW - generalised langevin equation KW - superstatistics KW - non-Gaussian diffusion Y1 - 2018 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-409315 ER - TY - JOUR A1 - Ślęzak, Jakub A1 - Metzler, Ralf A1 - Magdziarz, Marcin T1 - Superstatistical generalised Langevin equation BT - non-Gaussian viscoelastic anomalous diffusion JF - New Journal of Physics N2 - Recent advances in single particle tracking and supercomputing techniques demonstrate the emergence of normal or anomalous, viscoelastic diffusion in conjunction with non-Gaussian distributions in soft, biological, and active matter systems. We here formulate a stochastic model based on a generalised Langevin equation in which non-Gaussian shapes of the probability density function and normal or anomalous diffusion have a common origin, namely a random parametrisation of the stochastic force. We perform a detailed analysis demonstrating how various types of parameter distributions for the memory kernel result in exponential, power law, or power-log law tails of the memory functions. The studied system is also shown to exhibit a further unusual property: the velocity has a Gaussian one point probability density but non-Gaussian joint distributions. This behaviour is reflected in the relaxation from a Gaussian to a non-Gaussian distribution observed for the position variable. We show that our theoretical results are in excellent agreement with stochastic simulations. KW - anomalous diffusion KW - generalised langevin equation KW - superstatistics KW - non-Gaussian diffusion Y1 - 2018 U6 - https://doi.org/10.1088/1367-2630/aaa3d4 SN - 1367-2630 VL - 20 IS - 023026 SP - 1 EP - 25 PB - Deutsche Physikalische Gesellschaft / Institute of Physics CY - Bad Honnef und London ER - TY - GEN A1 - Ślęzak, Jakub A1 - Burnecki, Krzysztof A1 - Metzler, Ralf T1 - Random coefficient autoregressive processes describe Brownian yet non-Gaussian diffusion in heterogeneous systems T2 - Postprints der Universität Potsdam Mathematisch-Naturwissenschaftliche Reihe N2 - Many studies on biological and soft matter systems report the joint presence of a linear mean-squared displacement and a non-Gaussian probability density exhibiting, for instance, exponential or stretched-Gaussian tails. This phenomenon is ascribed to the heterogeneity of the medium and is captured by random parameter models such as ‘superstatistics’ or ‘diffusing diffusivity’. Independently, scientists working in the area of time series analysis and statistics have studied a class of discrete-time processes with similar properties, namely, random coefficient autoregressive models. In this work we try to reconcile these two approaches and thus provide a bridge between physical stochastic processes and autoregressive models.Westart from the basic Langevin equation of motion with time-varying damping or diffusion coefficients and establish the link to random coefficient autoregressive processes. By exploring that link we gain access to efficient statistical methods which can help to identify data exhibiting Brownian yet non-Gaussian diffusion. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 765 KW - diffusion KW - Langevin equation KW - Brownian yet non-Gaussian diffusion KW - diffusing diffusivity KW - superstatistics KW - autoregressive models KW - time series analysis KW - codifference Y1 - 2019 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-437923 SN - 1866-8372 IS - 765 ER - TY - JOUR A1 - Ślęzak, Jakub A1 - Burnecki, Krzysztof A1 - Metzler, Ralf T1 - Random coefficient autoregressive processes describe Brownian yet non-Gaussian diffusion in heterogeneous systems JF - New Journal of Physics N2 - Many studies on biological and soft matter systems report the joint presence of a linear mean-squared displacement and a non-Gaussian probability density exhibiting, for instance, exponential or stretched-Gaussian tails. This phenomenon is ascribed to the heterogeneity of the medium and is captured by random parameter models such as ‘superstatistics’ or ‘diffusing diffusivity’. Independently, scientists working in the area of time series analysis and statistics have studied a class of discrete-time processes with similar properties, namely, random coefficient autoregressive models. In this work we try to reconcile these two approaches and thus provide a bridge between physical stochastic processes and autoregressive models.Westart from the basic Langevin equation of motion with time-varying damping or diffusion coefficients and establish the link to random coefficient autoregressive processes. By exploring that link we gain access to efficient statistical methods which can help to identify data exhibiting Brownian yet non-Gaussian diffusion. KW - diffusion KW - Langevin equation KW - Brownian yet non-Gaussian diffusion KW - diffusing diffusivity KW - superstatistics KW - autoregressive models KW - time series analysis KW - codifference Y1 - 2019 U6 - https://doi.org/10.1088/1367-2630/ab3366 SN - 1367-2630 VL - 21 PB - Deutsche Physikalische Gesellschaft ; IOP, Institute of Physics CY - Bad Honnef und London ER - TY - THES A1 - Önel, Hakan T1 - Electron acceleration in a flare plasma via coronal circuits T1 - Elektronenbeschleunigung im Flareplasma modelliert mit koronalen Schaltkreisen N2 - The Sun is a star, which due to its proximity has a tremendous influence on Earth. Since its very first days mankind tried to "understand the Sun", and especially in the 20th century science has uncovered many of the Sun's secrets by using high resolution observations and describing the Sun by means of models. As an active star the Sun's activity, as expressed in its magnetic cycle, is closely related to the sunspot numbers. Flares play a special role, because they release large energies on very short time scales. They are correlated with enhanced electromagnetic emissions all over the spectrum. Furthermore, flares are sources of energetic particles. Hard X-ray observations (e.g., by NASA's RHESSI spacecraft) reveal that a large fraction of the energy released during a flare is transferred into the kinetic energy of electrons. However the mechanism that accelerates a large number of electrons to high energies (beyond 20 keV) within fractions of a second is not understood yet. The thesis at hand presents a model for the generation of energetic electrons during flares that explains the electron acceleration based on real parameters obtained by real ground and space based observations. According to this model photospheric plasma flows build up electric potentials in the active regions in the photosphere. Usually these electric potentials are associated with electric currents closed within the photosphere. However as a result of magnetic reconnection, a magnetic connection between the regions of different magnetic polarity on the photosphere can establish through the corona. Due to the significantly higher electric conductivity in the corona, the photospheric electric power supply can be closed via the corona. Subsequently a high electric current is formed, which leads to the generation of hard X-ray radiation in the dense chromosphere. The previously described idea is modelled and investigated by means of electric circuits. For this the microscopic plasma parameters, the magnetic field geometry and hard X-ray observations are used to obtain parameters for modelling macroscopic electric components, such as electric resistors, which are connected with each other. This model demonstrates that such a coronal electric current is correlated with large scale electric fields, which can accelerate the electrons quickly up to relativistic energies. The results of these calculations are encouraging. The electron fluxes predicted by the model are in agreement with the electron fluxes deduced from the measured photon fluxes. Additionally the model developed in this thesis proposes a new way to understand the observed double footpoint hard X-ray sources. N2 - Die Sonne ist ein Stern, der aufgrund seiner räumlichen Nähe einen großen Einfluss auf die Erde hat. Seit jeher hat die Menschheit versucht die "Sonne zu verstehen" und besonders im 20. Jahrhundert gelang es der Wissenschaft viele der offenen Fragen mittels Beobachtungen zu beantworten und mit Modellen zu beschreiben. Die Sonne ist ein aktiver Stern, dessen Aktivität sich in seinem magnetischen Zyklus ausdrückt, welcher in enger Verbindung zu den Sonnenfleckenzahlen steht. Flares spielen dabei eine besondere Rolle, da sie hohe Energien auf kurzen Zeitskalen freisetzen. Sie werden begleitet von erhöhter Strahlungsemission über das gesamte Spektrum hinweg und setzen darüber hinaus auch energetische Teilchen frei. Beobachtungen von harter Röntgenstrahlung (z.B. mit der RHESSI Raumsonde der NASA) zeigen, dass ein großer Teil der freigesetzten Energie in die kinetische Energie von Elektronen transferiert wird. Allerdings ist nach wie vor nicht verstanden, wie die Beschleunigung der vielen Elektronen auf hohe Energien (jenseits von 20 keV) in Bruchteilen einer Sekunde erfolgt. Die vorliegende Arbeit präsentiert ein Model für die Erzeugung von energetischen Elektronen während solarer Flares, das auf mit realen Beobachtungen gewonnenen Parametern basiert. Danach bauen photosphärische Plasmaströmungen elektrische Spannungen in den aktiven Regionen der Photosphäre auf. Für gewöhnlich sind diese Potentiale mit elektrischen Strömen verbunden, die innerhalb der Photosphäre geschlossen sind. Allerdings kann infolge von magnetischer Rekonnektion eine magnetische Verbindung in der Korona aufgebaut werden, die die Regionen von magnetisch unterschiedlicher Polarität miteinander verbindet. Wegen der deutlich höheren koronalen elektrischen Leitfähigkeit, kann darauf die photosphärische Spannungsquelle über die Korona geschlossen werden. Das auf diese Weise generierte elektrische Feld führt nachfolgend zur Erzeugung eines hohen elektrischen Stromes, der in der dichten Chromosphäre harte Röntgenstrahlung generiert. Die zuvor erläuterte Idee wird mit elektrischen Schaltkreisen modelliert und untersucht. Dafür werden die mikroskopischen Plasmaparameter, die Geometrie des Magnetfeldes und Beobachtungen der harten Röntgenstrahlung verwendet, um makroskopische elektronische Komponenten, wie z.B. elektrische Widerstände zu modellieren und miteinander zu verbinden. Es wird gezeigt, dass der auftretende koronale Strom mit hohen elektrischen Feldern verbunden ist, welche Elektronen schnell auf hohe relativistische Energien beschleunigen können. Die Ergebnisse dieser Berechnungen sind ermutigend. Die vorhergesagten Elektronenflüsse stehen im Einklang mit aus gemessenen Photonenflüssen gewonnenen Elektronenflüssen. Zudem liefert das Model einen neuen Ansatz für das Verständnis der harten Röntgendoppelquellen in den Fußpunkten. KW - Elektronenbeschleunigung KW - Flarephysik KW - koronale Stromsysteme KW - electron acceleration KW - physics of flares KW - coronal currents Y1 - 2008 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-29035 ER - TY - JOUR A1 - Öhberg, P. A1 - Surkov, E. L. A1 - Tittonen, I. A1 - Stenholm, Stig A1 - Wilkens, Martin A1 - Shlyapnikov, G. V. T1 - Low-energy elementary excitations of a trapped Bose-condensed gas N2 - We develop a method of finding analytical sotutions of the Bogolyubov-de Gennes equations for the excitations of a Bose condensate in the Thomas-Fermi regime in harmonic traps of any asymmetry and introduce a classification of eigenstates. In the case of cylindrical symmetry we emphasize the presence of an accidental degeneracy in the excitation spectrum at certain values of the projection of orbital angular momentum on the symmetry axis and discuss possible consequences of the degeneracy in the context of new signatures of Bose- Einstein condensation Y1 - 1997 ER - TY - JOUR A1 - Zöller, Gert A1 - Hainzl, Sebastian A1 - Kurths, Jürgen T1 - Observation of growing correlation length as an indicator for critical point behavior prior to large earthquakes Y1 - 2001 ER - TY - THES A1 - Zöller, Gert T1 - Critical states of seismicity : modeling and data analysis T1 - Kritische Zustände seismischer Dynamik : Modellierung und Datenanalyse N2 - The occurrence of earthquakes is characterized by a high degree of spatiotemporal complexity. Although numerous patterns, e.g. fore- and aftershock sequences, are well-known, the underlying mechanisms are not observable and thus not understood. Because the recurrence times of large earthquakes are usually decades or centuries, the number of such events in corresponding data sets is too small to draw conclusions with reasonable statistical significance. Therefore, the present study combines both, numerical modeling and analysis of real data in order to unveil the relationships between physical mechanisms and observational quantities. The key hypothesis is the validity of the so-called "critical point concept" for earthquakes, which assumes large earthquakes to occur as phase transitions in a spatially extended many-particle system, similar to percolation models. New concepts are developed to detect critical states in simulated and in natural data sets. The results indicate that important features of seismicity like the frequency-size distribution and the temporal clustering of earthquakes depend on frictional and structural fault parameters. In particular, the degree of quenched spatial disorder (the "roughness") of a fault zone determines whether large earthquakes occur quasiperiodically or more clustered. This illustrates the power of numerical models in order to identify regions in parameter space, which are relevant for natural seismicity. The critical point concept is verified for both, synthetic and natural seismicity, in terms of a critical state which precedes a large earthquake: a gradual roughening of the (unobservable) stress field leads to a scale-free (observable) frequency-size distribution. Furthermore, the growth of the spatial correlation length and the acceleration of the seismic energy release prior to large events is found. The predictive power of these precursors is, however, limited. Instead of forecasting time, location, and magnitude of individual events, a contribution to a broad multiparameter approach is encouraging. N2 - Das Auftreten von Erdbeben zeichnet sich durch eine hohe raumzeitliche Komplexität aus. Obwohl zahlreiche Muster, wie Vor- und Nachbeben bekannt sind, weiß man wenig über die zugrundeliegenden Mechanismen, da diese sich direkter Beobachtung entziehen. Die Zeit zwischen zwei starken Erdbeben in einer seismisch aktiven Region beträgt Jahrzehnte bis Jahrhunderte. Folglich ist die Anzahl solcher Ereignisse in einem Datensatz gering und es ist kaum möglich, allein aus Beobachtungsdaten statistisch signifikante Aussagen über deren Eigenschaften abzuleiten. Die vorliegende Arbeit nutzt daher numerische Modellierungen einer Verwerfungszone in Verbindung mit Datenanalyse, um die Beziehung zwischen physikalischen Mechanismen und beobachteter Seismizität zu studieren. Die zentrale Hypothese ist die Gültigkeit des sogenannten "kritischen Punkt Konzeptes" für Seismizität, d.h. starke Erdbeben werden als Phasenübergänge in einem räumlich ausgedehnten Vielteilchensystem betrachtet, ähnlich wie in Modellen aus der statistischen Physik (z.B. Perkolationsmodelle). Es werden praktische Konzepte entwickelt, die es ermöglichen, kritische Zustände in simulierten und in beobachteten Daten sichtbar zu machen. Die Resultate zeigen, dass wesentliche Eigenschaften von Seismizität, etwa die Magnitudenverteilung und das raumzeitliche Clustern von Erdbeben, durch Reibungs- und Bruchparameter bestimmt werden. Insbesondere der Grad räumlicher Unordnung (die "Rauhheit") einer Verwerfungszone hat Einfluss darauf, ob starke Erdbeben quasiperiodisch oder eher zufällig auftreten. Dieser Befund zeigt auf, wie numerische Modelle genutzt werden können, um den Parameterraum für reale Verwerfungen einzugrenzen. Das kritische Punkt Konzept kann in synthetischer und in beobachteter Seismizität verifiziert werden. Dies artikuliert sich auch in Vorläuferphänomenen vor großen Erdbeben: Die Aufrauhung des (unbeobachtbaren) Spannungsfeldes führt zu einer Skalenfreiheit der (beobachtbaren) Größenverteilung; die räumliche Korrelationslänge wächst und die seismische Energiefreisetzung wird beschleunigt. Ein starkes Erdbeben kann in einem zusammenhängenden Bruch oder in einem unterbrochenen Bruch (Vorbeben und Hauptbeben) stattfinden. Die beobachtbaren Vorläufer besitzen eine begrenzte Prognosekraft für die Auftretenswahrscheinlichkeit starker Erdbeben - eine präzise Vorhersage von Ort, Zeit, und Stärke eines nahenden Erdbebens ist allerdings nicht möglich. Die genannten Parameter erscheinen eher vielversprechend als Beitrag zu einem umfassenden Multiparameteransatz für eine verbesserte zeitabhängige Gefährdungsabschätzung. KW - Seismizität KW - Erdbebenvorhersage KW - statistische Physik KW - mathematische Modellierung KW - Datenanalyse KW - seismicity KW - earthquake prediction KW - statistical physics KW - mathematical modeling KW - data analysis Y1 - 2005 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-7427 ER - TY - THES A1 - Zöller, Gert T1 - Analyse raumzeitlicher Muster in Erdbebendaten N2 - Die vorliegende Arbeit beschäftigt sich mit der Charakterisierung von Seismizität anhand von Erdbebenkatalogen. Es werden neue Verfahren der Datenanalyse entwickelt, die Aufschluss darüber geben sollen, ob der seismischen Dynamik ein stochastischer oder ein deterministischer Prozess zugrunde liegt und was daraus für die Vorhersagbarkeit starker Erdbeben folgt. Es wird gezeigt, dass seismisch aktive Regionen häufig durch nichtlinearen Determinismus gekennzeichent sind. Dies schließt zumindest die Möglichkeit einer Kurzzeitvorhersage ein. Das Auftreten seismischer Ruhe wird häufig als Vorläuferphaenomen für starke Erdbeben gedeutet. Es wird eine neue Methode präsentiert, die eine systematische raumzeitliche Kartierung seismischer Ruhephasen ermöglicht. Die statistische Signifikanz wird mit Hilfe des Konzeptes der Ersatzdaten bestimmt. Als Resultat erhält man deutliche Korrelationen zwischen seismischen Ruheperioden und starken Erdbeben. Gleichwohl ist die Signifikanz dafür nicht hoch genug, um eine Vorhersage im Sinne einer Aussage über den Ort, die Zeit und die Stärke eines zu erwartenden Hauptbebens zu ermöglichen. KW - Erdbeben KW - nichtlineare Dynamik Y1 - 1999 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-0000122 ER - TY - JOUR A1 - Zykov, Vladimir A1 - Bordyugov, Grigory A1 - Lentz, Hartmut A1 - Engel, Harald T1 - Hysteresis phenomenon in the dynamics of spiral waves rotating around a hole N2 - Hysteresis in the pinning-depinning transitions of spiral waves rotating around a hole in a circular shaped two- dimensional excitable medium is studied both by use of the continuation software AUTO and by direct numerical integration of the reaction-diffusion equations for the FitzHugh-Nagumo model. In order to clarify the role of different factors in this phenomenon, a kinematical description is applied. It is found that the hysteresis phenomenon computed for the reaction-diffusion model can be reproduced qualitatively only when a nonlinear eikonal equation (i.e. velocity- curvature relationship) is assumed. However, to obtain quantitative agreement, the dispersion relation has to be taken into account. Y1 - 2010 UR - http://www.sciencedirect.com/science/journal/01672789 U6 - https://doi.org/10.1016/j.physd.2009.07.018 SN - 0167-2789 ER - TY - JOUR A1 - Zurita-Sánchez, Jorge R. A1 - Henkel, Carsten T1 - Lossy electrical transmission lines: Thermal fluctuations and quantization N2 - We present a theoretical framework for the analysis of the statistical properties of thermal fluctuations on a lossy transmission line. A quantization scheme of the electrical signals in the transmission line is formulated. We discuss two applications in detail. Noise spectra at finite temperature for voltage and current are shown to deviate significantly from the Johnson-Nyquist limit, and they depend on the position on the transmission line. We analyze the spontaneous emission, at low temperature, of a Rydberg atom and its resonant enhancement due to vacuum fluctuations in a capacitively coupled transmission line. The theory can also be applied to study the performance of microscale and nanoscale devices, including high-resolution sensors and quantum information processors Y1 - 2006 UR - http://pra.aps.org/pdf/PRA/v73/i6/e063825 U6 - https://doi.org/10.1103/Physreva.73.063825 SN - 1050-2947 ER - TY - JOUR A1 - Zurita-Sanchez, Jorge R. A1 - Henkel, Carsten T1 - Acoustic waves from mechanical impulses due to fluorescence resonant energy (Forster) transfer Blowing a whistle with light JF - epl : a letters journal exploring the frontiers of physics N2 - We present a momentum transfer mechanism mediated by electromagnetic fields that originates in a system of two nearby molecules: one excited (donor D*) and the other in ground state (acceptor A). An intermolecular force related to fluorescence resonant energy or Forster transfer (FRET) arises in the unstable D* A molecular system, which differs from the equilibrium van der Waals interaction. Due to the its finite lifetime, a mechanical impulse is imparted to the relative motion in the system. We analyze the FRET impulse when the molecules are embedded in free space and find that its magnitude can be much greater than the single recoil photon momentum, getting comparable with the thermal momentum (Maxwell-Boltzmann distribution) at room temperature. In addition, we propose that this FRET impulse can be exploited in the generation of acoustic waves inside a film containing layers of donor and acceptor molecules, when a picosecond laser pulse excites the donors. This acoustic transient is distinguishable from that produced by thermal stress due to laser absorption, and may therefore play a role in photoacoustic spectroscopy. The effect can be seen as exciting a vibrating system like a string or organ pipe with light; it may be used as an opto-mechanical transducer. Y1 - 2012 U6 - https://doi.org/10.1209/0295-5075/97/43002 SN - 0295-5075 VL - 97 IS - 4 PB - EDP Sciences CY - Mulhouse ER - TY - JOUR A1 - Zuo, Guangzheng A1 - Shoaee, Safa A1 - Kemerink, Martijn A1 - Neher, Dieter T1 - General rules for the impact of energetic disorder and mobility on nongeminate recombination in phase-separated organic solar cells JF - Physical review applied N2 - State-of-the-art organic solar cells exhibit power conversion efficiencies of 18% and above. These devices benefit from the suppression of free charge recombination with regard to the Langevin limit of charge encounter in a homogeneous medium. It is recognized that the main cause of suppressed free charge recombination is the reformation and resplitting of charge-transfer (CT) states at the interface between donor and acceptor domains. Here, we use kinetic Monte Carlo simulations to understand the interplay between free charge motion and recombination in an energetically disordered phase-separated donor-acceptor blend. We identify conditions for encounter-dominated and resplitting-dominated recombination. In the former regime, recombination is proportional to mobility for all parameters tested and only slightly reduced with respect to the Langevin limit. In contrast, mobility is not the decisive parameter that determines the nongeminate recombination coefficient, k(2), in the latter case, where k2 is a sole function of the morphology, CT and charge-separated (CS) energetics, and CT-state decay properties. Our simulations also show that free charge encounter in the phase-separated disordered blend is determined by the average mobility of all carriers, while CT reformation and resplitting involves mostly states near the transport energy. Therefore, charge encounter is more affected by increased disorder than the resplitting of the CT state. As a consequence, for a given mobility, larger energetic disorder, in combination with a higher hopping rate, is preferred. These findings have implications for the understanding of suppressed recombination in solar cells with nonfullerene acceptors, which are known to exhibit lower energetic disorder than that of fullerenes. Y1 - 2021 U6 - https://doi.org/10.1103/PhysRevApplied.16.034027 SN - 2331-7019 VL - 16 IS - 3 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Zunkovic, Bojan A1 - Prosen, Tomaz T1 - Explicit solution of the Lindblad equation for nearly isotropic boundary driven XY spin 1/2 chain N2 - Explicit solution for the two-point correlation function in a non-equilibrium steady state of a nearly isotropic boundary driven open XY spin 1/2 chain in the Lindblad formulation is provided. A non-equilibrium quantum phase transition from exponentially decaying correlations to long range order is discussed analytically. In the regime of long range order a new phenomenon of correlation resonances is reported, where the correlation response of the system is unusually high for certain discrete values of the external bulk parameter, e.g. the magnetic field. Y1 - 2010 UR - http://iopscience.iop.org/1742-5468/ U6 - https://doi.org/10.1088/1742-5468/2010/08/P08016 SN - 1742-5468 ER - TY - JOUR A1 - Zu, Fengshuo A1 - Wolff, Christian Michael A1 - Ralaiarisoa, Maryline A1 - Amsalem, Patrick A1 - Neher, Dieter A1 - Koch, Norbert T1 - Unraveling the Electronic Properties of Lead Halide Perovskites with Surface Photovoltage in Photoemission Studies JF - ACS applied materials & interfaces N2 - The tremendous success of metal-halide perovskites, especially in the field of photovoltaics, has triggered a substantial number of studies in understanding their optoelectronic properties. However, consensus regarding the electronic properties of these perovskites is lacking due to a huge scatter in the reported key parameters, such as work function (Φ) and valence band maximum (VBM) values. Here, we demonstrate that the surface photovoltage (SPV) is a key phenomenon occurring at the perovskite surfaces that feature a non-negligible density of surface states, which is more the rule than an exception for most materials under study. With ultraviolet photoelectron spectroscopy (UPS) and Kelvin probe, we evidence that even minute UV photon fluxes (500 times lower than that used in typical UPS experiments) are sufficient to induce SPV and shift the perovskite Φ and VBM by several 100 meV compared to dark. By combining UV and visible light, we establish flat band conditions (i.e., compensate the surface-state-induced surface band bending) at the surface of four important perovskites, and find that all are p-type in the bulk, despite a pronounced n-type surface character in the dark. The present findings highlight that SPV effects must be considered in all surface studies to fully understand perovskites’ photophysical properties. KW - lead halide perovskite films KW - ultraviolet photoelectron spectroscopy KW - Kelvin probe KW - surface band bending KW - surface photovoltage KW - surface states Y1 - 2019 U6 - https://doi.org/10.1021/acsami.9b05293 SN - 1944-8244 SN - 1944-8252 VL - 11 IS - 24 SP - 21578 EP - 21583 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Zu, Fengshuo A1 - Warby, Jonathan A1 - Stolterfoht, Martin A1 - Li, Jinzhao A1 - Shin, Dongguen A1 - Unger, Eva A1 - Koch, Norbert T1 - Photoinduced energy-level realignment at interfaces between organic semiconductors and metal-halide perovskites JF - Physical review letters N2 - In contrast to the common conception that the interfacial energy-level alignment is affixed once the interface is formed, we demonstrate that heterojunctions between organic semiconductors and metal-halide perovskites exhibit huge energy-level realignment during photoexcitation. Importantly, the photoinduced level shifts occur in the organic component, including the first molecular layer in direct contact with the perovskite. This is caused by charge-carrier accumulation within the organic semiconductor under illumination and the weak electronic coupling between the junction components. Y1 - 2021 U6 - https://doi.org/10.1103/PhysRevLett.127.246401 SN - 0031-9007 SN - 1079-7114 VL - 127 IS - 24 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Zu, Fengshuo A1 - Schultz, Thorsten A1 - Wolff, Christian Michael A1 - Shin, Dongguen A1 - Frohloff, Lennart A1 - Neher, Dieter A1 - Amsalem, Patrick A1 - Koch, Norbert T1 - Position-locking of volatile reaction products by atmosphere and capping layers slows down photodecomposition of methylammonium lead triiodide perovskite JF - RSC Advances N2 - The remarkable progress of metal halide perovskites in photovoltaics has led to the power conversion efficiency approaching 26%. However, practical applications of perovskite-based solar cells are challenged by the stability issues, of which the most critical one is photo-induced degradation. Bare CH3NH3PbI3 perovskite films are known to decompose rapidly, with methylammonium and iodine as volatile species and residual solid PbI2 and metallic Pb, under vacuum under white light illumination, on the timescale of minutes. We find, in agreement with previous work, that the degradation is non-uniform and proceeds predominantly from the surface, and that illumination under N-2 and ambient air (relative humidity 20%) does not induce substantial degradation even after several hours. Yet, in all cases the release of iodine from the perovskite surface is directly identified by X-ray photoelectron spectroscopy. This goes in hand with a loss of organic cations and the formation of metallic Pb. When CH3NH3PbI3 films are covered with a few nm thick organic capping layer, either charge selective or non-selective, the rapid photodecomposition process under ultrahigh vacuum is reduced by more than one order of magnitude, and becomes similar in timescale to that under N-2 or air. We conclude that the light-induced decomposition reaction of CH3NH3PbI3, leading to volatile methylammonium and iodine, is largely reversible as long as these products are restrained from leaving the surface. This is readily achieved by ambient atmospheric pressure, as well as a thin organic capping layer even under ultrahigh vacuum. In addition to explaining the impact of gas pressure on the stability of this perovskite, our results indicate that covalently "locking" the position of perovskite components at the surface or an interface should enhance the overall photostability. Y1 - 2020 U6 - https://doi.org/10.1039/d0ra03572f SN - 2046-2069 VL - 10 IS - 30 SP - 17534 EP - 17542 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Zou, Yong A1 - Thiel, M. A1 - Romano, Maria Carmen A1 - Kurths, Jürgen A1 - Bi, Q. T1 - Shrimp structure and associated dynamics in parametrically excited oscillators JF - International journal of bifurcation and chaos : in applied sciences and engineering N2 - We investigate the bifurcation structures in a two-dimensional parameter space (PS) of a parametrically excited system with two degrees of freedom both analytically and numerically. By means of the Renyi entropy of second order K-2, which is estimated from recurrence plots, we uncover that regions of chaotic behavior are intermingled with many complex periodic windows, such as shrimp structures in the PS. A detailed numerical analysis shows that, the stable solutions lose stability either via period doubling, or via intermittency when the parameters leave these shrimps in different directions, indicating different bifurcation properties of the boundaries. The shrimps of different sizes offer promising ways to control the dynamics of such a complex system. KW - bifurcation analysis KW - recurrence plot KW - period doubling KW - intermittency Y1 - 2006 U6 - https://doi.org/10.1142/S0218127406016987 SN - 0218-1274 VL - 16 IS - 12 SP - 3567 EP - 3579 PB - World Scientific Publ. Co CY - Singapore ER - TY - THES A1 - Zou, Yong T1 - Exploring recurrences in quasiperiodic systems T1 - Untersuchung des Wiederkehrverhaltens in quasiperiodischen dynamischen Systemen N2 - In this work, some new results to exploit the recurrence properties of quasiperiodic dynamical systems are presented by means of a two dimensional visualization technique, Recurrence Plots(RPs). Quasiperiodicity is the simplest form of dynamics exhibiting nontrivial recurrences, which are common in many nonlinear systems. The concept of recurrence was introduced to study the restricted three body problem and it is very useful for the characterization of nonlinear systems. I have analyzed in detail the recurrence patterns of systems with quasiperiodic dynamics both analytically and numerically. Based on a theoretical analysis, I have proposed a new procedure to distinguish quasiperiodic dynamics from chaos. This algorithm is particular useful in the analysis of short time series. Furthermore, this approach demonstrates to be efficient in recognizing regular and chaotic trajectories of dynamical systems with mixed phase space. Regarding the application to real situations, I have shown the capability and validity of this method by analyzing time series from fluid experiments. N2 - In dieser Arbeit stelle ich neue Resultate vor, welche zeigen, wie man Rekurrenzeigenschaften quasiperiodischer, dynamischer Systeme für eine Datenanalyse ausnutzen kann. Die vorgestellten Algorithmen basieren auf einer zweidimensionalen Darstellungsmethode, den Rekurrenz-Darstellungen. Quasiperiodizität ist die einfachste Dynamik, die nicht-triviale Rekurrenzen zeigt und tritt häufig in nichtlinearen Systemen auf. Nicht-triviale Rekurrenzen wurden im Zusammenhang mit dem eingeschränkten Dreikörper-problem eingeführt. In dieser Arbeit, habe ich mehrere Systeme mit quasiperiodischem Verhalten analytisch untersucht. Die erhaltenen Ergebnisse helfen die Wiederkehreigenschaften dieser Systeme im Detail zu verstehen. Basierend auf den analytischen Resultaten, schlage ich einen neuen Algorithmus vor, mit dessen Hilfe selbst in kurzen Zeitreihen zwischen chaotischem und quasiperiodischem Verhalten unterschieden werden kann. Die vorgeschlagene Methode ist besonders effizient zur Unterscheidung regulärer und chaotischer Trajektorien mischender dynamischer Systeme.Die praktische Anwendbarkeit der vorgeschlagenen Analyseverfahren auf Messdaten, habe ich gezeigt, indem ich erfolgreich Zeitreihen aus fluid-dynamischen Experimenten untersucht habe. KW - Wiederkehrverhalten KW - quasiperiodisches dynamisches System KW - Recurrence Plot KW - recurrence KW - quasiperiodic dynamical systems KW - recurrence plots Y1 - 2007 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-16497 ER - TY - JOUR A1 - Zoller, Peter A1 - Beth, Thomas A1 - Binosi, D. A1 - Blatt, Rainer A1 - Briegel, Hans J. A1 - Bruss, D. A1 - Calarco, Tommaso A1 - Cirac, Juan Ignacio A1 - Deutsch, David A1 - Eisert, Jens A1 - Ekert, Artur A1 - Fabre, Claude A1 - Gisin, Nicolas A1 - Grangiere, P. A1 - Grassl, Markus A1 - Haroche, Serge A1 - Imamoglu, Atac A1 - Karlson, A. A1 - Kempe, Julia A1 - Kouwenhoven, Leo P. A1 - Kröll, S. A1 - Leuchs, Gerd A1 - Lewenstein, Maciej A1 - Loss, Daniel A1 - Lütkenhaus, Norbert A1 - Massar, Serge A1 - Mooij, J. E. A1 - Plenio, Martin Bodo A1 - Polzik, Eugene A1 - Popescu, Sandu A1 - Rempe, Gerhard A1 - Sergienko, Alexander A1 - Suter, David A1 - Twamley, John A1 - Wendin, Göran A1 - Werner, Reinhard F. A1 - Winter, Andreas A1 - Wrachtrup, Jörg A1 - Zeilinger, Anton T1 - Quantum information processing and communication : Strategic report on current status, visions and goals for research in Europe N2 - We present an excerpt of the document "Quantum Information Processing and Communication: Strategic report on current status, visions and goals for research in Europe", which has been recently published in electronic form at the website of FET (the Future and Emerging Technologies Unit of the Directorate General Information Society of the European Commission, http://www.cordis.lu/ist/fet/qipc-sr.htm). This document has been elaborated, following a former suggestion by FET, by a committee of QIPC scientists to provide input towards the European Commission for the preparation of the Seventh Framework Program. Besides being a document addressed to policy makers and funding agencies (both at the European and national level), the document contains a detailed scientific assessment of the state-of-the-art, main research goals, challenges, strengths, weaknesses, visions and perspectives of all the most relevant QIPC sub-fields, that we report here Y1 - 2005 SN - 1434-6060 ER - TY - JOUR A1 - Zoller, Gert A1 - Holschneider, Matthias A1 - Ben-Zion, Yehuda T1 - The role of heterogeneities as a tuning parameter of earthquake dynamics N2 - We investigate the influence of spatial heterogeneities on various aspects of brittle failure and seismicity in a model of a large strike-slip fault. The model dynamics is governed by realistic boundary conditions consisting of constant velocity motion of regions around the fault, static/kinetic friction laws, creep with depth-dependent coefficients, and 3-D elastic stress transfer. The dynamic rupture is approximated on a continuous time scale using a finite stress propagation velocity ("quasidynamic model''). The model produces a "brittle- ductile'' transition at a depth of about 12.5 km, realistic hypocenter distributions, and other features of seismicity compatible with observations. Previous work suggested that the range of size scales in the distribution of strength-stress heterogeneities acts as a tuning parameter of the dynamics. Here we test this hypothesis by performing a systematic parameter-space study with different forms of heterogeneities. In particular, we analyze spatial heterogeneities that can be tuned by a single parameter in two distributions: ( 1) high stress drop barriers in near- vertical directions and ( 2) spatial heterogeneities with fractal properties and variable fractal dimension. The results indicate that the first form of heterogeneities provides an effective means of tuning the behavior while the second does not. In relatively homogeneous cases, the fault self-organizes to large-scale patches and big events are associated with inward failure of individual patches and sequential failures of different patches. The frequency-size event statistics in such cases are compatible with the characteristic earthquake distribution and large events are quasi-periodic in time. In strongly heterogeneous or near-critical cases, the rupture histories are highly discontinuous and consist of complex migration patterns of slip on the fault. In such cases, the frequency-size and temporal statistics follow approximately power-law relations Y1 - 2005 SN - 0033-4553 ER - TY - JOUR A1 - Zoller, Gert A1 - Hainzl, Sebastian A1 - Holschneider, Matthias A1 - Ben-Zion, Yehuda T1 - Aftershocks resulting from creeping sections in a heterogeneous fault N2 - We show that realistic aftershock sequences with space-time characteristics compatible with observations are generated by a model consisting of brittle fault segments separated by creeping zones. The dynamics of the brittle regions is governed by static/kinetic friction, 3D elastic stress transfer and small creep deformation. The creeping parts are characterized by high ongoing creep velocities. These regions store stress during earthquake failures and then release it in the interseismic periods. The resulting postseismic deformation leads to aftershock sequences following the modified Omori law. The ratio of creep coefficients in the brittle and creeping sections determines the duration of the postseismic transients and the exponent p of the modified Omori law Y1 - 2005 SN - 0094-8276 ER - TY - JOUR A1 - Zink, Christof A1 - Werner, Nils A1 - Jechow, Andreas A1 - Heuer, Axel A1 - Menzel, Ralf T1 - Multi-wavelength operation of a single broad area diode laser by spectral beam combining JF - IEEE photonics technology letters N2 - Stabilized multi-wavelength emission from a single emitter broad area diode laser (BAL) is realized by utilizing an external cavity with a spectral beam combining architecture. Self-organized emitters that are equidistantly spaced across the slow axis are enforced by the spatially distributed wavelength selectivity of the external cavity. This resulted in an array like near-field emission although the BAL is physically a single emitter without any epitaxial sub-structuring and only one electrical contact. Each of the self-organized emitters is operated at a different wavelength and the emission is multiplexed into one spatial mode with near-diffraction limited beam quality. With this setup, multi-line emission of 31 individual spectral lines centered around and a total spectral width of 3.6 nm is realized with a 1000 mu m wide BAL just above threshold. To the best of our knowledge, this is the first demonstration of such a self-organization of emitters by optical feedback utilizing a spectral beam combining architecture. KW - Laser resonators KW - semiconductor lasers KW - optical feedback Y1 - 2014 U6 - https://doi.org/10.1109/LPT.2013.2291963 SN - 1041-1135 SN - 1941-0174 VL - 26 IS - 3 SP - 253 EP - 256 PB - Inst. of Electr. and Electronics Engineers CY - Piscataway ER - TY - JOUR A1 - Zink, Christof A1 - Niebuhr, Mario A1 - Jechow, Andreas A1 - Heuer, Axel A1 - Menzel, Ralf T1 - Broad area diode laser with on-chip transverse Bragg grating stabilized in an off-axis external cavity JF - Optics express : the international electronic journal of optics N2 - The emission characteristics of a novel, specially designed broad area diode laser (BAL) with on-chip transversal Bragg resonance (TBR) grating in lateral direction were investigated in an off-axis external cavity setup. The internal TBR grating defines a low loss transversal mode at a specific angle of incidence and a certain wavelength. By providing feedback at this specific angle with an external mirror, it is possible to select this low loss transverse mode and stabilize