@phdthesis{Mandal2020, author = {Mandal, Partha Sarathi}, title = {Controlling the surface band gap in topological states of matter}, doi = {10.25932/publishup-48045}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-480459}, school = {Universit{\"a}t Potsdam}, pages = {122}, year = {2020}, abstract = {In the present study, we employ the angle-resolved photoemission spectroscopy (ARPES) technique to study the electronic structure of topological states of matter. In particular, the so-called topological crystalline insulators (TCIs) Pb1-xSnxSe and Pb1-xSnxTe, and the Mn-doped Z2 topological insulators (TIs) Bi2Te3 and Bi2Se3. The Z2 class of strong topological insulators is protected by time-reversal symmetry and is characterized by an odd number of metallic Dirac type surface states in the surface Brillouin zone. The topological crystalline insulators on the other hand are protected by the individual crystal symmetries and exhibit an even number of Dirac cones. The topological properties of the lead tin chalcogenides topological crystalline insulators can be tuned by temperature and composition. Here, we demonstrate that Bi-doping of the Pb1-xSnxSe(111) epilayers induces a quantum phase transition from a topological crystalline insulator to a Z2 topological insulator. This occurs because Bi-doping lifts the fourfold valley degeneracy in the bulk. As a consequence a gap appears at ⌈¯, while the three Dirac cones at the M̅ points of the surface Brillouin zone remain intact. We interpret this new phase transition is caused by lattice distortion. Our findings extend the topological phase diagram enormously and make strong topological insulators switchable by distortions or electric field. In contrast, the bulk Bi doping of epitaxial Pb1-xSnxTe(111) films induces a giant Rashba splitting at the surface that can be tuned by the doping level. Tight binding calculations identify their origin as Fermi level pinning by trap states at the surface. Magnetically doped topological insulators enable the quantum anomalous Hall effect (QAHE) which provide quantized edge states for lossless charge transport applications. The edge states are hosted by a magnetic energy gap at the Dirac point which has not been experimentally observed to date. Our low temperature ARPES studies unambiguously reveal the magnetic gap of Mn-doped Bi2Te3. Our analysis shows a five times larger gap size below the Tc than theoretically predicted. We assign this enhancement to a remarkable structure modification induced by Mn doping. Instead of a disordered impurity system, a self-organized alternating sequence of MnBi2Te4 septuple and Bi2Te3quintuple layers is formed. This enhances the wave-function overlap and gives rise to a large magnetic gap. Mn-doped Bi2Se3 forms similar heterostructure, but only a nonmagnetic gap is observed in this system. This correlates with the difference in magnetic anisotropy due to the much larger spin-orbit interaction in Bi2Te3 compared to Bi2Se3. These findings provide crucial insights for pushing lossless transport in topological insulators towards room-temperature applications.}, language = {en} } @phdthesis{Kniepert2015, author = {Kniepert, Juliane}, title = {Correlation between dynamic parameters and device performance of organic solar cells}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-90087}, school = {Universit{\"a}t Potsdam}, pages = {129}, year = {2015}, abstract = {Organic bulk heterojunction (BHJ) solar cells based on polymer:fullerene blends are a promising alternative for a low-cost solar energy conversion. Despite significant improvements of the power conversion efficiency in recent years, the fundamental working principles of these devices are yet not fully understood. In general, the current output of organic solar cells is determined by the generation of free charge carriers upon light absorption and their transport to the electrodes in competition to the loss of charge carriers due to recombination. The object of this thesis is to provide a comprehensive understanding of the dynamic processes and physical parameters determining the performance. A new approach for analyzing the characteristic current-voltage output was developed comprising the experimental determination of the efficiencies of charge carrier generation, recombination and transport, combined with numerical device simulations. Central issues at the beginning of this work were the influence of an electric field on the free carrier generation process and the contribution of generation, recombination and transport to the current-voltage characteristics. An elegant way to directly measure the field dependence of the free carrier generation is the Time Delayed Collection Field (TDCF) method. In TDCF charge carriers are generated by a short laser pulse and subsequently extracted by a defined rectangular voltage pulse. A new setup was established with