the BAL. Near diffraction limited emission with an almost single lobed far field pattern could be realized, in contrast to the double lobed far field pattern of similar setups using standard BALs or phase-locked diode laser arrays. Furthermore, we could achieve a narrow bandwidth emission with a simplified setup without external frequency selective elements. (C) 2014 Optical Society of America Y1 - 2014 U6 - https://doi.org/10.1364/OE.22.014108 SN - 1094-4087 VL - 22 IS - 12 SP - 14108 EP - 14113 PB - Optical Society of America CY - Washington ER - TY - JOUR A1 - Zillmer, Rüdiger A1 - Pikovskij, Arkadij T1 - Continuous approach for the random-field Ising chain N2 - We study the random-field Ising chain in the limit of strong exchange coupling. In order to calculate the free energy we apply a continuous Langevin-type approach. This continuous model can be solved exactly, whereupon we are able to locate the crossover between an exponential and a power-law decay of the free energy with increasing coupling strength. In terms of magnetization, this crossover restricts the validity of the linear scaling. The known analytical results for the free energy are recovered in the corresponding limits. The outcomes of numerical computations for the free energy are presented, which confirm the results of the continuous approach. We also discuss the validity of the replica method which we then utilize to investigate the sample-to-sample fluctuations of the finite size free energy Y1 - 2005 ER - TY - JOUR A1 - Zillmer, Rüdiger A1 - Ahlers, Volker A1 - Pikovskij, Arkadij T1 - Scaling of Lyapunov exponents of coupled chaotic systems N2 - We develop a statistical theory of the coupling sensitivity of chaos. The effect was first described by Daido [Prog. Theor. Phys. 72, 853 (1984)]; it appears as a logarithmic singularity in the Lyapunov exponent in coupled chaotic systems at very small couplings. Using a continuous-time stochastic model for the coupled systems we derive a scaling relation for the largest Lyapunov exponent. The singularity is shown to depend on the coupling and the systems' mismatch. Generalizations to the cases of asymmetrical coupling and three interacting oscillators are considered, too. The analytical results are confirmed by numerical simulations. Y1 - 2000 ER - TY - JOUR A1 - Zillmer, Rüdiger A1 - Ahlers, Volker A1 - Pikovskij, Arkadij T1 - Stochastic approach to Lapunov exponents in coupled chaotic systems Y1 - 2000 SN - 3-540-41074-0 ER - TY - THES A1 - Zillmer, Rüdiger T1 - Statistical properties and scaling of the Lyapunov exponents in stochastic systems N2 - Die vorliegende Arbeit umfaßt drei Abhandlungen, welche allgemein mit einer stochastischen Theorie für die Lyapunov-Exponenten befaßt sind. Mit Hilfe dieser Theorie werden universelle Skalengesetze untersucht, die in gekoppelten chaotischen und ungeordneten Systemen auftreten. Zunächst werden zwei zeitkontinuierliche stochastische Modelle für schwach gekoppelte chaotische Systeme eingeführt, um die Skalierung der Lyapunov-Exponenten mit der Kopplungsstärke ('coupling sensitivity of chaos') zu untersuchen. Mit Hilfe des Fokker-Planck-Formalismus werden Skalengesetze hergeleitet, die von Ergebnissen numerischer Simulationen bestätigt werden. Anschließend wird gezeigt, daß 'coupling sensitivity' im Fall gekoppelter ungeordneter Ketten auftritt, wobei der Effekt sich durch ein singuläres Anwachsen der Lokalisierungslänge äußert. Numerische Ergebnisse für gekoppelte Anderson-Modelle werden bekräftigt durch analytische Resultate für gekoppelte raumkontinuierliche Schrödinger-Gleichungen. Das resultierende Skalengesetz für die Lokalisierungslänge ähnelt der Skalierung der Lyapunov-Exponenten gekoppelter chaotischer Systeme. Schließlich wird die Statistik der exponentiellen Wachstumsrate des linearen Oszillators mit parametrischem Rauschen studiert. Es wird gezeigt, daß die Verteilung des zeitabhängigen Lyapunov-Exponenten von der Normalverteilung abweicht. Mittels der verallgemeinerten Lyapunov-Exponenten wird der Parameterbereich bestimmt, in welchem die Abweichungen von der Normalverteilung signifikant sind und Multiskalierung wesentlich wird. N2 - This work incorporates three treatises which are commonly concerned with a stochastic theory of the Lyapunov exponents. With the help of this theory universal scaling laws are investigated which appear in coupled chaotic and disordered systems. First, two continuous-time stochastic models for weakly coupled chaotic systems are introduced to study the scaling of the Lyapunov exponents with the coupling strength (coupling sensitivity of chaos). By means of the the Fokker-Planck formalism scaling relations are derived, which are confirmed by results of numerical simulations. Next, coupling sensitivity is shown to exist for coupled disordered chains, where it appears as a singular increase of the localization length. Numerical findings for coupled Anderson models are confirmed by analytic results for coupled continuous-space Schrödinger equations. The resulting scaling relation of the localization length resembles the scaling of the Lyapunov exponent of coupled chaotic systems. Finally, the statistics of the exponential growth rate of the linear oscillator with parametric noise are studied. It is shown that the distribution of the finite-time Lyapunov exponent deviates from a Gaussian one. By means of the generalized Lyapunov exponents the parameter range is determined where the non-Gaussian part of the distribution is significant and multiscaling becomes essential. KW - Lyapunov-Exponenten KW - Chaos KW - ungeordnete Systeme KW - Lokalisierung KW - stochastische Systeme KW - 'coupling sensitivity' KW - parametrisch erregter Oszillator KW - Lyapunov exponents KW - chaos KW - disordered systems KW - localization KW - stochastic systems KW - coupling sensitivity KW - parametrically excited oscillator Y1 - 2003 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-0001147 ER - TY - JOUR A1 - Zienicke, Egbert A1 - Seehafer, Norbert A1 - Li, B.-W. A1 - Schumacher, Jörg A1 - Politano, H. A1 - Thess, H. T1 - Voltage-driven instability of electrically conducting fluids Y1 - 2003 ER - TY - JOUR A1 - Zienicke, Egbert A1 - Seehafer, Norbert A1 - Feudel, Fred T1 - Bifurcations in two-dimensional Rayleigh-Bénard convection Y1 - 1998 ER - TY - BOOK A1 - Zienicke, Egbert A1 - Seehafer, Norbert A1 - Feudel, Fred T1 - Bifurcations in two-dimensional Rayleigh-Bénard convection T3 - Preprint NLD Y1 - 1997 SN - 1432-2935 VL - 42 PB - Univ. Potsdam CY - Potsdam ER - TY - JOUR A1 - Ziehmann, Christine A1 - Smith, Leonard A. A1 - Kurths, Jürgen T1 - Localized Lyapunov exponents and the prediction of predictability Y1 - 2000 ER - TY - JOUR A1 - Ziehmann, Christine A1 - Smith, L. A1 - Kurths, Jürgen T1 - The bootstrap and lyapunov exponents in deterministic chaos Y1 - 1999 ER - TY - JOUR A1 - Ziegler, A. A1 - Fischer, Thomas M. A1 - Menzel, Henning A1 - Stumpe, Joachim T1 - Photo-Induced modification and photo-orientation in LB-multilayers of thermotropic polymers containing azobenzene side groups Y1 - 1998 ER - TY - THES A1 - Zickfeld, Kirsten T1 - Modeling large-scale singular climate events for integrated assessment N2 - Erkenntnisse aus paläoklimatologischen Studien, theoretischen Betrachtungen und Modellsimulationen deuten darauf hin, dass anthropogene Emissionen von Treibhausgasen und Aerosolen zu großskaligen, singulären Klimaereignissen führen könnten. Diese bezeichnen stark nichtlineare, abrupte Klimaänderungen, mit regionalen bis hin zu globalen Auswirkungen. Ziel dieser Arbeit ist die Entwicklung von Modellen zweier maßgeblicher Komponenten des Klimasystems, die singuläres Verhalten aufweisen könnten: die atlantische thermohaline Zirkulation (THC) und der indische Monsun. Diese Modelle sind so konzipiert, dass sie den Anforderungen der "Integrated Assessment"-Modellierung genügen, d.h., sie sind realistisch, recheneffizient, transparent und flexibel. Das THC-Modell ist ein einfaches, interhemisphärisches Boxmodell, das anhand von Daten kalibriert wird, die mit einem gekoppelten Klimamodell mittlerer Komplexität erzeugt wurden. Das Modell wird durch die globale Mitteltemperatur angetrieben, die mit Hilfe eines linearen Downscaling-Verfahrens in regionale Wärme- und Süßwasserflüsse übersetzt wird. Die Ergebnisse einer Vielzahl von zeitabhängigen Simulationen zeigen, dass das Modell in der Lage ist, maßgebliche Eigenschaften des Verhaltens komplexer Klimamodelle wiederzugeben, wie die Sensitivität bezüglich des Ausmaßes, der regionalen Verteilung und der Rate der Klimaänderung. Der indische Monsun wird anhand eines neuartigen eindimensionalen Boxmodells der tropischen Atmosphäre beschrieben. Dieses enthält Parmetrisierungen der Oberflächen- und Strahlungsflüsse, des hydrologischen Kreislaufs und derHydrologie der Landoberfläche. Trotz des hohen Idealisierungsgrades ist das Modell in der Lage, relevante Aspekte der beobachteten Monsundynamik, wie z.B. den Jahresgang des Niederschlags und das Eintritts- sowie Rückzugsdatum des Sommermonsuns, zufrieden stellend zu simulieren. Außerdem erfasst das Modell die Sensitivitätdes Monsuns bezüglich Änderungen der Treibhausgas- und Aerosolkonzentrationen, die aus komplexeren Modellen bekannt sind. Eine vereinfachte Version des Monsunmodells wird für die Untersuchung des qualitativen Systemverhaltens in Abhängigkeit von Änderungen der Randbedingungen eingesetzt. Das bemerkenswerteste Ergebnis ist das Auftreten einer Sattelknotenbifurkation des Sommermonsuns für kritische Werte der Albedo oder der Sonneneinstrahlung. Darüber hinaus weist das Modell zwei stabile Zustände auf: neben dem niederschlagsreichen Sommermonsun besteht ein Zustand, der sich durch einen schwachen hydrologischen Kreislauf auszeichnet. Das Beachtliche an diesen Ergebnissen ist, dass anthropogene Störungen der plantetaren Albedo, wie Schwefelemissionen und/oder Landnutzungsänderungen, zu einer Destabilisierung des indischen Monsuns führen könnten. Das THC-Boxmodell findet exemplarische Anwendung in einem "Integrated Assessment" von Klimaschutzstrategien. Basierend auf dem konzeptionellen und methodischen Gerüst des Leitplankenansatzes werden Emissionskorridore (d.h. zulässige Spannen an CO2-Emissionen) berechnet, die das Risiko eines THC-Zusammenbruchs begrenzen sowie sozioökonomische Randbedingungen berücksichtigen. Die Ergebnisse zeigen u.a. eine starke Abhängigkeit der Breite der Emissionskorridore von der Klima- und hydrologischen Sensitivität. Für kleine Werte einer oder beider Sensitivitäten liegt der obere Korridorrand bei weit höheren Emissionswerten als jene, die von plausiblen Emissionsszenarien für das 21. Jahrhundert erreicht werden. Für große Werte der Sensitivitäten hingegen, verlassen schon niedrige Emissionsszenarien den Korridor in den frühen Jahrzehnten des 21. Jahrhunderts. Dies impliziert eine Abkehr von den gegenwärtigen Emissionstrends innherhalb der kommenden Jahrzehnte, wenn das Risko eines THC Zusammenbruchs gering gehalten werden soll. Anhand einer Vielzahl von Anwendungen - von Sensitivitäts- über Bifurkationsanalysen hin zu integrierter Modellierung - zeigt diese Arbeit den Wert reduzierter Modelle auf. Die Ergebnisse und die daraus zu ziehenden Schlussfolgerungen liefern einen wertvollen Beitrag zu der wissenschaftlichen und politischen Diskussion bezüglich der Folgen des anthropogenen Klimawandels und der langfristigen Klimaschutzziele. N2 - Concerns have been raised that anthropogenic climate change could lead to large-scale singular climate events, i.e., abrupt nonlinear climate changes with repercussions on regional to global scales. One central goal of this thesis is the development of models of two representative components of the climate system that could exhibit singular behavior: the Atlantic thermohaline circulation (THC) and the Indian monsoon. These models are conceived so as to fulfill the main requirements of integrated assessment modeling, i.e., reliability, computational efficiency, transparency and flexibility. The model of the THC is an interhemispheric four-box model calibrated against data generated with a coupled climate model of intermediate complexity. It is designed to be driven by global mean temperature change which is translated into regional fluxes of heat and freshwater through a linear down-scaling procedure. Results of a large number of transient climate change simulations indicate that the reduced-form THC model is able to emulate key features of the behavior of comprehensive climate models such as the sensitivity of the THC to the amount, regional distribution and rate of change in the heat and freshwater fluxes. The Indian monsoon is described by a novel one-dimensional box model of the tropical atmosphere. It includes representations of the radiative and surface fluxes, the hydrological cycle and surface hydrology. Despite its high degree of idealization, the model satisfactorily captures relevant aspects of the observed monsoon dynamics, such as the annual course of precipitation and the onset and withdrawal of the summer monsoon. Also, the model exhibits the sensitivity to changes in greenhouse gas and sulfate aerosol concentrations that are known from comprehensive models. A simplified version of the monsoon model is employed for the identification of changes in the qualitative system behavior against changes in boundary conditions. The most notable result is that under summer conditions a saddle-node bifurcation occurs at critical values of the planetary albedo or insolation. Furthermore, the system exhibits two stable equilibria: besides the wet summer monsoon, a stable state exists which is characterized by a weak hydrological cycle. These results are remarkable insofar, as they indicate that anthropogenic perturbations of the planetary albedo such as sulfur emissions and/or land-use changes could destabilize the Indian summer monsoon. The reduced-form THC model is employed in an exemplary integrated assessment application. Drawing on the conceptual and methodological framework of the tolerable windows approach, emissions corridors (i.e., admissible ranges of CO2- emissions) are derived that limit the risk of a THC collapse while considering expectations about the socio-economically acceptable pace of emissions reductions. Results indicate, for example, a large dependency of the width of the emissions corridor on climate and hydrological sensitivity: for low values of climate and/or hydrological sensitivity, the corridor boundaries are far from being transgressed by any plausible emissions scenario for the 21st century. In contrast, for high values of both quantities low non-intervention scenarios leave the corridor already in the early decades of the 21st century. This implies that if the risk of a THC collapse is to be kept low, business-as-usual paths would need to be abandoned within the next two decades. All in all, this thesis highlights the value of reduced-form modeling by presenting a number of applications of this class of models, ranging from sensitivity and bifurcation analysis to integrated assessment. The results achieved and conclusions drawn provide a useful contribution to the scientific and policy debate about the consequences of anthropogenic climate change and the long-term goals of climate protection. --- Anmerkung: Die Autorin ist Trägerin des von der Mathematisch-Naturwissenschaftlichen Fakultät der Universität Potsdam vergebenen Michelson-Preises für die beste Promotion des Jahres 2003/2004. KW - Nichtlineare Dynamik KW - 'Reduced-Form' Modellierung KW - Thermohaline Atlantikzirkulation KW - Indischer Monsun KW - Integrated Assessment KW - Nonlinear Dynamics KW - Reduced-Form Modeling KW - Atlantic Thermohaline Circulation KW - Indian Monsoon KW - Integrated Assessment Y1 - 2003 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-0001176 ER - TY - JOUR A1 - Zhuang, Y. A1 - Schelling, Christoph A1 - Stangl, Jochen A1 - Penn, C. A1 - Senz, S. A1 - Schäffler, Friedrich A1 - Roche, T. A1 - Daniel, A. A1 - Grenzer, Jörg A1 - Pietsch, Ullrich A1 - Bauer, Günther T1 - Structural and optical properties of Si/Si{1-x}Ge{x} wires Y1 - 2000 ER - TY - JOUR A1 - Zhuang, Y. A1 - Pietsch, Ullrich A1 - Stangl, Jochen A1 - Holý, Vaclav A1 - Darowski, Nora A1 - Grenzer, Jörg A1 - Zerlauth, S. A1 - Schäffler, F. A1 - Bauer, Günther T1 - In-plane strain and shape analysis of Si/SiGe nanostructures by grazing incidence diffraction Y1 - 2000 ER - TY - JOUR A1 - Zhuang, Y. A1 - Holý, Václav A1 - Stangl, Jochen A1 - Darhuber, A. A1 - Mikulik, P. A1 - Zerlauth, S. A1 - Schäffler, F. A1 - Bauer, Günther A1 - Darowski, Nora A1 - Lübbert, Daniel A1 - Pietsch, Ullrich T1 - Strain relaxation in periodic arrays of Si/SiGe quantum wires determined by coplanar high-resolution x-ray diffrcation and grazing-incidence diffraction Y1 - 1999 ER - TY - JOUR A1 - Zhu, Shigen A1 - Schulz, Burkhard A1 - Bruma, Maria A1 - Brehmer, Ludwig T1 - Comparative study of the thermal properties of related aromatic polyhydrazides and poly(1,3,4-oxadiazole)s Y1 - 1996 UR - http://www3.interscience.wiley.com/journal/5401/home?CRETRY=1&SRETRY=0 U6 - https://doi.org/10.1002/(SICI)1099-1581(199612)7:12<879::AID-PAT600>3.0.CO;2-X SN - 1042-7147 ER - TY - JOUR A1 - Zhu, M. Q. A1 - Armbruster, Dieter A1 - Katzorke, Ines T1 - Does synchronization of networks of chaotic maps lead to control? N2 - We consider networks of chaotic maps with different network topologies. In each case, they are coupled in such a way as to generate synchronized chaotic solutions. By using the methods of control of chaos we are controlling a single map into a predetermined trajectory. We analyze the reaction of the network to such a control. Specifically we show that a line of one-dimensional logistic maps that are unidirectionally coupled can be controlled from the first oscillator whereas a ring of diffusively coupled maps cannot be controlled for more than 5 maps. We show that rings with more elements can be controlled if every third map is controlled. The dependence of unidirectionally coupled maps on noise is studied. The noise level leads to a finite synchronization lengths for which maps can be controlled by a single location. A two-dimensional lattice is also studied. (C) 2005 American Institute of Physics Y1 - 2005 SN - 1054-1500 ER - TY - JOUR A1 - Zhu, Jian A1 - Stoyanov, Hristiyan A1 - Kofod, Guggi A1 - Suo, Zhigang T1 - Large deformation and electromechanical instability of a dielectric elastomer tube actuator N2 - This paper theoretically analyzes a dielectric elastomer tube actuator (DETA). Subject to a voltage difference between the inner and outer surfaces, the actuator reduces in thickness and expands in length, so that the same voltage will induce an even higher electric field. This positive feedback may cause the actuator to thin down drastically, resulting in electrical breakdown. We obtain an analytical solution of the actuator undergoing finite deformation when the elastomer obeys the neo-Hookean model. The critical strain of actuation is calculated in terms of various parameters of design. We also discuss the effect of the strain-stiffening on electromechanical behavior of DETAs by using the model of freely joined links. (C) 2010 American Institute of Physics. [doi:10.1063/1.3490186] Y1 - 2010 UR - http://jap.aip.org/ U6 - https://doi.org/10.1063/1.3490186 SN - 0021-8979 ER - TY - JOUR A1 - Zhu, Jian A1 - Kollosche, Matthias A1 - Lu, Tongqing A1 - Kofod, Guggi A1 - Suo, Zhigang T1 - Two types of transitions to wrinkles in dielectric elastomers JF - Soft matter N2 - A membrane of a dielectric elastomer coated with compliant electrodes may form wrinkles as the applied voltage is ramped up. We present a combination of experiment and theory to investigate the transition to wrinkles using a clamped membrane subject to a constant force and a voltage ramp. Two types of transitions are identified. In type-I transition, the voltage-stretch curve is N-shaped, and flat and wrinkled regions coexist in separate areas of the membrane. The type-I transition progresses by nucleation of small wrinkled regions, followed by the growth of the wrinkled regions at the expense of the flat regions, until the entire membrane is wrinkled. By contrast, in type-II transition, the voltage-stretch curve is monotonic, and the entire flat membrane becomes wrinkled with no nucleation barrier. The two types of transitions are analogous to the first and the second order phase transitions. While the type-I transition is accompanied by a jump in the vertical displacement, type-II transition is accompanied by a continuous change in the vertical displacement. Such transitions may enable applications in muscle-like actuation and energy harvesting, where large deformation and large energy of conversion are desired. Y1 - 2012 U6 - https://doi.org/10.1039/c2sm26034d SN - 1744-683X VL - 8 IS - 34 SP - 8840 EP - 8846 PB - Royal Society of Chemistry CY - Cambridge ER - TY - JOUR A1 - Zhu, Hui A1 - Shprits, Yuri Y. A1 - Spasojevic, M. A1 - Drozdov, Alexander T1 - New hiss and chorus waves diffusion coefficient parameterizations from the Van Allen Probes and their effect on long-term relativistic electron radiation-belt VERB simulations JF - Journal of Atmospheric and Solar-Terrestrial Physics N2 - New wave frequency and amplitude models for the nightside and dayside chorus waves are built based on measurements from the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) instrument onboard the Van Allen Probes. The corresponding 3D diffusion coefficients are systematically obtained. Compared with previous commonly-used (typical) parameterizations, the new parameterizations result in differences in diffusion rates that depend on the energy and pitch angle. Furthermore, one-year 3D diffusive simulations are performed using the Versatile Electron Radiation Belt (VERB) code. Both typical and new wave parameterizations simulation results are in a good agreement with observations at 0.9 MeV. However, the new parameterizations for nightside chorus better reproduce the observed electron fluxes. These parameterizations will be incorporated into future modeling efforts. KW - Inner magnetosphere KW - Radiation belts KW - Chorus waves KW - Diffusion coefficients KW - VERB code Y1 - 2019 U6 - https://doi.org/10.1016/j.jastp.2019.105090 SN - 1364-6826 SN - 1879-1824 VL - 193 PB - Elsevier CY - Oxford ER - TY - THES A1 - Zhou, Xu T1 - Atmospheric interactions with land surface in the arctic based on regional climate model solutions Y1 - 2014 ER - TY - JOUR A1 - Zhou, Changsong A1 - Zemanova, Lucia A1 - Zamora, Gorka A1 - Hilgetag, Claus C. A1 - Kurths, Jürgen T1 - Hierarchical organization unveiled by functional connectivity in complex brain networks JF - Physical review letters N2 - How do diverse dynamical patterns arise from the topology of complex networks? We study synchronization dynamics in the cortical brain network of the cat, which displays a hierarchically clustered organization, by modeling each node (cortical area) with a subnetwork of interacting excitable neurons. We find that in the biologically plausible regime the dynamics exhibits a hierarchical modular organization, in particular, revealing functional clusters coinciding with the anatomical communities at different scales. Our results provide insights into the relationship between network topology and functional organization of complex brain networks. Y1 - 2006 U6 - https://doi.org/10.1103/PhysRevLett.97.238103 SN - 0031-9007 SN - 1079-7114 VL - 97 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Zhou, Changsong A1 - Motter, Adilson E. A1 - Kurths, Jürgen T1 - Universality in the synchronization of weighted random networks N2 - Realistic networks display not only a complex topological structure, but also a heterogeneous distribution of weights in the connection strengths. Here we study synchronization in weighted complex networks and show that the synchronizability of random networks with a large minimum degree is determined by two leading parameters: the mean degree and the heterogeneity of the distribution of node's intensity, where the intensity of a node, defined as the total strength of input connections, is a natural combination of topology and weights. Our results provide a possibility for the control of synchronization in complex networks by the manipulation of a few parameters Y1 - 2006 UR - http://prl.aps.org/pdf/PRL/v96/i3/e034101 U6 - https://doi.org/10.1103/Physrevlett.96.034101 ER - TY - JOUR A1 - Zhou, Changsong A1 - Kurths, Jürgen A1 - Kiss, Istvan Z. A1 - Hudson, J. L. T1 - Noise-enhanced phase synchronization of chaotic oscillators Y1 - 2002 ER - TY - JOUR A1 - Zhou, Changsong A1 - Kurths, Jürgen A1 - Hu, B. T1 - Array-enhanced coherence resonance: Nontrivial effects of heterogeneity and spatial independence of noise Y1 - 2001 ER - TY - JOUR A1 - Zhou, Changsong A1 - Kurths, Jürgen T1 - Noise-sustained and controlled synchronization of stirred excitable media by external forcing N2 - Most of the previous studies on constructive effects of noise in spatially extended systems have focused on static media, e.g., of the reaction diffusion type. Because many active chemical or biological processes occur in a fluid environment with mixing, we investigate here the interplay among noise, excitability, mixing and external forcing in excitable media advected by a chaotic flow, in a two-dimensional FitzHugh-Nagumo model described by a set of reaction- advection-diffusion equations. In the absence of external forcing, noise may generate sustained coherent oscillations of the media in a range of noise intensities and stirring rates. We find that these noise-sustained oscillations can be synchronized by external periodic signals much smaller than the threshold. Analysis of the locking regions in the parameter space of the signal period, stirring rate and noise intensity reveals that the mechanism underlying the synchronization behaviour is a matching between the time scales of the forcing signal and the noise-sustained oscillations. The results demonstrate that, in the presence of a suitable level of noise, the stirred excitable media act as self-sustained oscillatory systems and become much easier to be entrained by weak external forcing. Our results may be verified in experiments and are useful to understand the synchronization of population dynamics of oceanic ecological systems by annual cycles Y1 - 2005 SN - 1367-2630 ER - TY - JOUR A1 - Zhou, Changsong A1 - Kurths, Jürgen T1 - Resonant patterns in noisy active media N2 - We investigate noise-controlled resonant response of active media to weak periodic forcing, both in excitable and oscillatory regimes. In the excitable regime, we find that noise-induced irregular wave structures can be reorganized into frequency-locked resonant patterns by weak signals with suitable frequencies. The resonance occurs due to a matching condition between the signal frequency and the noise-induced inherent time scale of the media. m:1 resonant regions similar to the Arnold tongues in frequency locking of self-sustained oscillatory media are observed. In the self-sustained oscillatory regime, noise also controls the oscillation frequency and reshapes significantly the Arnold tongues. The combination of noise and weak signal thus could provide an efficient tool to manipulate active extended systems in experiments Y1 - 2004 SN - 1063-651X ER - TY - JOUR A1 - Zhou, Changsong A1 - Kurths, Jürgen T1 - Dynamical weights and enhanced synchronization in adaptive complex networks N2 - Dynamical organization of connection weights is studied in scale-free networks of chaotic oscillators, where the coupling strength of a node from its neighbors develops adaptively according to the local synchronization property between the node and its neighbors. We find that when complete synchronization is achieved, the coupling strength becomes weighted and correlated with the topology due to a hierarchical transition to synchronization in heterogeneous networks. Importantly, such an adaptive process enhances significantly the synchronizability of the networks, which could have meaningful implications in the manipulation of dynamical networks Y1 - 2006 UR - http://link.aps.org/doi/10.1103/PhysRevLett.96.164102 U6 - https://doi.org/10.1103/Physrevlett.96.164102 ER - TY - JOUR A1 - Zhou, Changsong A1 - Kurths, Jürgen T1 - Hierarchical synchronization in complex networks with heterogeneous degrees N2 - We study synchronization behavior in networks of coupled chaotic oscillators with heterogeneous connection degrees. Our focus is on regimes away from the complete synchronization state, when the coupling is not strong enough, when the oscillators are under the influence of noise or when the oscillators are nonidentical. We have found a hierarchical organization of the synchronization behavior with respect to the collective dynamics of the network. Oscillators with more connections (hubs) are synchronized more closely by the collective dynamics and constitute the dynamical core of the network. The numerical observation of this hierarchical synchronization is supported with an analysis based on a mean field approximation and the master stability function. (C) 2006 American Institute of Physics Y1 - 2006 UR - http://scitation.aip.org/getpdf/servlet/ GetPDFServlet?filetype=pdf&id=CHAOEH000016000001015104000001&idtype=cvips&doi=10.1063/1.2150381&prog=normal U6 - https://doi.org/10.1063/1.2150381 SN - 1054-1500 ER - TY - JOUR A1 - Zhou, Changsong A1 - Kurths, Jürgen T1 - Noise-induced phase synchronization and synchronization transitions in chaotic oscillators Y1 - 2002 ER - TY - JOUR A1 - Zhong-Fu, X. A1 - Shan-Shan, M. A1 - Jia-Qian, Z. A1 - Xun-Lin, Q. A1 - Ye-Wen, Z. A1 - Gerhard, Reimund A1 - Künstler, Wolfgang T1 - Piezoelectric activity and its stability of polytetrafluoroethylene (PTFE) films Y1 - 2003 ER - TY - JOUR A1 - Zhong, Yufei A1 - Causa, Martina A1 - Moore, Gareth John A1 - Krauspe, Philipp A1 - Xiao, Bo A1 - Günther, Florian A1 - Kublitski, Jonas A1 - BarOr, Eyal A1 - Zhou, Erjun A1 - Banerji, Natalie T1 - Sub-picosecond charge-transfer at near-zero driving force in polymer:non-fullerene acceptor blends and bilayers JF - Nature Communications N2 - Organic photovoltaics based on non-fullerene acceptors (NFAs) show record efficiency of 16 to 17% and increased photovoltage owing to the low driving force for interfacial charge-transfer. However, the low driving force potentially slows down charge generation, leading to a tradeoff between voltage and current. Here, we disentangle the intrinsic charge-transfer rates from morphology-dependent exciton diffusion for a series of polymer:NFA systems. Moreover, we establish the influence of the interfacial energetics on the electron and hole transfer rates separately. We demonstrate that charge-transfer timescales remain at a few hundred femtoseconds even at near-zero driving force, which is consistent with the rates predicted by Marcus theory in the normal region, at moderate electronic coupling and at low re-organization energy. Thus, in the design of highly efficient devices, the energy offset at the donor:acceptor interface can be minimized without jeopardizing the charge-transfer rate and without concerns about a current-voltage tradeoff. KW - organic solar cell KW - electron-transfer KW - Donor-Acceptor (DA) interface KW - transfer dynamics KW - donor KW - seperation KW - efficiency KW - impact KW - energy KW - photovoltaics Y1 - 2020 U6 - https://doi.org/10.1038/s41467-020-14549-w SN - 2041-1723 VL - 11 IS - 1 SP - 1 EP - 10 PB - Nature Publishing Group UK CY - London ER - TY - GEN A1 - Zhong, Yufei A1 - Causa, Martina A1 - Moore, Gareth John A1 - Krauspe, Philipp A1 - Xiao, Bo A1 - Günther, Florian A1 - Kublitski, Jonas A1 - BarOr, Eyal A1 - Zhou, Erjun A1 - Banerji, Natalie T1 - Sub-picosecond charge-transfer at near-zero driving force in polymer:non-fullerene acceptor blends and bilayers T2 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - Organic photovoltaics based on non-fullerene acceptors (NFAs) show record efficiency of 16 to 17% and increased photovoltage owing to the low driving force for interfacial charge-transfer. However, the low driving force potentially slows down charge generation, leading to a tradeoff between voltage and current. Here, we disentangle the intrinsic charge-transfer rates from morphology-dependent exciton diffusion for a series of polymer:NFA systems. Moreover, we establish the influence of the interfacial energetics on the electron and hole transfer rates separately. We demonstrate that charge-transfer timescales remain at a few hundred femtoseconds even at near-zero driving force, which is consistent with the rates predicted by Marcus theory in the normal region, at moderate electronic coupling and at low re-organization energy. Thus, in the design of highly efficient devices, the energy offset at the donor:acceptor interface can be minimized without jeopardizing the charge-transfer rate and without concerns about a current-voltage tradeoff. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1422 KW - organic solar cell KW - electron-transfer KW - Donor-Acceptor (DA) interface KW - transfer dynamics KW - donor KW - seperation KW - efficiency KW - impact KW - energy KW - photovoltaics Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-511936 SN - 1866-8372 IS - 1 ER - TY - JOUR A1 - Zhong, Qi A1 - Wang, Weinan A1 - Adelsberger, Joseph A1 - Golosova, Anastasia A1 - Koumba, Achille M. Bivigou A1 - Laschewsky, André A1 - Funari, Sergio S. A1 - Perlich, Jan A1 - Roth, Stephan V. A1 - Papadakis, Christine M. A1 - Müller-Buschbaum, Peter T1 - Collapse transition in thin films of poly(methoxydiethylenglycol acrylate) JF - Colloid and polymer science : official journal of the Kolloid-Gesellschaft N2 - The thermal behavior of poly(methoxydiethylenglycol acrylate) (PMDEGA) is studied in thin hydrogel films on solid supports and is compared with the behavior in aqueous solution. The PMDEGA hydrogel film thickness is varied from 2 to 422 nm. Initially, these films are homogenous, as measured with optical microscopy, atomic force microscopy, X-ray reflectivity, and grazing-incidence small-angle X-ray scattering (GISAXS). However, they tend to de-wet when stored under ambient conditions. Along the surface normal, no long-ranged correlations between substrate and film surface are detected with GISAXS, due to the high mobility of the polymer at room temperature. The swelling of the hydrogel films as a function of the water vapor pressure and the temperature are probed for saturated water vapor pressures between 2,380 and 3,170 Pa. While the swelling capability is found to increase with water vapor pressure, swelling in dependence on the temperature revealed a collapse phase transition of a lower critical solution temperature type. The transition temperature decreases from 40.6 A degrees C to 36.6 A degrees C with increasing film thickness, but is independent of the thickness for very thin films below a thickness of 40 nm. The observed transition temperature range compares well with the cloud points observed in dilute (0.1 wt.%) and semi-dilute (5 wt.%) solution which decrease from 45 A degrees C to 39 A degrees C with increasing concentration. KW - Hydrogel KW - Thin film KW - Thermoresponsive KW - LCST behavior KW - GISAXS KW - AFM Y1 - 2011 U6 - https://doi.org/10.1007/s00396-011-2384-1 SN - 0303-402X VL - 289 IS - 5-6 SP - 569 EP - 581 PB - Springer CY - New York ER - TY - JOUR A1 - Zhirov, O. V. A1 - Pikovskij, Arkadij A1 - Shepelyansky, Dima L. T1 - Quantum vacuum of strongly nonlinear lattices JF - Physical review : E, Statistical, nonlinear and soft matter physics N2 - We study the properties of classical and quantum strongly nonlinear chains by means of extensive numerical simulations. Due to strong nonlinearity, the classical dynamics of such chains remains chaotic at arbitrarily low energies. We show that the collective excitations of classical chains are described by sound waves whose decay rate scales algebraically with the wave number with a generic exponent value. The properties of the quantum chains are studied by the quantum Monte Carlo method and it is found that the low-energy excitations are well described by effective phonon modes with the sound velocity dependent on an effective Planck constant. Our results show that at low energies the quantum effects lead to a suppression of chaos and drive the system to a quasi-integrable regime of effective phonon modes. Y1 - 2011 U6 - https://doi.org/10.1103/PhysRevE.83.016202 SN - 1539-3755 VL - 83 IS - 1 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Zheng, Chunming A1 - Toenjes, Ralf A1 - Pikovskij, Arkadij T1 - Transition to synchrony in a three-dimensional swarming model with helical trajectories JF - Physical review : E, Statistical, nonlinear and soft matter physics N2 - We investigate the transition from incoherence to global collective motion in a three-dimensional swarming model of agents with helical trajectories, subject to noise and global coupling. Without noise this model was recently proposed as a generalization of the Kuramoto model and it was found that alignment of the velocities occurs discontinuously for arbitrarily small attractive coupling. Adding noise to the system resolves this singular limit and leads to a continuous transition, either to a directed collective motion or to center-of-mass rotations. Y1 - 2021 U6 - https://doi.org/10.1103/PhysRevE.104.014216 SN - 2470-0045 SN - 2470-0053 VL - 104 IS - 1 PB - American Physical Society CY - College Park ER - TY - JOUR A1 - Zheng, Chunming A1 - Pikovskij, Arkadij T1 - Stochastic bursting in unidirectionally delay-coupled noisy excitable systems JF - Chaos : an interdisciplinary journal of nonlinear science N2 - We show that "stochastic bursting" is observed in a ring of unidirectional delay-coupled noisy excitable systems, thanks to the combinational action of time-delayed coupling and noise. Under the approximation of timescale separation, i.e., when the time delays in each connection are much larger than the characteristic duration of the spikes, the observed rather coherent spike pattern can be described by an idealized coupled point processwith a leader-follower relationship. We derive analytically the statistics of the spikes in each unit, the pairwise correlations between any two units, and the spectrum of the total output from the network. Theory is in good agreement with the simulations with a network of theta-neurons. Published under license by AIP Publishing. Y1 - 2019 U6 - https://doi.org/10.1063/1.5093180 SN - 1054-1500 SN - 1089-7682 VL - 29 IS - 4 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Zheng, Chunming A1 - Pikovskij, Arkadij T1 - Delay-induced stochastic bursting in excitable noisy systems JF - Physical review : E, Statistical, nonlinear and soft matter physics N2 - We show that a combined action of noise and delayed feedback on an excitable theta-neuron leads to rather coherent stochastic bursting. An idealized point process, valid if the characteristic timescales in the problem are well separated, is used to describe statistical properties such as the power spectral density and the interspike interval distribution. We show how the main parameters of the point process, the spontaneous excitation rate, and the probability to induce a spike during the delay action can be calculated from the solutions of a stationary and a forced Fokker-Planck equation. Y1 - 2018 U6 - https://doi.org/10.1103/PhysRevE.98.042148 SN - 2470-0045 SN - 2470-0053 VL - 98 IS - 4 PB - American Physical Society CY - College Park ER - TY - THES A1 - Zheng, Chunming T1 - Bursting and synchronization in noisy oscillatory systems T1 - Bursting und Synchronisation in verrauschten, oszillierenden Systemen N2 - Noise is ubiquitous in nature and usually results in rich dynamics in stochastic systems such as oscillatory systems, which exist in such various fields as physics, biology and complex networks. The correlation and synchronization of two or many oscillators are widely studied topics in recent years. In this thesis, we mainly investigate two problems, i.e., the stochastic bursting phenomenon in noisy excitable systems and synchronization in a three-dimensional Kuramoto model with noise. Stochastic bursting here refers to a sequence of coherent spike train, where each spike has random number of followers due to the combined effects of both time delay and noise. Synchronization, as a universal phenomenon in nonlinear dynamical systems, is well illustrated in the Kuramoto model, a prominent model in the description of collective motion. In the first part of this thesis, an idealized point process, valid if the characteristic timescales in the problem are well separated, is used to describe statistical properties such as the power spectral density and the interspike interval distribution. We show how the main parameters of the point process, the spontaneous excitation rate, and the probability to induce a spike during the delay action can be calculated from the solutions of a stationary and a forced Fokker-Planck equation. We extend it to the delay-coupled case and derive analytically the statistics of the spikes in each neuron, the pairwise correlations between any two neurons, and the spectrum of the total output from the network. In the second part, we investigate the three-dimensional noisy Kuramoto model, which can be used to describe the synchronization in a swarming model with helical trajectory. In the case without natural frequency, the Kuramoto model can be connected with the Vicsek model, which is widely studied in collective motion and swarming of active matter. We analyze the linear stability of the incoherent state and derive the critical coupling strength above which the incoherent state loses stability. In the limit of no natural frequency, an exact self-consistent