an improved time resolution compared to former reports in literature. It was found that charge generation is in general independent of the electric field, in contrast to the current view in literature and opposed to the expectations of the Braun-Onsager model that was commonly used to describe the charge generation process. Even in cases where the charge generation was found to be field-dependend, numerical modelling showed that this field-dependence is in general not capable to account for the voltage dependence of the photocurrent. This highlights the importance of efficient charge extraction in competition to non-geminate recombination, which is the second objective of the thesis. Therefore, two different techniques were combined to characterize the dynamics and efficiency of non-geminate recombination under device-relevant conditions. One new approach is to perform TDCF measurements with increasing delay between generation and extraction of charges. Thus, TDCF was used for the first time to measure charge carrier generation, recombination and transport with the same experimental setup. This excludes experimental errors due to different measurement and preparation conditions and demonstrates the strength of this technique. An analytic model for the description of TDCF transients was developed and revealed the experimental conditions for which reliable results can be obtained. In particular, it turned out that the \$RC\$ time of the setup which is mainly given by the sample geometry has a significant influence on the shape of the transients which has to be considered for correct data analysis. Secondly, a complementary method was applied to characterize charge carrier recombination under steady state bias and illumination, i.e. under realistic operating conditions. This approach relies on the precise determination of the steady state carrier densities established in the active layer. It turned out that current techniques were not sufficient to measure carrier densities with the necessary accuracy. Therefore, a new technique {Bias Assisted Charge Extraction} (BACE) was developed. Here, the charge carriers photogenerated under steady state illumination are extracted by applying a high reverse bias. The accelerated extraction compared to conventional charge extraction minimizes losses through non-geminate recombination and trapping during extraction. By performing numerical device simulations under steady state, conditions were established under which quantitative information on the dynamics can be retrieved from BACE measurements. The applied experimental techniques allowed to sensitively analyse and quantify geminate and non-geminate recombination losses along with charge transport in organic solar cells. A full analysis was exemplarily demonstrated for two prominent polymer-fullerene blends. The model system P3HT:PCBM spincast from chloroform (as prepared) exhibits poor power conversion efficiencies (PCE) on the order of 0.5\%, mainly caused by low fill factors (FF) and currents. It could be shown that the performance of these devices is limited by the hole transport and large bimolecular recombination (BMR) losses, while geminate recombination losses are insignificant. The low polymer crystallinity and poor interconnection between the polymer and fullerene domains leads to a hole mobility of the order of 10^-7 cm^2/Vs which is several orders of magnitude lower than the electron mobility in these devices. The concomitant build up of space charge hinders extraction of both electrons and holes and promotes bimolecular recombination losses. Thermal annealing of P3HT:PCBM blends directly after spin coating improves crystallinity and interconnection of the polymer and the fullerene phase and results in comparatively high electron and hole mobilities in the order of 10^-3 cm^2/Vs and 10^-4 cm^2/Vs, respectively. In addition, a coarsening of the domain sizes leads to a reduction of the BMR by one order of magnitude. High charge carrier mobilities and low recombination losses result in comparatively high FF (>65\%) and short circuit current (J_SC ≈ 10 mA/cm^2). The overall device performance (PCE ≈ 4\%) is only limited by a rather low spectral overlap of absorption and solar emission and a small V_OC, given by the energetics of the P3HT. From this point of view the combination of the low bandgap polymer PTB7 with PCBM is a promising approach. In BHJ solar cells, this polymer leads to a higher V_OC due to optimized energetics with PCBM. However, the J_SC in these (unoptimized) devices is similar to the J_SC in the optimized blend with P3HT and the FF is rather low (≈ 50\%). It turned out that the unoptimized PTB7:PCBM blends suffer from high BMR, a low electron mobility of the order of 10^-5 cm^2/Vs and geminate recombination losses due to field dependent charge carrier generation. The use of the solvent additive DIO optimizes the blend