equation of the mean field is derived and extended straightforward to any high-dimensional case. N2 - Rauschen ist in der Natur allgegenwärtig und führt zu einer reichen Dynamik in stochastischen Systemen von gekoppelten Oszillatoren, die in so unterschiedlichen Bereichen wie Physik, Biologie und in komplexen Netzwerken existieren. Korrelation und Synchronisation von zwei oder vielen Oszillatoren ist in den letzten Jahren ein aktives Forschungsfeld. In dieser Arbeit untersuchen wir hauptsächlich zwei Probleme, d.h. das stochastische Burst-Phänomen in verrauschten anregbaren Systemen und die Synchronisation in einem dreidimensionalen Kuramoto-Modell mit Rauschen. Stochastisches Bursting bezieht sich hier auf eine Folge von kohärenten Spike-Zügen, bei denen jeder Spike aufgrund der kombinierten Effekte von Zeitverzögerung und Rauschen eine zufällige Anzahl von Folge Spikes aufweist. Die Synchronisation als universelles Phänomen in nichtlinearen dynamischen Systemen kann anhand des Kuramoto-Modells, einem grundlegenden Modell bei der gekoppelter Oszillatoren und kollektiver Bewegung, gut demonstriert und analysiert werden. Im ersten Teil dieser Arbeit wird ein idealisierter Punktprozess betrachtet, der gültig ist, wenn die charakteristischen Zeitskalen im Problem gut voneinander getrennt sind,um statistische Eigenschaften wie die spektrale Leistungsdichte und die Intervallverteilung zwischen Neuronen Impulsen zu beschreiben. Wir zeigen, wie die Hauptparameter des Punktprozesses, die spontane Anregungsrate und die Wahrscheinlichkeit, während der Verzögerungsaktion einen Impuls zu induzieren, aus den Lösungen einer stationären und einer getriebenen Fokker-Planck-Gleichung berechnet werden können. Wir erweitern dieses Ergebnis auf den verzögerungsgekoppelten Fall und leiten analytisch die Statistiken der Impulse in jedem Neuron, die paarweisen Korrelationen zwischen zwei beliebigen Neuronen und das Spektrum der Zeitreihe alle Impulse aus dem Netzwerk ab. Im zweiten Teil untersuchen wir das dreidimensionale verrauschte Kuramoto-Modell, mit dem die Synchronisation eines Schwarmmodells mit schraubenförmigen Flugbahnen beschrieben werden kann. Im Fall ohne Eigenfrequenz jedes Teilchensist das System äquivalent zum Vicsek Modell, welches in der Beschreibung der kollektiven Bewegung von Schwärmen und aktiver Materie eine breite Anwendung findet. Wir analysieren die lineare Stabilität des inkohärenten Zustands und leiten die kritische Kopplungsstärke ab, oberhalb derer der inkohärente Zustand an Stabilität verliert. Im Fall ohne Eigenfrequenz wird eine exakte selbstkonsistente Gleichung für das mittlere Feld abgeleitet und direkt für höherdimensionale Bewegungen verallgemeinert. KW - Synchronization KW - Kuramoto model KW - Oscillation KW - stochastic bursting KW - Synchronisation KW - Kuramoto-Modell KW - Oszillatoren KW - Stochastisches Bursting Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-500199 ER - TY - JOUR A1 - Zhelavskaya, Irina A1 - Spasojevic, M. A1 - Shprits, Yuri Y. A1 - Kurth, William S. T1 - Automated determination of electron density from electric field measurements on the Van Allen Probes spacecraft JF - Journal of geophysical research : Space physics N2 - We present the Neural-network-based Upper hybrid Resonance Determination (NURD) algorithm for automatic inference of the electron number density from plasma wave measurements made on board NASA's Van Allen Probes mission. A feedforward neural network is developed to determine the upper hybrid resonance frequency, fuhr, from electric field measurements, which is then used to calculate the electron number density. In previous missions, the plasma resonance bands were manually identified, and there have been few attempts to do robust, routine automated detections. We describe the design and implementation of the algorithm and perform an initial analysis of the resulting electron number density distribution obtained by applying NURD to 2.5 years of data collected with the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) instrumentation suite of the Van Allen Probes mission. Densities obtained by NURD are compared to those obtained by another recently developed automated technique and also to an existing empirical plasmasphere and trough density model. KW - Van Allen Probes KW - electron number density KW - neural networks Y1 - 2016 U6 - https://doi.org/10.1002/2015JA022132 SN - 2169-9380 SN - 2169-9402 VL - 121 SP - 4611 EP - 4625 PB - American Geophysical Union CY - Washington ER - TY - JOUR A1 - Zhelavskaya, Irina A1 - Shprits, Yuri Y. A1 - Spasojevic, Maria T1 - Empirical Modeling of the Plasmasphere Dynamics Using Neural Networks JF - Journal of geophysical research : Space physics N2 - We present the PINE (Plasma density in the Inner magnetosphere Neural network‐based Empirical) model ‐ a new empirical model for reconstructing the global dynamics of the cold plasma density distribution based only on solar wind data and geomagnetic indices. Utilizing the density database obtained using the NURD (Neural‐network‐based Upper hybrid Resonance Determination) algorithm for the period of 1 October 2012 to 1 July 2016, in conjunction with solar wind data and geomagnetic indices, we develop a neural network model that is capable of globally reconstructing the dynamics of the cold plasma density distribution for 2≤L≤6 and all local times. We validate and test the model by measuring its performance on independent data sets withheld from the training set and by comparing the model‐predicted global evolution with global images of He+ distribution in the Earth's plasmasphere from the IMAGE Extreme UltraViolet (EUV) instrument. We identify the parameters that best quantify the plasmasphere dynamics by training and comparing multiple neural networks with different combinations of input parameters (geomagnetic indices, solar wind data, and different durations of their time history). The optimal model is based on the 96 h time history of Kp, AE, SYM‐H, and F10.7 indices. The model successfully reproduces erosion of the plasmasphere on the nightside and plume formation and evolution. We demonstrate results of both local and global plasma density reconstruction. This study illustrates how global dynamics can be reconstructed from local in situ observations by using machine learning techniques. Y1 - 2017 U6 - https://doi.org/10.1002/2017JA024406 SN - 2169-9380 SN - 2169-9402 VL - 122 SP - 11227 EP - 11244 PB - American Geophysical Union CY - Washington ER - TY - THES A1 - Zhelavskaya, Irina T1 - Modeling of the Plasmasphere Dynamics T1 - Modellierung der Plasmasphärendynamik N2 - The plasmasphere is a dynamic region of cold, dense plasma surrounding the Earth. Its shape and size are highly susceptible to variations in solar and geomagnetic conditions. Having an accurate model of plasma density in the plasmasphere is important for GNSS navigation and for predicting hazardous effects of radiation in space on spacecraft. The distribution of cold plasma and its dynamic dependence on solar wind and geomagnetic conditions remain, however, poorly quantified. Existing empirical models of plasma density tend to be oversimplified as they are based on statistical averages over static parameters. Understanding the global dynamics of the plasmasphere using observations from space remains a challenge, as existing density measurements are sparse and limited to locations where satellites can provide in-situ observations. In this dissertation, we demonstrate how such sparse electron density measurements can be used to reconstruct the global electron density distribution in the plasmasphere and capture its dynamic dependence on solar wind and geomagnetic conditions. First, we develop an automated algorithm to determine the electron density from in-situ measurements of the electric field on the Van Allen Probes spacecraft. In particular, we design a neural network to infer the upper hybrid resonance frequency from the dynamic spectrograms obtained with the Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) instrumentation suite, which is then used to calculate the electron number density. The developed Neural-network-based Upper hybrid Resonance Determination (NURD) algorithm is applied to more than four years of EMFISIS measurements to produce the publicly available electron density data set. We utilize the obtained electron density data set to develop a new global model of plasma density by employing a neural network-based modeling approach. In addition to the location, the model takes the time history of geomagnetic indices and location as inputs, and produces electron density in the equatorial plane as an output. It is extensively validated using in-situ density measurements from the Van Allen Probes mission, and also by comparing the predicted global evolution of the plasmasphere with the global IMAGE EUV images of He+ distribution. The model successfully reproduces erosion of the plasmasphere on the night side as well as plume formation and evolution, and agrees well with data. The performance of neural networks strongly depends on the availability of training data, which is limited during intervals of high geomagnetic activity. In order to provide reliable density predictions during such intervals, we can employ physics-based modeling. We develop a new approach for optimally combining the neural network- and physics-based models of the plasmasphere by means of data assimilation. The developed approach utilizes advantages of both neural network- and physics-based modeling and produces reliable global plasma density reconstructions for quiet, disturbed, and extreme geomagnetic conditions. Finally, we extend the developed machine learning-based tools and apply them to another important problem in the field of space weather, the prediction of the geomagnetic index Kp. The Kp index is one of the most widely used indicators for space weather alerts and serves as input to various models, such as for the thermosphere, the radiation belts and the plasmasphere. It is therefore crucial to predict the Kp index accurately. Previous work in this area has mostly employed artificial neural networks to nowcast and make short-term predictions of Kp, basing their inferences on the recent history of Kp and solar wind measurements at L1. We analyze how the performance of neural networks compares to other machine learning algorithms for nowcasting and forecasting Kp for up to 12 hours ahead. Additionally, we investigate several machine learning and information theory methods for selecting the optimal inputs to a predictive model of Kp. The developed tools for feature selection can also be applied to other problems in space physics in order to reduce the input dimensionality and identify the most important drivers. Research outlined in this dissertation clearly demonstrates that machine learning tools can be used to develop empirical models from sparse data and also can be used to understand the underlying physical processes. Combining machine learning, physics-based modeling and data assimilation allows us to develop novel methods benefiting from these different approaches. N2 - Die Plasmasphäre ist eine die Erde umgebende dynamische Region aus kaltem, dichtem Plasma. Ihre Form und Größe sind sehr anfällig für Schwankungen der solaren und geomagnetischen Bedingungen. Ein präzises Modell der Plasmadichte in der Plasmasphäre ist wichtig für die GNSS-Navigation und für die Vorhersage gefährlicher Auswirkungen der kosmischen Strahlung auf Raumfahrzeuge. Die Verteilung des kalten Plasmas und seine dynamische Abhängigkeit vom Sonnenwind und den geomagnetischen Bedingungen sind jedoch nach wie vor nur unzureichend quantifiziert. Bestehende empirische Modelle der Plasmadichte sind in der Regel zu stark vereinfacht, da sie auf statistischen Durchschnittswerten statischer Parameter basieren. Das Verständnis der globalen Dynamik der Plasmasphäre anhand von Beobachtungen aus dem Weltraum bleibt eine Herausforderung, da vorhandene Dichtemessungen spärlich sind und sich auf Orte beschränken, an denen Satelliten In-situ-Beobachtungen liefern können. In dieser Dissertation zeigen wir, wie solche spärlichen Elektronendichtemessungen verwendet werden können, um die globale Elektronendichteverteilung in der Plasmasphäre zu rekonstruieren und ihre dynamische Abhängigkeit vom Sonnenwind und den geomagnetischen Bedingungen zu erfassen. Zunächst entwickeln wir einen automatisierten Algorithmus zur Bestimmung der Elektronendichte aus In-situ-Messungen des elektrischen Feldes der Van Allen Probes Raumsonden. Insbesondere entwerfen wir ein neuronales Netzwerk, um die obere Hybridresonanzfrequenz aus den dynamischen Spektrogrammen abzuleiten, die wir durch die Instrumentensuite „Electric and Magnetic Field Instrument Suite“ (EMFISIS) erhielten, welche dann zur Berechnung der Elektronenzahldichte verwendet wird. Der entwickelte „Neural-network-based Upper Hybrid Resonance Determination“ (NURD)-Algorithmus wird auf mehr als vier Jahre der EMFISIS-Messungen angewendet, um den öffentlich verfügbaren Elektronendichte-Datensatz zu erstellen. Wir verwenden den erhaltenen Elektronendichte-Datensatz, um ein neues globales Modell der Plasmadichte zu entwickeln, indem wir einen auf einem neuronalen Netzwerk basierenden Modellierungsansatz verwenden. Zusätzlich zum Ort nimmt das Modell den zeitlichen Verlauf der geomagnetischen Indizes und des Ortes als Eingabe und erzeugt als Ausgabe die Elektronendichte in der äquatorialebene. Dies wird ausführlich anhand von In-situ-Dichtemessungen der Van Allen Probes-Mission und durch den Vergleich der vom Modell vorhergesagten globalen Entwicklung der Plasmasphäre mit den globalen IMAGE EUV-Bildern der He+ -Verteilung validiert. Das Modell reproduziert erfolgreich die Erosion der Plasmasphäre auf der Nachtseite sowie die Bildung und Entwicklung von Fahnen und stimmt gut mit den Daten überein. Die Leistung neuronaler Netze hängt stark von der Verfügbarkeit von Trainingsdaten ab, die für Intervalle hoher geomagnetischer Aktivität nur spärlich vorhanden sind. Um zuverlässige Dichtevorhersagen während solcher Intervalle zu liefern, können wir eine physikalische Modellierung verwenden. Wir entwickeln einen neuen Ansatz zur optimalen Kombination der neuronalen Netzwerk- und physikbasierenden Modelle der Plasmasphäre mittels Datenassimilation. Der entwickelte Ansatz nutzt sowohl die Vorteile neuronaler Netze als auch die physikalischen Modellierung und liefert zuverlässige Rekonstruktionen der globalen Plasmadichte für ruhige, gestörte und extreme geomagnetische Bedingungen. Schließlich erweitern wir die entwickelten auf maschinellem Lernen basierten Werkzeuge und wenden sie auf ein weiteres wichtiges Problem im Bereich des Weltraumwetters an, die Vorhersage des geomagnetischen Index Kp. Der Kp-Index ist einer der am häufigsten verwendeten Indikatoren für Weltraumwetterwarnungen und dient als Eingabe für verschiedene Modelle, z.B. für die Thermosphäre, die Strahlungsgürtel und die Plasmasphäre. Es ist daher wichtig, den Kp-Index genau vorherzusagen. Frühere Arbeiten in diesem Bereich verwendeten hauptsächlich künstliche neuronale Netze, um Kurzzeit-Kp-Vorhersagen zu treffen, wobei deren Schlussfolgerungen auf der jüngsten Vergangenheit von Kp- und Sonnenwindmessungen am L1-Punkt beruhten. Wir analysieren, wie sich die Leistung neuronaler Netze im Vergleich zu anderen Algorithmen für maschinelles Lernen verhält, um kurz- und längerfristige Kp-Voraussagen von bis zu 12 Stunden treffen zu können. Zusätzlich untersuchen wir verschiedene Methoden des maschinellen Lernens und der Informationstheorie zur Auswahl der optimalen Eingaben für ein Vorhersagemodell von Kp. Die entwickelten Werkzeuge zur Merkmalsauswahl können auch auf andere Probleme in der Weltraumphysik angewendet werden, um die Eingabedimensionalität zu reduzieren und die wichtigsten Treiber zu identifizieren. Die in dieser Dissertation skizzierten Untersuchungen zeigen deutlich, dass Werkzeuge für maschinelles Lernen sowohl zur Entwicklung empirischer Modelle aus spärlichen Daten als auch zum Verstehen zugrunde liegender physikalischer Prozesse genutzt werden können. Die Kombination von maschinellem Lernen, physikbasierter Modellierung und Datenassimilation ermöglicht es uns, kombinierte Methoden zu entwickeln, die von unterschiedlichen Ansätzen profitieren. KW - Plasmasphere KW - Inner magnetosphere KW - Neural networks KW - Machine learning KW - Modeling KW - Kp index KW - Geomagnetic activity KW - Data assimilation KW - Validation KW - IMAGE EUV KW - Kalman filter KW - Plasmasphäre KW - Innere Magnetosphäre KW - Neuronale Netze KW - Maschinelles Lernen KW - Modellieren KW - Forecasting KW - Kp-Index KW - Geomagnetische Aktivität KW - Datenassimilation KW - Validierung KW - Kalman Filter KW - Prognose Y1 - 2020 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-482433 ER - TY - JOUR A1 - Zhao, Yuhang A1 - Sarhan, Radwan Mohamed A1 - Eljarrat, Alberto A1 - Kochovski, Zdravko A1 - Koch, Christoph A1 - Schmidt, Bernd A1 - Koopman, Wouter-Willem Adriaan A1 - Lu, Yan T1 - Surface-functionalized Au-Pd nanorods with enhanced photothermal conversion and catalytic performance JF - ACS applied materials & interfaces N2 - Bimetallic nanostructures comprising plasmonic and catalytic components have recently emerged as a promising approach to generate a new type of photo-enhanced nanoreactors. Most designs however concentrate on plasmon-induced charge separation, leaving photo-generated heat as a side product. This work presents a photoreactor based on Au-Pd nanorods with an optimized photothermal conversion, which aims to effectively utilize the photo-generated heat to increase the rate of Pd-catalyzed reactions. Dumbbell-shaped Au nanorods were fabricated via a seed-mediated growth method using binary surfactants. Pd clusters were selectively grown at the tips of the Au nanorods, using the zeta potential as a new synthetic parameter to indicate the surfactant remaining on the nanorod surface. The photothermal conversion of the Au-Pd nanorods was improved with a thin layer of polydopamine (PDA) or TiO2. As a result, a 60% higher temperature increment of the dispersion compared to that for bare Au rods at the same light intensity and particle density could be achieved. The catalytic performance of the coated particles was then tested using the reduction of 4-nitrophenol as the model reaction. Under light, the PDA-coated Au-Pd nanorods exhibited an improved catalytic activity, increasing the reaction rate by a factor 3. An analysis of the activation energy confirmed the photoheating effect to be the dominant mechanism accelerating the reaction. Thus, the increased photothermal heating is responsible for the reaction acceleration. Interestingly, the same analysis shows a roughly 10% higher reaction rate for particles under illumination compared to under dark heating, possibly implying a crucial role of localized heat gradients at the particle surface. Finally, the coating thickness was identified as an essential parameter determining the photothermal conversion efficiency and the reaction acceleration. KW - Au-Pd nanorods KW - PDA KW - photothermal conversion KW - surface plasmon KW - 4-nitrophenol Y1 - 2022 U6 - https://doi.org/10.1021/acsami.2c00221 SN - 1944-8244 SN - 1944-8252 VL - 14 IS - 15 SP - 17259 EP - 17272 PB - American Chemical Society CY - Washington, DC ER - TY - JOUR A1 - Zhao, Siqi Q. A1 - Yan, Huirong A1 - Liu, Terry Z. A1 - Liu, Mingzhe A1 - Wang, Huizi T1 - Multispacecraft analysis of the properties of magnetohydrodynamic fluctuations in Sub-Alfvenic solar wind turbulence at 1 au JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We present observations of three-dimensional magnetic power spectra in wavevector space to investigate the anisotropy and scalings of sub-Alfvenic solar wind turbulence at magnetohydrodynamic (MHD) scale using the Magnetospheric Multiscale spacecraft. The magnetic power distributions are organized in a new coordinate determined by wavevectors ((kappa) over cap) and background magnetic field ((b) over cap (0)) in Fourier space. This study utilizes two approaches to determine wavevectors: the singular value decomposition method and multispacecraft timing analysis. The combination of the two methods allows an examination of the properties of magnetic field fluctuations in terms of mode compositions without any spatiotemporal hypothesis. Observations show that fluctuations (delta B-perpendicular to 1) in the direction perpendicular to (kappa) over cap and (b) over cap (0) prominently cascade perpendicular to (b) over cap (0), and such anisotropy increases with wavenumbers. The reduced power spectra of 6.8 11 follow Goldreich-Sridhar scalings: (P) over cap (k(perpendicular to)) proportional to k(perpendicular to)(-5/3) and (P) over cap (k(parallel to)) proportional to k(parallel to)(-2). In contrast, fluctuations within the (k) over cap(b) over cap (0) plane show isotropic behaviors: perpendicular power distributions are approximately the same as parallel distributions. The reduced power spectra of fluctuations within the (k) over cap(b) over cap (0) plane follow the scalings (P) over cap (k(perpendicular to)) proportional to k(perpendicular to)(-3/2) and (P) over cap (k(parallel to)) proportional to k(parallel to)(-3/2). Comparing frequency-wavevector spectra with theoretical dispersion relations of MHD modes, we find that delta B-perpendicular to 1 are probably associated with Alfven modes. On the other hand, magnetic field fluctuations within the (k) over cap(b) over cap (0) plane more likely originate from fast modes based on their isotropic behaviors. The observations of anisotropy and scalings of different magnetic field components are consistent with the predictions of current compressible MHD theory. Moreover, for the Alfvenic component, the ratio of cascading time to the wave period is found to be a factor of a few, consistent with critical balance in the strong turbulence regime. These results are valuable for further studies of energy compositions of plasma turbulence and their effects on energetic particle transport. Y1 - 2022 U6 - https://doi.org/10.3847/1538-4357/ac822e SN - 0004-637X SN - 1538-4357 VL - 937 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Zhao, Qiang A1 - Dunlop, John William Chapman A1 - Qiu, Xunlin A1 - Huang, Feihe A1 - Zhang, Zibin A1 - Heyda, Jan A1 - Dzubiella, Joachim A1 - Antonietti, Markus A1 - Yuan, Jiayin T1 - An instant multi-responsive porous polymer actuator driven by solvent molecule sorption JF - Nature Communications N2 - Fast actuation speed, large-shape deformation and robust responsiveness are critical to synthetic soft actuators. A simultaneous optimization of all these aspects without trade-offs remains unresolved. Here we describe porous polymer actuators that bend in response to acetone vapour (24 kPa, 20 degrees C) at a speed of an order of magnitude faster than the state-of-the-art, coupled with a large-scale locomotion. They are meanwhile multi-responsive towards a variety of organic vapours in both the dry and wet states, thus distinctive from the traditional gel actuation systems that become inactive when dried. The actuator is easy-to-make and survives even after hydrothermal processing (200 degrees C, 24 h) and pressing-pressure (100 MPa) treatments. In addition, the beneficial responsiveness is transferable, being able to turn 'inert' objects into actuators through surface coating. This advanced actuator arises from the unique combination of porous morphology, gradient structure and the interaction between solvent molecules and actuator materials. Y1 - 2014 U6 - https://doi.org/10.1038/ncomms5293 SN - 2041-1723 VL - 5 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Zhang, Xiyun A1 - Pikovskij, Arkadij A1 - Liu, Zonghua T1 - Dynamics of oscillators globally coupled via two mean fields JF - Scientific reports N2 - Many studies of synchronization properties of coupled oscillators, based on the classical Kuramoto approach, focus on ensembles coupled via a mean field. Here we introduce a setup of Kuramoto-type phase oscillators coupled via two mean fields. We derive stability properties of the incoherent state and find traveling wave solutions with different locking patterns; stability properties of these waves are found numerically. Mostly nontrivial states appear when the two fields compete, i.e. one tends to synchronize oscillators while the other one desynchronizes them. Here we identify normal branches which bifurcate from the incoherent state in a usual way, and anomalous branches, appearance of which cannot be described as a bifurcation. Furthermore, hybrid branches combining properties of both are described. In the situations where no stable traveling wave exists, modulated quasiperiodic in time dynamics is observed. Our results indicate that a competition between two coupling channels can lead to a complex system behavior, providing a potential generalized framework for understanding of complex phenomena in natural oscillatory systems. Y1 - 2017 U6 - https://doi.org/10.1038/s41598-017-02283-1 SN - 2045-2322 VL - 7 PB - Nature Publ. Group CY - London ER - TY - JOUR A1 - Zhang, Xiaoqing A1 - Zhang, Xinwu A1 - You, Qiong A1 - Sessler, Gerhard M. T1 - Low- cost, large- area, stretchable piezoelectric films based on irradiation- crosslinked poly ( propylene) JF - Macromolecular materials and engineering N2 - Low cost, large area, lightweight, stretchable piezoelectric films, based on space-charge electret with a foam structure (i.e., ferroelectrets or piezoelectrets), have been fabricated by using commercially available irradiation cross-linked poly(propylene) (IXPP) foam sheets. Piezoelectric d(33) coefficients are as high as 100pCN(-1). The piezoelectric performance in such IXPP films is well preserved for repeated strains of less than 10%. Piezoelectric d(33) coefficients are frequency independent in the range from 2 to 100Hz. Such new class materials may be applied in sensory skins, smart clothing, bio-inspired systems, microenergy harvesters, and so on. KW - crosslinked poly(propylene) KW - ferroelectret KW - piezoelectricity KW - stretchability Y1 - 2014 U6 - https://doi.org/10.1002/mame.201300161 SN - 1438-7492 SN - 1439-2054 VL - 299 IS - 3 SP - 290 EP - 295 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Zhang, Shanshan A1 - Stolterfoht, Martin A1 - Armin, Ardalan A1 - Lin, Qianqian A1 - Zu, Fengshuo A1 - Sobus, Jan A1 - Jin, Hui A1 - Koch, Norbert A1 - Meredith, Paul A1 - Burn, Paul L. A1 - Neher, Dieter T1 - Interface Engineering of Solution-Processed Hybrid Organohalide Perovskite Solar Cells JF - ACS applied materials & interfaces N2 - Engineering the interface between the perovskite absorber and the charge-transporting layers has become an important method for improving the charge extraction and open-circuit voltage (V-OC) of hybrid perovskite solar cells. Conjugated polymers are particularly suited to form the hole-transporting layer, but their hydrophobicity renders it difficult to solution-process the perovskite absorber on top. Herein, oxygen plasma treatment is introduced as a simple means to change the surface energy and work function of hydrophobic polymer interlayers for use as p-contacts in perovskite solar cells. We find that upon oxygen plasma treatment, the hydrophobic surfaces of different prototypical p-type polymers became sufficiently hydrophilic to enable subsequent perovskite junction processing. In addition, the oxygen plasma treatment also increased the ionization potential of the polymer such that it became closer to the valance band energy of the perovskite. It was also found that the oxygen plasma treatment could increase the electrical conductivity of the p-type polymers, facilitating more efficient charge extraction. On the basis of this concept, inverted MAPbI(3) perovskite devices with different oxygen plasma-treated polymers such as P3HT, P3OT, polyTPD, or PTAA were fabricated with power conversion efficiencies of up to 19%. KW - organohalide lead perovskites KW - solar cells KW - surface wetting KW - work function KW - oxygen plasma KW - transport layer Y1 - 2018 U6 - https://doi.org/10.1021/acsami.8b02503 SN - 1944-8244 VL - 10 IS - 25 SP - 21681 EP - 21687 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Zhang, Shanshan A1 - Hosseini, Seyed Mehrdad A1 - Gunder, Rene A1 - Petsiuk, Andrei A1 - Caprioglio, Pietro A1 - Wolff, Christian Michael A1 - Shoaee, Safa A1 - Meredith, Paul A1 - Schorr, Susan A1 - Unold, Thomas A1 - Burn, Paul L. A1 - Neher, Dieter A1 - Stolterfoht, Martin T1 - The Role of Bulk and Interface Recombination in High-Efficiency Low-Dimensional Perovskite Solar Cells JF - Advanced materials N2 - 2D Ruddlesden-Popper perovskite (RPP) solar cells have excellent environmental stability. However, the power conversion efficiency (PCE) of RPP cells remains inferior to 3D perovskite-based cells. Herein, 2D (CH3(CH2)(3)NH3)(2)(CH3NH3)(n-1)PbnI3n+1 perovskite cells with different numbers of [PbI6](4-) sheets (n = 2-4) are analyzed. Photoluminescence quantum yield (PLQY) measurements show that nonradiative open-circuit voltage (V-OC) losses outweigh radiative losses in materials with n > 2. The n = 3 and n = 4 films exhibit a higher PLQY than the standard 3D methylammonium lead iodide perovskite although this is accompanied by increased interfacial recombination at the top perovskite/C-60 interface. This tradeoff results in a similar PLQY in all devices, including the n = 2 system where the perovskite bulk dominates the recombination properties of the cell. In most cases the quasi-Fermi level splitting matches the device V-OC within 20 meV, which indicates minimal recombination losses at the metal contacts. The results show that poor charge transport rather than exciton dissociation is the primary reason for the reduction in fill factor of the RPP devices. Optimized n = 4 RPP solar cells had PCEs of 13% with significant potential for further improvements. KW - 2D perovskites KW - interface recombination KW - perovskite solar cells KW - photoluminescence KW - V-OC loss Y1 - 2019 U6 - https://doi.org/10.1002/adma.201901090 SN - 0935-9648 SN - 1521-4095 VL - 31 IS - 30 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Zhang, Ning A1 - Hayer, Anna A1 - Al-Suti, Mohammed K. A1 - Al-Belushi, Rayya A. A1 - Khan, Muhammad S. A1 - Köhler, Anna T1 - The effect of delocalization on the exchange energy in meta- and para-linked Pt-containing carbazole polymers and monomers N2 - A series of novel platinum-containing carbazole monomers and polymers was synthesized and fully characterized by UV-VIS absorption, luminescence, and photoinduced absorption studies. In these compounds, a carbazole unit is incorporated into the main chain via either a para- or a meta-linkage. We discuss the effects of linkage and polymerization on the energy levels of S-1, T-1, and T-n. The S-1-T-1 splitting observed for the meta-linked monomer (0.4 eV) is only half of that in the para-linked monomer (0.8 eV). Upon polymerization, the exchange energy in the para- linked compound reduces, yet still remains larger than in the meta-linked polymer. We attribute the difference in exchange energy to the difference in wave function overlap between electron and hole in these compounds. (c) 2006 American Institute of Physics Y1 - 2006 UR - http://scitation.aip.org/getpdf/servlet/ GetPDFServlet?filetype=pdf&id=JCPSA6000124000024244701000001&idtype=cvips&doi=10.1063/1.2200351&prog=normal U6 - https://doi.org/10.1063/1.2200351 ER - TY - JOUR A1 - Zhang, Kai A1 - Chen, Zhiming A1 - Armin, Ardalan A1 - Dong, Sheng A1 - Xia, Ruoxi A1 - Yip, Hin-Lap A1 - Shoaee, Safa A1 - Huang, Fei A1 - Cao, Yong T1 - Efficient large area organic solar cells processed by blade-coating with single-component green solvent JF - Solar Rrl N2 - While the performance of laboratory-scale organic solar cells (OSCs) continues to grow, development of high efficiency large area OSCs remains a big challenge. Although a few attempts to produce large area organic solar cells (OSCs) have been reported, there are still challenges on the way to realizing efficient module devices, such as the low compatibility of the thickness-sensitive active layer with large area coating techniques, the frequent need for toxic solvents and tedious optimization processes used during device fabrication. In this work, highly efficient thickness-insensitive OSCs based on PTB7-Th:PC71BM that processed with single-component green solvent 2-methylanisole are presented, in which both junction thickness limitation and solvent toxicity issues are simultaneously addressed. Careful investigation reveals that this green solvent prevents the evolution of PC71BM into large area clusters resulting in reduced charge carrier recombination, and largely eliminates trapping centers, and thus improves the thickness tolerance of the films. These findings enable us to address the scalability and solvent toxicity issues and to fabricate a 16 cm(2) OSC with doctor-blade coating with a state-of-the-art power conversion efficiency of 7.5% using green solvent. KW - doctor-blade coating KW - green solvents KW - large area devices KW - organic solar cells KW - thickness insensitive active layers Y1 - 2017 U6 - https://doi.org/10.1002/solr.201700169 SN - 2367-198X VL - 2 IS - 1 PB - Wiley-VCH CY - Weinheim ER - TY - JOUR A1 - Zhang, Heshou A1 - Yan, Huirong A1 - Richter, Philipp T1 - The influence of atomic alignment on absorption and emission spectroscopy JF - Monthly notices of the Royal Astronomical Society N2 - Spectroscopic observations play essential roles in astrophysics. They are crucial for determining physical parameters in our Universe, providing information about the chemistry of various astronomical environments. The proper execution of the spectroscopic analysis requires accounting for all the physical effects that are compatible to the signal-to-noise ratio. We find in this paper the influence on spectroscopy from the atomic/ground state alignment owing to anisotropic radiation and modulated by interstellar magnetic field, has significant impact on the study of interstellar gas. In different observational scenarios, we comprehensively demonstrate how atomic alignment influences the spectral analysis and provide the expressions for correcting the effect. The variations are even more pronounced for multiplets and line ratios. We show the variation of the deduced physical parameters caused by the atomic alignment effect, including alpha-to-iron ratio ([X/Fe]) and ionization fraction. Synthetic observations are performed to illustrate the visibility of such effect with current facilities. A study of Photodissociation regions in rho Ophiuchi cloud is presented to demonstrate how to account for atomic alignment in practice. Our work has shown that due to its potential impact, atomic alignment has to be included in an accurate spectroscopic analysis of the interstellar gas with current observational capability. KW - magnetic fields KW - submillimetre: ISM KW - ultraviolet: ISM KW - ISM: abundances KW - ISM: lines and bands KW - techniques: spectroscopic Y1 - 2018 U6 - https://doi.org/10.1093/mnras/sty1594 SN - 0035-8711 SN - 1365-2966 VL - 479 IS - 3 SP - 3923 EP - 3935 PB - Oxford Univ. Press CY - Oxford ER - TY - JOUR A1 - Zhang, Heshou A1 - Yan, Huirong T1 - Polarization of submillimetre lines from interstellar medium JF - Monthly notices of the Royal Astronomical Society N2 - Magnetic fields play important roles in many astrophysical processes. However, there is no universal diagnostic for the magnetic fields in the interstellar medium (ISM) and each magnetic tracer has its limitation. Any new detection method is thus valuable. Theoretical studies have shown that submillimetre fine-structure lines are polarized due to atomic alignment by ultraviolet photon-excitation, which opens up a new avenue to probe interstellar magnetic fields. We will, for the first time, perform synthetic observations on the simulated three-dimensional ISM to demonstrate the measurability of the polarization of submillimetre atomic lines. The maximum polarization for different absorption and emission lines expected from various sources, including star-forming regions are provided. Our results demonstrate that the polarization of submillimetre atomic lines is a powerful magnetic tracer and add great value to the observational studies of the submilimetre astronomy. KW - polarization KW - turbulence KW - H II regions KW - ISM: magnetic fields KW - photodissociation region (PDR) KW - submillimetre: ISM Y1 - 2017 U6 - https://doi.org/10.1093/mnras/stx3164 SN - 0035-8711 SN - 1365-2966 VL - 475 IS - 2 SP - 2415 EP - 2420 PB - Oxford University Press CY - Oxford ER - TY - THES A1 - Zhang, Heshou T1 - Magnetic fields in the universe BT - Diagnostics, turbulent properties, and their implications N2 - The galactic interstellar medium is magnetized and turbulent. The magnetic field and turbulence play important roles in many astrophysical mechanisms, including cosmic ray transport, star formation, etc. Therefore, measurements of magnetic field and turbulence information are crucial for the proper interpretation of astronomical observations. Nonetheless, the magnetic field observation is quite challenging, especially, there is not universal magnetic tracer for diffuse medium. Moreover, the modelling of turbulence can be oversimplified due to the lack of observational tools to diagnose the plasma properties of the turbulence in the galactic interstellar medium. The studies presented in this thesis have addressed these challenges by bridging the theoretical studies of magnetic field and turbulence with numerical simulations and observations. The following research are presented in this thesis. The first observational evidence of the novel magnetic tracer, ground state alignment (GSA), is discovered, revealing the three-dimensional magnetic field as well as 2 orders of magnitude higher precision comparing to previous observational study in the stellar atmosphere of the post-AGB 89 Herculis. Moreover, the application of GSA in the sub-millimeter fine-structure lines is comprehensively studied for different elements and with magnetohydrodynamic simulations. Furthermore, the influence of GSA effect on the spectroscopy is analyzed and it is found that measurable variation will be produced on the spectral line intensity and the line ratio without accounting for the optical pumping process or magnetic field. Additionally, a novel method to measure plasma modes in the interstellar medium, Signatures from Polarization Analysis (SPA), is proposed and applied to real observations. Magneto-sonic modes are discovered in different types of interstellar medium. An explanation is provided for the long-standing mystery, the origin of γ-ray enhanced emission “Cygnus Cocoon”, based on the comparison between the outcome of SPA and multi-waveband observational data. These novel methods have strong potentials for broader observational applications and will play crucial roles in future multi-wavelength astronomy. N2 - Das interstellare Medium ist magnetisiert und turbulent. Das Magnetfeld und die Turbulenz spielen eine wichtige Rolle in vielen astrophysikalischen Prozessen, unter anderem beim Transport kosmischer Strahlung, bei der Entstehung von Sternen usw. Daher sind Messungen des Magnetfelds und der Turbulenz entscheidend bei der korrekten Interpretation astronomischer Beobachtungen. Dennoch ist die Messung des astronomischen Magnetfeldes eine große Herausforderung, besonders da es keinen universellen magnetischen Tracer für diffuse Medien gibt. Außerdem kann es aufgrund des Mangels an geeigneten Beobachtungswerkzeugen für die Plasmaeigenschaften der galaktischen Turbulenz zu übermäßigen Vereinfachungen bei der Turbulenzmodellierung kommen. Die in dieser Arbeit vorgestellten Forschungsergebnisse beschäftigen sich mit diesen Herausforderungen; die Theorien des Magnetfeldes und der Turbulenz werden durch numerische Simulationen und Beobachtungen anwendbar gemacht. Die erste Anwendung eines neuen magnetischen Markers, Ground State Alignment (GSA, deutsch „Grundzustandsausrichtung“), wird vorgestellt. Dabei wird das dreidimensionale Magnetfeld in der Analyse von Beobachtungsdaten aus der stellaren Atmosphäre des post-AGB 89 Herculis um zwei Größenordnungen genauer bestimmt als bei den bisherigen Methoden. Zusätzlich wird die Anwendung von GSA bei der Analyse von sub-millimeter Feinstrukturlinien ausführlich für verschiedene Elemente mit Hilfe von magnetohydrodynamischen Simulationen getestet. Auch der Einfluss von GSA-Effekten auf spektroskopische Analysen wird untersucht; dabei stellt sich heraus, dass messbare Variationen in der Linienbreite und im Verhältnis der Linien verursacht werden, ohne dass optische Pumpprozesse oder das Magnetfeld berücksichtigt werden. Des Weiteren wird die Signatures from Polarization Analysis (SPA, deutsch „Signatur-aus-Polarisationsanalyse“) als neue Messmethode für Plasmamoden im galaktischen Medium vorgestellt und auf reale Beobachtungen angewandt. In verschiedenen galaktischen Medien werden magneto-akustische Moden gefunden. Durch den Vergleich von Ergebnissen der SPA mit Beobachtungsdaten aus verschiedenen elektromagnetischen Frequenzbereichen kann die bisher ungeklärte Herkunft der gammastrahlenverstärkten Emission „Cygnus Cocoon“ erklärt werden. Diese neuen Methoden haben das Potential für eine breitere Anwendung bei der Analyse von Beobachtungen und werden in der Zukunft eine wichtige Rolle in der Multiwellenlängen-Astronomie spielen. KW - magnetic field KW - turbulence KW - polarization KW - interstellar medium KW - cosmic ray propagation Y1 - 2021 ER - TY - JOUR A1 - Zhang, Haocheng A1 - Chen, Xuhui A1 - Böttcher, Markus A1 - Guo, Fan A1 - Li, Hui T1 - Polarization swings reveal magnetic energy dissipation in blazars JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - The polarization signatures of blazar emissions are known to be highly variable. In addition to small fluctuations of the polarization angle around a mean value, large (greater than or similar to 180 degrees) polarization angle swings are sometimes observed. We suggest that such phenomena can be interpreted as arising from light travel time effects within an underlying axisymmetric emission region. We present the first simultaneous fitting of the multi-wavelength spectrum, variability, and time-dependent polarization features of a correlated optical and gamma-ray flaring event of the prominent blazar 3C279, which was accompanied by a drastic change in its polarization signatures. This unprecedented combination of spectral, variability, and polarization information in a coherent physical model allows us to place stringent constraints on the particle acceleration and magnetic field topology in the relativistic jet of a blazar, strongly favoring a scenario in which magnetic energy dissipation is the primary driver of the flare event. KW - galaxies: active KW - galaxies: jets KW - gamma-rays: galaxies KW - radiation mechanisms: nonthermal KW - relativistic processes Y1 - 2015 U6 - https://doi.org/10.1088/0004-637X/804/1/58 SN - 0004-637X SN - 1538-4357 VL - 804 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Zhang, Haocheng A1 - Chen, Xuhui A1 - Boettcher, Markus T1 - Synchrotron polarization in blazars JF - The astrophysical journal : an international review of spectroscopy and astronomical physics N2 - We present a detailed analysis of time-and energy-dependent synchrotron polarization signatures in a shock-in-jet model for gamma-ray blazars. Our calculations employ a full three-dimensional radiation transfer code, assuming a helical magnetic field throughout the jet. The code considers synchrotron emission from an ordered magnetic field, and takes into account all light-travel-time and other relevant geometric effects, while the relevant synchrotron self-Compton and external Compton effects are handled with the two-dimensional Monte-Carlo/Fokker-Planck (MCFP) code. We consider several possible mechanisms through which a relativistic shock propagating through the jet may affect the jet plasma to produce a synchrotron and high-energy flare. Most plausibly, the shock is expected to lead to a compression of the magnetic field, increasing the toroidal field component and thereby changing the direction of the magnetic field in the region affected by the shock. We find that such a scenario leads to correlated synchrotron + synchrotron-self-Compton flaring, associated with substantial variability in the synchrotron polarization percentage and position angle. Most importantly, this scenario naturally explains large polarization angle rotations by greater than or similar to 180 degrees, as observed in connection with gamma-ray flares in several blazars, without the need for bent or helical jet trajectories or other nonaxisymmetric jet features. KW - galaxies: active KW - galaxies: jets KW - gamma rays: galaxies KW - radiation mechanisms: non-thermal KW - relativistic processes Y1 - 2014 U6 - https://doi.org/10.1088/0004-637X/789/1/66 SN - 0004-637X SN - 1538-4357 VL - 789 IS - 1 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Zhang, H. A1 - Hu, B. A1 - Hu, G. A1 - Ouyang, Q. A1 - Kurths, Jürgen T1 - Turbulence control by developing a spiral wave with a periodic signal injection in the complex Ginzburg-Laundau equation Y1 - 2002 ER - TY - JOUR A1 - Zhang, Chaoli A1 - Li, Chengyuan A1 - de Grijs, Richard A1 - Bekki, Kenji A1 - Deng, Licai A1 - Zaggia, Simone A1 - Rubele, Stefano A1 - Piatti, Andres E. A1 - Cioni, Maria-Rosa L. A1 - Emerson, Jim A1 - For, Bi-Qing A1 - Ripepi, Vincenzo A1 - Marconi, Marcella A1 - Ivanov, Valentin D. A1 - Chen, Li T1 - The vmc survey. XVIII. radial dependence of the Low-Mass, 0.55-0.82M(circle dot) stellar mass function in the galactic globular cluster 47 tucanae JF - The astrophysical journal : an international review of spectroscopy and astronomical physics KW - galaxies: clusters: individual (47 Tucanae) KW - globular clusters: general KW - Hertzsprung-Russell and C-M diagrams KW - stars: low-mass KW - stars: luminosity function, mass function Y1 - 2015 U6 - https://doi.org/10.1088/0004-637X/815/2/95 SN - 0004-637X SN - 1538-4357 VL - 815 IS - 2 PB - IOP Publ. Ltd. CY - Bristol ER - TY - JOUR A1 - Zhang, Bo A1 - Henkel, Carsten T1 - Magnetic noise around metallic microstructures N2 - We compute the local spectrum of the magnetic field near a metallic microstructure at finite temperature. Our main focus is on deviations from a plane-layered geometry for which we review the main properties. Arbitrary geometries are handled with the help of numerical calculations based on surface integral equations. The magnetic noise shows a significant polarization anisotropy above flat wires with finite lateral width, in stark contrast to an infinitely wide wire. Within the limits of a two-dimensional setting, our results provide accurate estimates for loss and dephasing rates in so-called `atom chip traps' based on metallic wires. A simple approximation based on the incoherent summation of local current elements gives qualitative agreement with the numerics, but fails to describe current correlations among neighboring objects. Y1 - 2007 UR - http://dx.doi.org/10.1063/1.2800174 U6 - https://doi.org/10.1063/1.2800174 SN - 0021-8979 ER - TY - THES A1 - Zhang, Bo T1 - Magnetic fields near microstructured surfaces : application to atom chips T1 - Magnetische Felder in der Nähe von microstrukturierten Oberflächen : Anwendung auf Atomchips N2 - Microfabricated solid-state surfaces, also called atom chip', have become a well-established technique to trap and manipulate atoms. This has simplified applications in atom interferometry, quantum information processing, and studies of many-body systems. Magnetic trapping potentials with arbitrary geommetries are generated with atom chip by miniaturized current-carrying conductors integrated on a solid substrate. Atoms can be trapped and cooled to microKelvin and even nanoKelvin temperatures in such microchip trap. However, cold atoms can be significantly perturbed by the chip surface, typically held at room temperature. The magnetic field fluctuations generated by thermal currents in the chip elements may induce spin flips of atoms and result in loss, heating and decoherence. In this thesis, we extend previous work on spin flip rates induced by magnetic noise and consider the more complex geometries that are typically encountered in atom chips: layered structures and metallic wires of finite cross-section. We also discuss a few aspects of atom chips traps built with superconducting structures that have been suggested as a means to suppress magnetic field fluctuations. The thesis describes calculations of spin flip rates based on magnetic Green functions that are computed analytically and numerically. For a chip with a top metallic layer, the magnetic noise depends essentially on the thickness of that layer, as long as the layers below have a much smaller conductivity. Based on this result, scaling laws for loss rates above a thin metallic layer are derived. A good agreement with experiments is obtained in the regime where the atom-surface distance is comparable to the skin depth of metal. Since in the experiments, metallic layers are always etched to separate wires carrying different currents, the impact of the finite lateral wire size on the magnetic noise has been taken into account. The local spectrum of the magnetic field near a metallic microstructure has been investigated numerically with the help of boundary integral equations. The magnetic noise significantly depends on polarizations above flat wires with finite lateral width, in stark contrast to an infinitely wide wire. Correlations between multiple wires are also taken into account. In the last part, superconducting atom chips are considered. Magnetic traps generated by superconducting wires in the Meissner state and the mixed state are studied analytically by a conformal mapping method and also numerically. The properties of the traps created by superconducting wires are investigated and compared to normal conducting wires: they behave qualitatively quite similar and open a route to further trap miniaturization, due to the advantage of low magnetic noise. We discuss critical currents and fields for several geometries. N2 - Mikrotechnologische Oberflächen, sogenannte Atomchips, sind eine etablierte Methode zum Speichern und Manipulieren von Atomen geworden. Das hat Anwendungen in der Atom-Interferometrie, Quanteninformationsverarbeitung und Vielteilchensystemen vereinfacht. Magnetische Fallenpotentiale mit beliebigen Geometrien werden durch Atomchips mit miniaturisierten stromführenden Leiterbahnen auf einer Festkörperunterlage realisiert. Atome können bei Temperaturen im $\mu$ K oder sogar nK-Bereich in einer solchen Falle gespeichert und gekühlt werden. Allerdings können kalte Atome signifikant durch die Chip-Oberfläche gestört werden, die sich typischerweise auf Raumtemperatur befindet. Die durch thermische Ströme im Chip erzeugten magnetischen Feldfluktuationen können Spin-Flips der Atome induzieren und Verlust, Erwärmung und Dekohärenz zur Folge haben. In dieser Dissertation erweitern wir frühere Arbeiten über durch magnetisches Rauschen induzierte Spin-Flip-Ratenund betrachten kompliziertere Geometrien, wie sie typischerweise auf einem Atom-Chip anzutreffen sind: Geschichtete Strukturen und metallische Leitungen mit endlichem Querschnitt. Wir diskutieren auch einige Aspekte von Aomchips aus Supraleitenden Strukturen die als Mittel zur Unterdrückung magnetischer Feldfluktuationen vorgeschlagen wurden. Die Arbeit beschreibt analytische und numerische Rechnungen von Spin-Flip Raten auf Grundlage magnetischer Greensfunktionen. Für einen Chip mit einem metallischen Top-Layer hängt das magnetische Rauschen hauptsächlich von der Dicke des Layers ab, solange die unteren Layer eine deutlich kleinere Leitfähigkeit haben. Auf Grundlage dieses Ergebnisses werden Skalengesetze für Verlustraten über einem dünnen metallischen Leiter hergeleitet. Eine gute Übereinstimmung mit Experimenten wird in dem Bereich erreicht, wo der Abstand zwischen Atom und Oberfläche in der Größenordnung der Eindringtiefe des Metalls ist. Da in Experimenten metallische Layer immer geätzt werden, um verschiedene stromleitende Bahnen vonenander zu trennen, wurde der Einfluß eines endlichen Querschnittsauf das magnetische Rauschen berücksichtigt. Das lokale Spektrum des magnetischen Feldes in der Nähe einer metallischen Mikrostruktur wurde mit Hilfe von Randintegralen numerisch untersucht. Das magnetische Rauschen hängt signifikant von der Polarisierung über flachen Leiterbahnen mit endlichem Querschnitt ab, im Unterschied zu einem unendlich breiten Leiter. Es wurden auch Korrelationen zwischen mehreren Leitern berücksichtigt. Im letzten Teil werden supraleitende Atomchips betrachtet. Magnetische Fallen, die von supraleitenden Bahnen im Meissner Zustand und im gemischten Zustand sind werden analytisch durch die Methode der konformen Abbildung und numerisch untersucht. Die Eigenschaften der durch supraleitende Bahnen erzeugten Fallen werden erforscht und mit normal leitenden verglichen: Sie verhalten sich qualitativ sehr ähnlich und öffnen einen Weg zur weiteren Miniaturisierung von Fallen, wegen dem Vorteil von geringem magnetischem Rauschen. Wir diskutieren kritische Ströme und Felder für einige Geometrien. KW - Magnetische Felder KW - Atom chip KW - Supraleiter KW - magnetisches Rauschen KW - microstrukturierte Oberfläche KW - magnetic fields KW - atom chip KW - superconductors KW - magnetic noise KW - microstructured surface Y1 - 2008 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-28984 ER - TY - JOUR A1 - Zhang, B. A1 - Henkel, Carsten A1 - Haller, E. A1 - Wildermuth, S. A1 - Hofferberth, S. A1 - Kruger, P. A1 - Schmiedmayer, Jörg T1 - Relevance of sub-surface chip layers for the lifetime of magnetically trapped atoms N2 - We investigate the lifetime of magnetically trapped atoms above a planar, layered atom chip structure. Numerical calculations of the thermal magnetic noise spectrum are performed, based on the exact magnetic Green function and multi layer reflection coefficients. We have performed lifetime measurements where the center of a side guide trap is laterally shifted with respect to the current carrying wire using additional bias fields. Comparing the experiment to theory, we find a fair agreement and demonstrate that for a chip whose topmost layer is metallic, the magnetic noise depends essentially on the thickness of that layer, as long as the layers below have a, much smaller conductivity; essentially the same magnetic noise would be obtained with a metallic membrane suspended in vacuum. Based on our theory we give general scaling laws of how to reduce the effect of surface magnetic noise on the trapped atoms Y1 - 2005 ER - TY - JOUR A1 - Zeuschner, Steffen Peer A1 - Wang, Xi-Guang A1 - Deb, Marwan A1 - Popova, Elena A1 - Malinowski, Gregory A1 - Hehn, Michel A1 - Keller, Niels A1 - Berakdar, Jamal A1 - Bargheer, Matias T1 - Standing spin wave excitation in Bi BT - YIG films via temperature-induced anisotropy changes and magneto-elastic coupling JF - Physical review : B, Condensed matter and materials physics N2 - Based on micromagnetic simulations and experimental observations of the magnetization and lattice dynamics after the direct optical excitation of the magnetic insulator Bi : YIG or indirect excitation via an optically opaque Pt/Cu double layer, we disentangle the dynamical effects of magnetic anisotropy and magneto-elastic coupling. The strain and temperature of the lattice are quantified via modeling ultrafast x-ray diffraction data. Measurements of the time-resolved magneto-optical Kerr effect agree well with the magnetization dynamics simulated according to the excitation via two mechanisms: the magneto-elastic coupling to the experimentally verified strain dynamics and the ultrafast temperature-induced transient change in the magnetic anisotropy. The numerical modeling proves that, for direct excitation, both mechanisms drive the fundamental mode with opposite phase. The relative ratio of standing spin wave amplitudes of higher-order modes indicates that both mechanisms are substantially active. Y1 - 2022 U6 - https://doi.org/10.1103/PhysRevB.106.134401 SN - 2469-9950 SN - 2469-9969 VL - 106 IS - 13 PB - American Physical Society CY - College Park ER - TY - GEN A1 - Zeuschner, Steffen Peer A1 - Mattern, Maximilian A1 - Pudell, Jan-Etienne A1 - Reppert, Alexander von A1 - Rössle, Matthias A1 - Leitenberger, Wolfram A1 - Schwarzkopf, Jutta A1 - Boschker, Jos A1 - Herzog, Marc A1 - Bargheer, Matias T1 - Reciprocal space slicing BT - a time-efficient approach to femtosecond x-ray diffraction T2 - Postprints der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe N2 - An experimental technique that allows faster assessment of out-of-plane strain dynamics of thin film heterostructures via x-ray diffraction is presented. In contrast to conventional high-speed reciprocal space-mapping setups, our approach reduces the measurement time drastically due to a fixed measurement geometry with a position-sensitive detector. This means that neither the incident (ω) nor the exit (2θ) diffraction angle is scanned during the strain assessment via x-ray diffraction. Shifts of diffraction peaks on the fixed x-ray area detector originate from an out-of-plane strain within the sample. Quantitative strain assessment requires the determination of a factor relating the observed shift to the change in the reciprocal lattice vector. The factor depends only on the widths of the peak along certain directions in reciprocal space, the diffraction angle of the studied reflection, and the resolution of the instrumental setup. We provide a full theoretical explanation and exemplify the concept with picosecond strain dynamics of a thin layer of NbO2. T3 - Zweitveröffentlichungen der Universität Potsdam : Mathematisch-Naturwissenschaftliche Reihe - 1137 Y1 - 2021 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-499761 SN - 1866-8372 IS - 1137 ER - TY - JOUR A1 - Zeuschner, Steffen Peer A1 - Mattern, Maximilian A1 - Pudell, Jan-Etienne A1 - Reppert, Alexander von A1 - Rössle, Matthias A1 - Leitenberger, Wolfram A1 - Schwarzkopf, Jutta A1 - Boschker, Jos A1 - Herzog, Marc A1 - Bargheer, Matias T1 - Reciprocal space slicing BT - a time-efficient approach to femtosecond x-ray diffraction JF - Structural dynamics N2 - An experimental technique that allows faster assessment of out-of-plane strain dynamics of thin film heterostructures via x-ray diffraction is presented. In contrast to conventional high-speed reciprocal space-mapping setups, our approach reduces the measurement time drastically due to a fixed measurement geometry with a position-sensitive detector. This means that neither the incident (ω) nor the exit (2θ) diffraction angle is scanned during the strain assessment via x-ray diffraction. Shifts of diffraction peaks on the fixed x-ray area detector originate from an out-of-plane strain within the sample. Quantitative strain assessment requires the determination of a factor relating the observed shift to the change in the reciprocal lattice vector. The factor depends only on the widths of the peak along certain directions in reciprocal space, the diffraction angle of the studied reflection, and the resolution of the instrumental setup. We provide a full theoretical explanation and exemplify the concept with picosecond strain dynamics of a thin layer of NbO2. Y1 - 2020 U6 - https://doi.org/10.1063/4.0000040 SN - 2329-7778 VL - 8 IS - 1 PB - AIP Publishing LLC CY - Melville, NY ER - TY - THES A1 - Zeuschner, Steffen Peer T1 - Magnetoacoustics observed with ultrafast x-ray diffraction N2 - In the present thesis I investigate the lattice dynamics of thin film hetero structures of magnetically ordered materials upon femtosecond laser excitation as a probing and manipulation scheme for the spin system. The quantitative assessment of laser induced thermal dynamics as well as generated picosecond acoustic pulses and their respective impact on the magnetization dynamics of thin films is a challenging endeavor. All the more, the development and implementation of effective experimental tools and comprehensive models are paramount to propel future academic and technological progress. In all experiments in the scope of this cumulative dissertation, I examine the crystal lattice of nanoscale thin films upon the excitation with femtosecond laser pulses. The relative change of the lattice constant due to thermal expansion or picosecond strain pulses is directly monitored by an ultrafast X-ray diffraction (UXRD) setup with a femtosecond laser-driven plasma X-ray source (PXS). Phonons and spins alike exert stress on the lattice, which responds according to the elastic properties of the material, rendering the lattice a versatile sensor for all sorts of ultrafast interactions. On the one hand, I investigate materials with strong magneto-elastic properties; The highly magnetostrictive rare-earth compound TbFe2, elemental Dysprosium or the technological relevant Invar material FePt. On the other hand I conduct a comprehensive study on the lattice dynamics of Bi1Y2Fe5O12 (Bi:YIG), which exhibits high-frequency coherent spin dynamics upon femtosecond laser excitation according to the literature. Higher order standing spinwaves (SSWs) are triggered by coherent and incoherent motion of atoms, in other words phonons, which I quantified with UXRD. We are able to unite the experimental observations of the lattice and magnetization dynamics qualitatively and quantitatively. This is done with a combination of multi-temperature, elastic, magneto-elastic, anisotropy and micro-magnetic modeling. The collective data from UXRD, to probe the lattice, and time-resolved magneto-optical Kerr effect (tr-MOKE) measurements, to monitor the magnetization, were previously collected at different experimental setups. To improve the precision of the quantitative assessment of lattice and magnetization dynamics alike, our group implemented a combination of UXRD and tr-MOKE in a singular experimental setup, which is to my knowledge, the first of its kind. I helped with the conception and commissioning of this novel experimental station, which allows the simultaneous observation of lattice and magnetization dynamics on an ultrafast timescale under identical excitation conditions. Furthermore, I developed a new X-ray diffraction measurement routine which significantly reduces the measurement time of UXRD experiments by up to an order of magnitude. It is called reciprocal space slicing (RSS) and utilizes an area detector to monitor the angular motion of X-ray diffraction peaks, which is