morphology, mainly by suppressing the formation of very large fullerene domains and by forming a more uniform structure of well interconnected donor and acceptor domains of the order of a few nanometers. Our analysis shows that this results in an increase of the electron mobility by about one order of magnitude (3 x 10^-4 cm^2/Vs), while BMR and geminate recombination losses are significantly reduced. In total these effects improve the J_SC (≈ 17 mA/cm^2) and the FF (> 70\%). In 2012 this polymer/fullerene combination resulted in a record PCE for a single junction OSC of 9.2\%. Remarkably, the numerical device simulations revealed that the specific shape of the J-V characteristics depends very sensitively to the variation of not only one, but all dynamic parameters. On the one hand this proves that the experimentally determined parameters, if leading to a good match between simulated and measured J-V curves, are realistic and reliable. On the other hand it also emphasizes the importance to consider all involved dynamic quantities, namely charge carrier generation, geminate and non-geminate recombination as well as electron and hole mobilities. The measurement or investigation of only a subset of these parameters as frequently found in literature will lead to an incomplete picture and possibly to misleading conclusions. Importantly, the comparison of the numerical device simulation employing the measured parameters and the experimental \$J-V\$ characteristics allows to identify loss channels and limitations of OSC. For example, it turned out that inefficient extraction of charge carriers is a criticical limitation factor that is often disobeyed. However, efficient and fast transport of charges becomes more and more important with the development of new low bandgap materials with very high internal quantum efficiencies. Likewise, due to moderate charge carrier mobilities, the active layer thicknesses of current high-performance devices are usually limited to around 100 nm. However, larger layer thicknesses would be more favourable with respect to higher current output and robustness of production. Newly designed donor materials should therefore at best show a high tendency to form crystalline structures, as observed in P3HT, combined with the optimized energetics and quantum efficiency of, for example, PTB7.}, language = {en} } @phdthesis{Schemenz2022, author = {Schemenz, Victoria}, title = {Correlations between osteocyte lacuno-canalicular network and material characteristics in bone adaptation and regeneration}, doi = {10.25932/publishup-55959}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-559593}, school = {Universit{\"a}t Potsdam}, pages = {3, xii, 146}, year = {2022}, abstract = {The complex hierarchical structure of bone undergoes a lifelong remodeling process, where it adapts to mechanical needs. Hereby, bone resorption by osteoclasts and bone formation by osteoblasts have to be balanced to sustain a healthy and stable organ. Osteocytes orchestrate this interplay by sensing mechanical strains and translating them into biochemical signals. The osteocytes are located in lacunae and are connected to one another and other bone cells via cell processes through small channels, the canaliculi. Lacunae and canaliculi form a network (LCN) of extracellular spaces that is able to transport ions and enables cell-to-cell communication. Osteocytes might also contribute to mineral homeostasis by direct interactions with the surrounding matrix. If the LCN is acting as a transport system, this should be reflected in the mineralization pattern. The central hypothesis of this thesis is that osteocytes are actively changing their material environment. Characterization methods of material science are used to achieve the aim of detecting traces of this interaction between osteocytes and the extracellular matrix. First, healthy murine bones were characterized. The properties analyzed were then compared with three murine model systems: 1) a loading model, where a bone of the mouse was loaded during its life time; 2) a healing model, where a bone of the mouse was cut to induce a healing response; and 3) a disease model, where the Fbn1 gene is dysfunctional causing defects in the formation of the extracellular tissue. The measurement strategy included routines that make it possible to analyze the organization of the LCN and the material components (i.e., the organic collagen matrix and the mineral particles) in the same bone volumes and compare the spatial distribution of different data sets. The three-dimensional network architecture of the LCN is visualized by confocal laser scanning microscopy (CLSM) after rhodamine staining and is then subsequently quantified. The calcium content is determined via quantitative backscattered electron imaging (qBEI), while small- and wide-angle X-ray scattering (SAXS and WAXS) are employed to determine