associated with lattice constant changes, without a time-consuming scan of the diffraction angles with the goniometer. RSS is particularly useful for ultrafast diffraction experiments, since measurement time at large scale facilities like synchrotrons and free electron lasers is a scarce and expensive resource. However, RSS is not limited to ultrafast experiments and can even be extended to other diffraction techniques with neutrons or electrons. N2 - In der vorliegenden Arbeit untersuche ich die Gitterdynamik von magnetisch geordneten und dünnen Filmen, deren Spinsystem mit Femtosekunden-Laserpulsen angeregt und untersucht wird. Die Quantifizierung der laserinduzierten thermischen Dynamik, der erzeugten Pikosekunden-Schallpulse sowie deren jeweiliger Einfluss auf die Magnetisierungsdynamik ist ein schwieriges Unterfangen. Umso mehr ist die Entwicklung und Anwendung von effizienten experimentellen Konzepten und umfangreichen Modellen grundlegend für das Antreiben des zukünftigen wissenschaftlichen und technologischen Fortschritt. In jedem Experiment dieser kummulativen Dissertation untersuche ich das Kristallgitter von Nanometer dünnen Filmen nach der Anregung mit Femtosekunden-Laserpulsen. Die relative Änderung der Gitterkonstante, hervorgerufen durch thermische Ausdehnung oder Pikosekunden-Schallpulse, wird dabei direkt mittels ultraschneller Röntgenbeugung (UXRD) gemessen. Der Aufbau nutzt zur Bereitstellung von ultrakurzen Röntgenpulsen eine lasergetriebene Plasma-Röntgenquelle (PXS). Phononen und Spins üben gleichermaßen einen Druck auf das Gitter aus, welches entsprechend der elastsischen Eigenschaften des Materials reagiert, was das Gitter zu einem vielseitigen Sensor für ultraschenlle Wechselwirkungen macht. Zum einen untersuche ich Materialien mit starken magnetoelastischen Eigentschaften: die stark magnetostriktive Seltenen-Erden-Verbindung TbFe2, elementares Dysprosium oder das technologisch relavante Invar-Material FePt. Zum anderen habe ich eine umfangreiche Studie der Gitterdynamik von Bi1Y2Fe5O12 (Bi:YIG) angestellt, in dem der Literatur zufolge hochfrequente kohärente Spindynamiken durch Femtosekunden-Laseranregung zu beobachten sind. Diese stehenden Spinwellen (SSWs) höherer Ordnung entstehen durch die kohärente und inkohärente Bewegung von Atomen, in anderen Worten Phononen, welche ich durch UXRD vermessen habe. Somit sind wir in der Lage, die experimentellen Beobachtungen der Gitter- und Spindynamik qualitativ und quantitativ zu vereinigen. Dies geschieht durch eine Kombination von Viel-Temperatur- und Anisotropiemodellierung sowie elastische, magnetoelastische, und mikromagnetsiche Modelle. Die gemeinsamen Daten von UXRD und der zeitaufgelösten magnetooptischen Kerr-Effekt Messungen (tr-MOKE), um jeweils die Gitter- und Spindynamik zu messen, wurden in der Vergangenheit noch an unterschiedlichen experimentellen Aufbauten gemessen. Um die Quantifizierung präziser zu gestalten, haben wir in unserer Arbeitsgruppe UXRD und tr-MOKE in einem einzigen Aufbau kombiniert, welcher somit meines Wissens der erste seiner Art ist. Ich half bei dem Entwurf und der Inbetriebnahme des neuen Aufbaus, welcher die gleichzeitige Messung von Gitter- und Spindynamik auf einer ultraschnellen Zeitskala unter identischen Anregungsbedingungen ermöglicht. Außerdem entwickelte ich eine neue Messroutine für Röntgenbeugung, welche die Messzeit von UXRD-Experimenten um bis zu einer Größenordnungen reduziert. Es nennt sich das Schneiden des reziproken Raumes (reciprocal space slicing, RSS) und nutzt den Vorteil von Flächendetektoren die Bewegung von Beugungsreflexen zu detektieren, was von einer Änderung der Gitterkonstante einhergeht, ohne zeitintensive Scans der Beugungswinkel mit dem Goniometer durchzuführen. RSS ist besonders nützlich für ultraschnelle Beugungsexperimente, weil die Messzeit an Großgeräten wie Synchrotrons oder Freie Elektronen Laser eine seltene und teure Ressource ist. Darüber hinaus ist RSS nicht zwangsläufig auf die Anwendung in ultraschnellen Experimenten beschränkt und kann sogar auf andere Beugungsexperimente, wie die mit Neutronen und Elektronen, ausgeweitet werden. KW - ultrafast KW - X-ray diffraction KW - thin films KW - magnetoelasticity KW - ultraschnell KW - Röntgenbeugung KW - dünne Filme KW - Magnetoelastizität Y1 - 2022 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus4-561098 ER - TY - JOUR A1 - Zerson, Mario A1 - Neumann, Martin A1 - Steyrleuthner, Robert A1 - Neher, Dieter A1 - Magerle, Robert T1 - Surface Structure of Semicrystalline Naphthalene Diimide-Bithiophene Copolymer Films Studied with Atomic Force Microscopy JF - Macromolecules : a publication of the American Chemical Society Y1 - 2016 U6 - https://doi.org/10.1021/acs.macromol.6b00988 SN - 0024-9297 SN - 1520-5835 VL - 49 SP - 6549 EP - 6557 PB - American Chemical Society CY - Washington ER - TY - JOUR A1 - Zen, Achmad A1 - Saphiannikova, Marina A1 - Neher, Dieter A1 - Asawapirom, Udom A1 - Scherf, Ullrich T1 - Comparative study of the field-effect mobility of a copolymer and a binary blend based on poly(3- alkylthiophene)s N2 - The performance of highly soluble regioregular poly[ (3-hexylthiophene)-co-(3-octylthiophetie)] (P3HTOT) as a semiconducting material in organic field-effect transistors (OFETs) is presented in comparison to that of the corresponding homopolymers. Transistors made from as-prepared layers of P3HTOT exhibit a mobility of ca. 7 x 10(-3) cm(2) V-1 s(-1), which is comparable to the performance of transistors made from as-prepared poly(3-hexylthiophene) (P3HT) and almost 6 times larger than the mobility of transistors prepared with poly(3-octylthiophene) (P3OT). On the other hand, the solubility parameter delta(p) of P3HTOT is close to that of the highly soluble P3OT. Moreover, compared to a physical blend of poly(3-hexylthiophene) and poly(3-octylthiophene), the mobility of P3HTOT devices is almost twice as large and the performance does not degrade upon annealing at elevated temperatures. Therefore, the copolymer approach outlined here may be one promising step toward an optimum balance between a Sufficient processability of the polymers from common organic solvents, a high solid state order, and applicable OFET performances Y1 - 2005 SN - 0897-4756 ER - TY - JOUR A1 - Zen, Achmad A1 - Pflaum, J. A1 - Hirschmann, S. A1 - Zhuang, W. A1 - Jaiser, Frank A1 - Asawapirom, Udom A1 - Rabe, J. P. A1 - Scherf, Ullrich A1 - Neher, Dieter T1 - Effect of molecular weight and annealing of poly (3-hexylthiophene)s on the performance of organic field-effect transistors N2 - The optical, structural, and electrical properties of thin layers made from poly(3-hexylthiophene) (P3HT) samples of different molecular weights are presented. As reported in a previous paper by Kline et al., Adv. Mater 2003, 15, 1519, the mobilities of these layers are a strong function of the molecular weight, with the largest mobility found for the largest molecular weight. Atomic force microscopy studies reveal a complex polycrystalline morphology which changes considerably upon annealing. X-ray studies show the occurrence of a layered phase for all P3HT fractions, especially after annealing at 1.50 degreesC . However, there is no clear correlation between the differences in the transport properties and the data from structural investigations. In order to reveal the processes limiting the mobility in these layers, the transistor properties were investigated as a function of temperature. The mobility decreases continuously with increasing temperatures; with the same trend pronounced thermochromic effects of the P3HT films occur. Apparently, the polymer chains adopt a more twisted, disordered conformation at higher temperatures, leading to interchain transport barriers. We conclude that the backbone conformation of the majority of the bulk material rather than the crystallinity of the layer is the most crucial parameter controlling the charge transport in these P3HT layers. This interpretation is supported by the significant blue-shift of the solid-state absorption spectra with decreasing molecular weight, which is indicative of a larger distortion of the P3HT backbone in the low-molecular weight P3HT layers Y1 - 2004 ER - TY - JOUR A1 - Zen, Achmad A1 - Neher, Dieter A1 - Silmy, Kamel A1 - Hollander, A. A1 - Asawapirom, Udom A1 - Scherf, Ullrich T1 - Improving the performance of organic field effect transistor by optimizing the gate insulator surface N2 - The effect of oxygen plasma treatment and/or silanization with hexamethyldisilazane (HMDS) on the surface chemistry and the morphology of the SiO2-gate insulator were studied with respect to the performance of organic field effect transistors. Using X-ray photoelectron spectroscopy (XPS), it is shown that silanization leads to the growth of a polysiloxane interfacial layer and that longer silanization times increase the thickness of this layer. Most important, silanization reduces the signal from surface contaminations such as oxidized hydrocarbon molecules. In fact, the lowest concentration of these contaminations was found after a combined oxygen plasma/silanization treatment. The results of these investigations were correlated with the characteristic device parameters of polymer field effect transistors with poly(3-hexylthiophene)s as the semiconducting layer. We found that the field effect mobility correlates with the concentration of contaminations as measured by XPS. We, finally, demonstrate that silanization significantly improves the operational stability of the device in air compared to the untreated devices Y1 - 2005 ER - TY - JOUR A1 - Zen, Achmad A1 - Neher, Dieter A1 - Bauer, C. A1 - Asawapirom, Udom A1 - Scherf, Ullrich A1 - Hagen, R. A1 - Kostromine, S. A1 - Mahrt, R. F. T1 - Polarization-sensitive photoconductivity in aligned polyfluorene layers Y1 - 2002 ER - TY - JOUR A1 - Zen, Achmad A1 - Bilge, Askin A1 - Galbrecht, Frank A1 - Alle, Ronald A1 - Meerholz, Klaus A1 - Grenzer, Jörg A1 - Neher, Dieter A1 - Scherf, Ullrich A1 - Farrell, Tony T1 - Solution processable organic field-effect transistors utilizing an alpha,alpha '-dihexylpentathiophene- based swivel cruciform Y1 - 2006 UR - http://pubs.acs.org/doi/full/10.1021/ja0573357 U6 - https://doi.org/10.1021/Ja0573357 ER - TY - GEN A1 - Zen, Achmad T1 - Charge Transport in Poly(3-hexylthiophene) and in Highly Soluble Obligothiophene Field-Effect Transistors Y1 - 2006 CY - Potsdam ER - TY - THES A1 - Zemanová, Lucia T1 - Structure-function relationship in hierarchical model of brain networks T1 - Das Verhältnis von Struktur und Funktion in hierarchischem Model der Hirn-Netzwerken N2 - The mammalian brain is, with its numerous neural elements and structured complex connectivity, one of the most complex systems in nature. Recently, large-scale corticocortical connectivities, both structural and functional, have received a great deal of research attention, especially using the approach of complex networks. Here, we try to shed some light on the relationship between structural and functional connectivities by studying synchronization dynamics in a realistic anatomical network of cat cortical connectivity. We model the cortical areas by a subnetwork of interacting excitable neurons (multilevel model) and by a neural mass model (population model). With weak couplings, the multilevel model displays biologically plausible dynamics and the synchronization patterns reveal a hierarchical cluster organization in the network structure. We can identify a group of brain areas involved in multifunctional tasks by comparing the dynamical clusters to the topological communities of the network. With strong couplings of multilevel model and by using neural mass model, the dynamics are characterized by well-defined oscillations. The synchronization patterns are mainly determined by the node intensity (total input strengths of a node); the detailed network topology is of secondary importance. The biologically improved multilevel model exhibits similar dynamical patterns in the two regimes. Thus, the study of synchronization in a multilevel complex network model of cortex can provide insights into the relationship between network topology and functional organization of complex brain networks. N2 - Das Gehirn von Säugetieren stellt mit seinen zahlreichen, hochgradig vernetzten Neuronen ein natürliches Netzwerk von immenser Komplexität dar. In der jüngsten Vergangenheit sind die großflächige kortikale Konnektivitäten, sowohl unter strukturellen wie auch funktionalen Gesichtspunkten, in den Fokus der Forschung getreten. Die Verwendung von komplexe Netzwerke spielt hierbei eine entscheidende Rolle. In der vorliegenden Dissertation versuchen wir, das Verhältnis von struktureller und funktionaler Konnektivität durch Untersuchung der Synchronisationsdynamik anhand eines realistischen Modells der Konnektivität im Kortex einer Katze näher zu beleuchten. Wir modellieren die Kortexareale durch ein Subnetzwerk interagierender, erregbarer Neuronen (multilevel model) und durch ein Modell von Neuronenensembles (population model). Bei schwacher Kopplung zeigt das multilevel model eine biologisch plausible Dynamik und die Synchronisationsmuster lassen eine hierarchische Organisation der Netzwerkstruktur erkennen. Indem wir die dynamischen Cluster mit den topologischen Einheiten des Netzwerks vergleichen, sind wir in der Lage die Hirnareale, die an der Bewältigung komplexer Aufgaben beteiligt sind, zu identifizieren. Bei starker Kopplung im multilevel model und unter Verwendung des Ensemblemodells weist die Dynamik klare Oszillationen auf. Die Synchronisationsmuster werden hauptsächlich durch die Eingangsstärke an den einzelnen Knoten bestimmt, während die genaue Netzwerktopologie zweitrangig ist. Eine Erweiterung des Modells auf andere biologisch relevante Faktoren bestätigt die vorherigen Ergebnisse. Die Untersuchung der Synchronisation in einem multilevel model des Kortex ermöglicht daher tiefere Einblicke in die Zusammenhänge zwischen Netzwerktopologie und funktionaler Organisation in komplexen Hirn-Netzwerken. KW - komplexe Hirnnetzwerke KW - Verhältnis der Struktur und Funktion KW - hierarchisches Model KW - Synchronization KW - complex brain networks KW - structur-function relationship KW - hierarchical model KW - synchronization Y1 - 2007 U6 - http://nbn-resolving.de/urn/resolver.pl?urn:nbn:de:kobv:517-opus-18400 ER - TY - JOUR A1 - Zemanova, Lucia A1 - Zhou, Changsong A1 - Kurths, Jürgen T1 - Structural and functional clusters of complex brain networks JF - Physica, D, Nonlinear phenomena N2 - Recent research using the complex network approach has revealed a rich and complicated network topology in the cortical connectivity of mammalian brains. It is of importance to understand the implications of such complex network structures in the functional organization of the brain activities. Here we study this problem from the viewpoint of dynamical complex networks. We investigate synchronization dynamics on the corticocortical network of the cat by modeling each node (cortical area) of the network with a sub-network of interacting excitable neurons. We find that the network displays clustered synchronization behavior, and the dynamical clusters coincide with the topological community structures observed in the anatomical network. Our results provide insights into the relationship between the global organization and the functional specialization of the brain cortex. KW - cortical network KW - anatomical connectivity KW - functional connectivity KW - topological community KW - dynamical cluster Y1 - 2006 U6 - https://doi.org/10.1016/j.physd.2006.09.008 SN - 0167-2789 SN - 1872-8022 VL - 224 IS - 1-2 SP - 202 EP - 212 PB - Elsevier CY - Amsterdam ER - TY - JOUR A1 - Zellmeier, M. A1 - Brenner, Thomas J. K. A1 - Janietz, Silvia A1 - Nickel, N. H. A1 - Rappich, J. T1 - Polythiophenes as emitter layers for crystalline silicon solar cells BT - parasitic absorption, interface passivation, and open circuit voltage JF - Journal of applied physics N2 - We investigated the influence of the emitter (amorphous-Si, a-Si, or polythiophene derivatives: poly(3-hexylthiophene), P3HT, and poly(3-[3,6-dioxaheptyl]-thiophene), P3DOT) and the interface passivation (intrinsic a-Si or SiOX and methyl groups or SiOX) on the c-Si based 1 × 1 cm2 planar hybrid heterojunction solar cell parameters. We observed higher short circuit currents for the P3HT or P3DOT/c-Si solar cells than those obtained for a-Si/c-Si devices, independent of the interface passivation. The obtained VOC of 659 mV for the P3DOT/SiOX/c-Si heterojunction solar cell with hydrophilic 3,6-dioxaheptyl side chains is among the highest reported for c-Si/polythiophene devices. The maximum power conversion efficiency, PCE, was 11% for the P3DOT/SiOX/c-Si heterojunction solar cell. Additionally, our wafer lifetime measurements reveal a field effect passivation in the wafer induced by the polythiophenes when deposited on c-Si. Y1 - 2018 U6 - https://doi.org/10.1063/1.5006625 SN - 0021-8979 SN - 1089-7550 VL - 123 IS - 3 PB - American Institute of Physics CY - Melville ER - TY - JOUR A1 - Zeitz, Maria A1 - Reese, Ronja A1 - Beckmann, Johanna A1 - Krebs-Kanzow, Uta A1 - Winkelmann, Ricarda T1 - Impact of the melt-albedo feedback on the future evolution of the Greenland Ice Sheet with PISM-dEBM-simple JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - Surface melting of the Greenland Ice Sheet contributes a large amount to current and future sea level rise. Increased surface melt may lower the reflectivity of the ice sheet surface and thereby increase melt rates: the so-called melt-albedo feedback describes this self-sustaining increase in surface melting. In order to test the effect of the melt-albedo feedback in a prognostic ice sheet model, we implement dEBM-simple, a simplified version of the diurnal Energy Balance Model dEBM, in the Parallel Ice Sheet Model (PISM). The implementation includes a simple representation of the melt-albedo feedback and can thereby replace the positive-degree-day melt scheme. Using PISM-dEBM-simple, we find that this feedback increases ice loss through surface warming by 60 % until 2300 for the high-emission scenario RCP8.5 when compared to a scenario in which the albedo remains constant at its present-day values. With an increase of 90 % compared to a fixed-albedo scenario, the effect is more pronounced for lower surface warming under RCP2.6. Furthermore, assuming an immediate darkening of the ice surface over all summer months, we estimate an upper bound for this effect to be 70 % in the RCP8.5 scenario and a more than 4-fold increase under RCP2.6. With dEBM-simple implemented in PISM, we find that the melt-albedo feedback is an essential contributor to mass loss in dynamic simulations of the Greenland Ice Sheet under future warming. Y1 - 2021 U6 - https://doi.org/10.5194/tc-15-5739-2021 SN - 1994-0416 SN - 1994-0424 VL - 15 IS - 12 SP - 5739 EP - 5764 PB - Copernicus CY - Katlenburg-Lindau ER - TY - JOUR A1 - Zeitz, Maria A1 - Levermann, Anders A1 - Winkelmann, Ricarda T1 - Sensitivity of ice loss to uncertainty in flow law parameters in an idealized one-dimensional geometry JF - The Cryosphere : TC ; an interactive open access journal of the European Geosciences Union N2 - Acceleration of the flow of ice drives mass losses in both the Antarctic and the Greenland Ice Sheet. The projections of possible future sea-level rise rely on numerical ice-sheet models, which solve the physics of ice flow, melt, and calving. While major advancements have been made by the ice-sheet modeling community in addressing several of the related uncertainties, the flow law, which is at the center of most process-based ice-sheet models, is not in the focus of the current scientific debate. However, recent studies show that the flow law parameters are highly uncertain and might be different from the widely accepted standard values. Here, we use an idealized flow-line setup to investigate how these uncertainties in the flow law translate into uncertainties in flow-driven mass loss. In order to disentangle the effect of future warming on the ice flow from other effects, we perform a suite of experiments with the Parallel Ice Sheet Model (PISM), deliberately excluding changes in the surface mass balance. We find that changes in the flow parameters within the observed range can lead up to a doubling of the flow-driven mass loss within the first centuries of warming, compared to standard parameters. The spread of ice loss due to the uncertainty in flow parameters is on the same order of magnitude as the increase in mass loss due to surface warming. While this study focuses on an idealized flow-line geometry, it is likely that this uncertainty carries over to realistic three-dimensional simulations of Greenland and Antarctica. Y1 - 2020 U6 - https://doi.org/10.5194/tc-14-3537-2020 SN - 1994-0416 SN - 1994-0424 VL - 14 IS - 10 SP - 3537 EP - 3550 PB - Copernicus CY - Göttingen ER -