the thickness and length of local mineral particles. First, tibiae cortices of healthy mice were characterized to investigate how changes in LCN architecture can be attributed to interactions of osteocytes with the surrounding bone matrix. The tibial mid-shaft cross-sections showed two main regions, consisting of a band with unordered LCN surrounded by a region with ordered LCN. The unordered region is a remnant of early bone formation and exhibited short and thin mineral particles. The surrounding, more aligned bone showed ordered and dense LCN as well as thicker and longer mineral particles. The calcium content was unchanged between the two regions. In the mouse loading model, the left tibia underwent two weeks of mechanical stimulation, which results in increased bone formation and decreased resorption in skeletally mature mice. Here the specific research question addressed was how do bone material characteristics change at (re)modeling sites? The new bone formed in response to mechanical stimulation showed similar properties in terms of the mineral particles, like the ordered calcium region but lower calcium content compared to the right, non-loaded control bone of the same mice. There was a clear, recognizable border between mature and newly formed bone. Nevertheless, some canaliculi went through this border connecting the LCN of mature and newly formed bone. Additionally, the question should be answered whether the LCN topology and the bone matrix material properties adapt to loading. Although, mechanically stimulated bones did not show differences in calcium content compared to controls, different correlations were found between the local LCN density and the local Ca content depending on whether the bone was loaded or not. These results suggest that the LCN may serve as a mineral reservoir. For the healing model, the femurs of mice underwent an osteotomy, stabilized with an external fixator and were allowed to heal for 21 days. Thus, the spatial variations in the LCN topology with mineral properties within different tissue types and their interfaces, namely calcified cartilage, bony callus and cortex, could be simultaneously visualized and compared in this model. All tissue types showed structural differences across multiple length scales. Calcium content increased and became more homogeneous from calcified cartilage to bony callus to lamellar cortical bone. The degree of LCN organization increased as well, while the lacunae became smaller, as did the lacunar density between these different tissue types that make up the callus. In the calcified cartilage, the mineral particles were short and thin. The newly formed callus exhibited thicker mineral particles, which still had a low degree of orientation. While most of the callus had a woven-like structure, it also served as a scaffold for more lamellar tissue at the edges. The lamelar bone callus showed thinner mineral particles, but a higher degree of alignment in both, mineral particles and the LCN. The cortex showed the highest values for mineral length, thickness and degree of orientation. At the same time, the lacunae number density was 34\% lower and the lacunar volume 40\% smaller compared to bony callus. The transition zone between cortical and callus regions showed a continuous convergence of bone mineral properties and lacunae shape. Although only a few canaliculi connected callus and the cortical region, this indicates that communication between osteocytes of both tissues should be possible. The presented correlations between LCN architecture and mineral properties across tissue types may suggest that osteocytes have an active role in mineralization processes of healing. A mouse model for the disease marfan syndrome, which includes a genetic defect in the fibrillin-1 gene, was investigated. In humans, Marfan syndrome is characterized by a range of clinical symptoms such as long bone overgrowth, loose joints, reduced bone mineral density, compromised bone microarchitecture, and increased fracture rates. Thus, fibrillin-1 seems to play a role in the skeletal homeostasis. Therefore, the present work studied how marfan syndrome alters LCN architecture and the surrounding bone matrix. The mice with marfan syndrome showed longer tibiae than their healthy littermates from an age of seven weeks onwards. In contrast, the cortical development appeared retarded, which was observed across all measured characteristics, i. e. lower endocortical bone formation, looser and less organized lacuno-canalicular network, less collagen orientation, thinner and shorter mineral particles. In each of the three model systems, this study found that changes in the LCN architecture spatially correlated with bone matrix material parameters. While not knowing the exact mechanism, these results provide indications that osteocytes can actively manipulate a mineral reservoir located around the canaliculi to make a quickly accessible contribution to mineral homeostasis. However, this interaction is most likely not one-sided, but could be understood as an interplay between osteocytes and extra-cellular matrix, since the bone matrix contains biochemical signaling molecules (e.g. non-collagenous proteins) that can change osteocyte behavior. Bone (re)modeling can therefore not only be understood as a method for removing defects or adapting to external mechanical stimuli, but also for increasing the efficiency of possible osteocyte-mineral interactions during bone homeostasis. With these findings, it seems reasonable to consider osteocytes as a target for drug development related to bone diseases that cause changes in bone composition and mechanical properties. It will most likely require the combined effort of materials scientists, cell biologists, and molecular biologists to gain a deeper understanding of how bone cells respond to their material environment.}, language = {en} } @phdthesis{Schroen2016, author = {Schr{\"o}n, Martin}, title = {Cosmic-ray neutron sensing and its applications to soil and land surface hydrology}, publisher = {Verlag Dr. Hut GmbH}, address = {M{\"u}nchen}, isbn = {978-3-8439-3139-7}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-395433}, school = {Universit{\"a}t Potsdam}, pages = {223}, year = {2016}, abstract = {Water scarcity, adaption on climate change, and risk assessment of droughts and floods are critical topics for science and society these days. Monitoring and modeling of the hydrological cycle are a prerequisite to understand and predict the consequences for weather and agriculture. As soil water storage plays a key role for partitioning of water fluxes between the atmosphere, biosphere, and lithosphere, measurement techniques are required to estimate soil moisture states from small to large scales. The method of cosmic-ray neutron sensing (CRNS) promises to close the gap between point-scale and remote-sensing observations, as its footprint was reported to be 30 ha. However, the methodology is rather young and requires highly interdisciplinary research to understand and interpret the response of neutrons to soil moisture. In this work, the signal of nine detectors has been systematically compared, and correction approaches have been revised to account for meteorological and geomagnetic variations. Neutron transport simulations have been consulted to precisely characterize the sensitive footprint area, which turned out to be 6--18 ha, highly local, and temporally dynamic. These results have been experimentally confirmed by the significant influence of water bodies and dry roads. Furthermore, mobile measurements on agricultural fields and across different land use types were able to accurately capture the various soil moisture states. It has been further demonstrated that the corresponding spatial and temporal neutron data can be beneficial for mesoscale hydrological modeling. Finally, first tests with a gyrocopter have proven the concept of airborne neutron sensing, where increased footprints are able to overcome local effects. This dissertation not only bridges the gap between scales of soil moisture measurements. It also establishes a close connection between the two worlds of observers and modelers, and further aims to combine the disciplines of particle physics, geophysics, and soil hydrology to thoroughly explore the potential and limits of the CRNS method.}, language = {en} } @phdthesis{Shaw2024, author = {Shaw, Vasundhara}, title = {Cosmic-ray transport and signatures in their local environment}, doi = {10.25932/publishup-62019}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-620198}, school = {Universit{\"a}t Potsdam}, pages = {143}, year = {2024}, abstract = {The origin and structure of magnetic fields in the Galaxy are largely unknown. What is known is that they are essential for several astrophysical processes, in particular the propagation of cosmic rays. Our ability to describe the propagation of cosmic rays through the Galaxy is severely limited by the lack of observational data needed to probe the structure of the Galactic magnetic field on many different length scales. This is particularly true for modelling the propagation of cosmic rays into the Galactic halo, where our knowledge of the magnetic field is particularly poor. In the last decade, observations of the Galactic halo in different frequency regimes have revealed the existence of out-of-plane bubble emission in the Galactic halo. In gamma rays these bubbles have been termed Fermi bubbles with a radial extent of ≈ 3 kpc and an azimuthal height of ≈ 6 kpc. The radio counterparts of the Fermi bubbles were seen by both the S-PASS telescopes and the Planck satellite, and showed a clear spatial overlap. The X-ray counterparts of the Fermi bubbles were named eROSITA bubbles after the eROSITA satellite, with a radial width of ≈ 7 kpc and an azimuthal height of ≈ 14 kpc. Taken together, these observations suggest the presence of large extended Galactic Halo Bubbles (GHB) and have stimulated interest in exploring the less explored Galactic halo. In this thesis, a new toy model (GHB model) for the magnetic field and non-thermal electron distribution in the Galactic halo has been proposed. The new toy model has been used to produce polarised synchrotron emission sky maps. Chi-square analysis was used to compare the synthetic skymaps with the Planck 30 GHz polarised skymaps. The obtained constraints on the strength and azimuthal height were found to be in agreement with the S-PASS radio observations. The upper, lower and best-fit values obtained from the above chi-squared analysis were used to generate three separate toy models. These three models were used to propagate ultra-high energy cosmic rays. This study was carried out for two potential sources, Centaurus A and NGC 253, to produce magnification maps and arrival direction skymaps. The simulated arrival direction skymaps were found to be consistent with the hotspots of Centaurus A and NGC 253 as seen in the observed arrival direction skymaps provided by the Pierre Auger Observatory (PAO). The turbulent magnetic field component of the GHB model was also used to investigate the extragalactic dipole suppression seen by PAO. UHECRs with an extragalactic dipole were forward-tracked through the turbulent GHB model at different field strengths. The suppression in the dipole due to the varying diffusion coefficient from the simulations was noted. The results could also be compared with an analytical analogy of electrostatics. The simulations of the extragalactic dipole suppression were in agreement with similar studies carried out for galactic cosmic rays.}, language = {en} } @phdthesis{Schmidt2000, author = {Schmidt, Robert W.}, title = {Cosmological applications of gravitational lensing}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0000261}, school = {Universit{\"a}t Potsdam}, year = {2000}, abstract = {In dieser Arbeit benutze ich den Gravitationslinseneffekt als ein Werkzeug, um zwei recht unterschiedliche kosmologische Fragestellungen zu bearbeiten: die Natur der dunklen Materie in Galaxienhalos und die Rotation des Universums. Zuerst untersuche ich den Mikrolinseneffekt in den Gravitationlinsensystemen Q0957+561 und Q2237+0305. In diesen Systemen scheint das Licht eines Quasars durch die Linsengalaxie hindurch. Aufgrund der Relativbewegung zwischen Quasar, Linsengalaxie und Beobachter verursachen kompakte Objekte innerhalb der Galaxie oder dem Galaxienhalo Helligkeitsfluktuationen des Hintergrundquasars. Ich vergleiche die am 3.5m Teleskop des Apache Point Observatory zwischen 1995 und 1998 gewonnene Lichtkurve des Doppelquasars Q0957+561 (Colley, Kundic \& Turner 2000) mit numerischen Simulationen, um zu untersuchen, ob der Halo der Linsengalaxie aus massiven kompakten Objekten (MACHOs) besteht. Dieser Test wurde zuerst von Gott (1981) vorgeschlagen. Ich kann MACHO-Massen von 10^-6 M_sun bis zu 10^-2 M_sun ausschliessen, sofern der Quasar kleiner ist als 3x10^14 h_60^-0.5 cm und MACHOs mehr als 50\% des dunklen Halos ausmachen. Im zweiten Teil der Arbeit stelle ich neue Beobachtungsdaten fuer den Vierfachquasar Q2237+0305 vor, die am 3.5m Teleskop des Apache Point Observatory zwischen Juni 1995 und Januar 1998 gewonnen wurden. Obwohl die Daten bei veraenderlichen, oft schlechten Seeing Bedingungen und grober Pixelaufloesung aufgenommen wurden, ist die Photometrie der beiden helleren Quasarbilder A und B mit Hilfe von HST-Beobachtungen moeglich. Ich finde ein Helligkeitsmaximum in Bild A mit einer Amplitude von 0.4 bis 0.5 mag und einer Dauer von wenigstens 100 Tagen. Dies zeigt, dass in der Linsengalaxie der Mikrolinseneffekt stattgefunden hat. Im abschliessenden Teil der Arbeit benutze ich dann den schwachen Gravitationslinseneffekt, um Grenzen fuer eine Klasse von rotierenden Kosmologien vom Goedel-Typ zu ermitteln, die von Korotky \& Obukhov (1996) beschrieben wurde. In Studien des schwachen Linseneffektes werden die Formen von tausenden von Hintergrundgalaxien vermessen und gemittelt. Dabei werden kohaerente Verzerrungen der Galaxienformen gemessen, die von Massenverteilungen im Vordergrund oder von der grossraeumigen Struktur der Raumzeit selbst verursacht werden. Ich berechne die vorhergesagte Scherung als Funktion der Rotverschiebung in rotierenden Kosmologien vom Goedel-Typ und vergleiche diese mit der oberen Grenze fuer die kosmische Scherung gamma_limit von 0.04, die in Studien des schwachen Linseneffektes gewonnen wurde. Dieser Vergleich zeigt, dass Modelle vom Goedel-Typ keine groesseren Rotationen omega als H_0=6.1x10^-11 h_60/Jahr haben koennen, wenn die Grenze fuer die kosmische Scherung fuer den ganzen Himmel gilt.}, language = {en} } @phdthesis{GilMerinoRubio2003, author = {Gil-Merino Rubio, Rodrigo}, title = {Cosmology through gravitational lenses}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0001030}, school = {Universit{\"a}t Potsdam}, year = {2003}, abstract = {In dieser Dissertation nutze ich den Gravitationslinseneffekt, um eine Reihe von kosmologischen Fragen zu untersuchen. Der Laufzeitunterschied des Gravitationslinsensystems HE1104-1805 wurde mit unterschiedlichen Methoden bestimmt. Zwischen den beiden Komponenten erhalte ich einen Unterschied von Delta_t(A-B) = -310 +-20 Tagen (2 sigma Konfidenzintervall). Außerdem nutze ich eine dreij{\"a}hrige Beobachtungskampagne, um den Doppelquasar Q0957+561 zu untersuchen. Die beobachteten Fluktuationen in den Differenzlichtkurven lassen sich durch Rauschen erkl{\"a}ren, ein Mikrogravitationslinseneffekt wird zur Erkl{\"a}rung nicht ben{\"o}tigt. Am Vierfachquasar Q2237+0305 untersuchte ich den Mikrogravitationslinseneffekt anhand der Daten der GLITP-Kollaboration (Okt. 1999-Feb. 2000). Durch die Abwesenheit eines starken Mikrogravitationslinsensignals konnte ich eine obere Grenze von v=600 km/s f f{\"u}r die effektive Transversalgeschwindigkeit der Linsengalaxie bestimmen (unter der Annahme von Mikrolinsen mit 0.1 Sonnenmassen). Im zweiten Teil der Arbeit untersuchte ich die Verteilung der Dunklen Materie in Galaxienhaufen. F{\"u}r den Galaxienhaufen Cl0024+1654 erhalte ich ein Masse-Leuchtkraft-Verh{\"a}ltnis von M/L = 200 M_sun/L_sun (innerhalb eines Radius von 3 Bogenminuten). Im Galaxienhaufen RBS380 finde ich eine relativ geringe R{\"o}ntgenleuchtkraft von L =2*10^(44) erg/s, obwohl im optischen eine große Anzahl von Galaxien gefunden wurde.}, language = {en} } @phdthesis{Sahlmann2002, author = {Sahlmann, Hanno}, title = {Coupling matter to loop quantum gravity}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0000602}, school = {Universit{\"a}t Potsdam}, year = {2002}, abstract = {Motiviert durch neuere Vorschl{\"a}ge zur experimentellen Untersuchung von Quantengravitationseffekten werden in der vorliegenden Arbeit Annahmen und Methoden untersucht, die f{\"u}r die Vorhersagen solcher Effekte im Rahmen der Loop-Quantengravitation verwendet werden k{\"o}nnen. Dazu wird als Modellsystem ein skalares Feld, gekoppelt an das Gravitationsfeld, betrachtet. Zun{\"a}chst wird unter bestimmten Annahmen {\"u}ber die Dynamik des gekoppelten Systems eine Quantentheorie f{\"u}r das Skalarfeld vorgeschlagen. Unter der Annahme, dass sich das Gravitationsfeld in einem semiklassischen Zustand befindet, wird dann ein \"QFT auf gekr{\"u}mmter Raumzeit-Limes\" dieser Theorie definiert. Im Gegensatz zur gew{\"o}hnlichen Quantenfeldtheorie auf gekr{\"u}mmter Raumzeit beschreibt die Theorie in diesem Grenzfall jedoch ein quantisiertes Skalarfeld, das auf einem (klassisch beschriebenen) Zufallsgitter propagiert. Sodann werden Methoden vorgeschlagen, den Niederenergieliemes einer solchen Gittertheorie, vor allem hinsichtlich der resultierenden modifizierten Dispersonsrelation, zu berechnen. Diese Methoden werden anhand von einfachen Modellsystemen untersucht. Schließlich werden die entwickelten Methoden unter vereinfachenden Annahmen und der Benutzung einer speziellen Klasse von semiklassischen Zust{\"a}nden angewandt, um Korrekturen zur Dispersionsrelation des skalaren und des elektromagnetischen Feldes im Rahmen der Loop-Quantengravitation zu berechnen. Diese Rechnungen haben vorl{\"a}ufigen Charakter, da viele Annahmen eingehen, deren G{\"u}ltigkeit genauer untersucht werden muss. Zumindest zeigen sie aber Probleme und M{\"o}glichkeiten auf, im Rahmen der Loop-Quantengravitation Vorhersagen zu machen, die sich im Prinzip experimentell verifizieren lassen.}, language = {en} } @phdthesis{Ganesan2010, author = {Ganesan, Lakshmi Meena}, title = {Coupling of the electrical, mechanical and optical response in polymer/liquid-crystal composites}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-41572}, school = {Universit{\"a}t Potsdam}, year = {2010}, abstract = {Micrometer-sized liquid-crystal (LC) droplets embedded in a polymer matrix may enable optical switching in the composite film through the alignment of the LC director along an external electric field. When a ferroelectric material is used as host polymer, the electric field generated by the piezoelectric effect can orient the director of the LC under an applied mechanical stress, making these materials interesting candidates for piezo-optical devices. In this work, polymer-dispersed liquid crystals (PDLCs) are prepared from poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) and a nematic liquid crystal (LC). The anchoring effect is studied by means of dielectric relaxation spectroscopy. Two dispersion regions are observed in the dielectric spectra of the pure P(VDF-TrFE) film. They are related to the glass transition and to a charge-carrier relaxation, respectively. In PDLC films containing 10 and 60 wt\% LC, an additional, bias-field-dependent relaxation peak is found that can be attributed to the motion of LC molecules. Due to the anchoring effect of the LC molecules, this relaxation process is slowed down considerably, when compared with the related process in the pure LC. The electro-optical and piezo-optical behavior of PDLC films containing 10 and 60 wt\% LCs is investigated. In addition to the refractive-index mismatch between the polymer matrix and the LC molecules, the interaction between the polymer dipoles and the LC molecules at the droplet interface influences the light-scattering behavior of the PDLC films. For the first time, it was shown that the electric field generated by the application of a mechanical stress may lead to changes in the transmittance of a PDLC film. Such a piezo-optical PDLC material may be useful e.g. in sensing and visualization applications. Compared to a non-polar matrix polymer, the polar matrix polymer exhibits a strong interaction with the LC molecules at the polymer/LC interface which affects the electro-optical effect of the PDLC films and prevents a larger increase in optical transmission.}, language = {en} } @phdthesis{Balzer2014, author = {Balzer, Arnim}, title = {Crab flare observations with H.E.S.S. phase II}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-72545}, school = {Universit{\"a}t Potsdam}, year = {2014}, abstract = {The H.E.S.S. array is a third generation Imaging Atmospheric Cherenkov Telescope (IACT) array. It is located in the Khomas Highland in Namibia, and measures very high energy (VHE) gamma-rays. In Phase I, the array started data taking in 2004 with its four identical 13 m telescopes. Since then, H.E.S.S. has emerged as the most successful IACT experiment to date. Among the almost 150 sources of VHE gamma-ray radiation found so far, even the oldest detection, the Crab Nebula, keeps surprising the scientific community with unexplained phenomena such as the recently discovered very energetic flares of high energy gamma-ray radiation. During its most recent flare, which was detected by the Fermi satellite in March 2013, the Crab Nebula was simultaneously observed with the H.E.S.S. array for six nights. The results of the observations will be discussed in detail during the course of this work. During the nights of the flare, the new 24 m × 32 m H.E.S.S. II telescope was still being commissioned, but participated in the data taking for one night. To be able to reconstruct and analyze the data of the H.E.S.S. Phase II array, the algorithms and software used by the H.E.S.S. Phase I array had to be adapted. The most prominent advanced shower reconstruction technique developed by de Naurois and Rolland, the template-based model analysis, compares real shower images taken by the Cherenkov telescope cameras with shower templates obtained using a semi-analytical model. To find the best fitting image, and, therefore, the relevant parameters that describe the air shower best, a pixel-wise log-likelihood fit is done. The adaptation of this advanced shower reconstruction technique to the heterogeneous H.E.S.S. Phase II array for stereo events (i.e. air showers seen by at least two telescopes of any kind), its performance using MonteCarlo simulations as well as its application to real data will be described.}, language = {en} } @phdthesis{Aue2024, author = {Aue, Lars}, title = {Cyclone impacts on sea ice in the Atlantic Arctic Ocean}, doi = {10.25932/publishup-63445}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-634458}, school = {Universit{\"a}t Potsdam}, pages = {VIII, 131}, year = {2024}, abstract = {The Arctic is the hot spot of the ongoing, global climate change. Over the last decades, near-surface temperatures in the Arctic have been rising almost four times faster than on global average. This amplified warming of the Arctic and the associated rapid changes of its environment are largely influenced by interactions between individual components of the Arctic climate system. On daily to weekly time scales, storms can have major impacts on the Arctic sea-ice cover and are thus an important part of these interactions within the Arctic climate. The sea-ice impacts of storms are related to high wind speeds, which enhance the drift and deformation of sea ice, as well as to changes in the surface energy budget in association with air mass advection, which impact the seasonal sea-ice growth and melt. The occurrence of storms in the Arctic is typically associated with the passage of transient cyclones. Even though the above described mechanisms how storms/cyclones impact the Arctic sea ice are in principal known, there is a lack of statistical quantification of these effects. In accordance with that, the overarching objective of this thesis is to statistically quantify cyclone impacts on sea-ice concentration (SIC) in the Atlantic Arctic Ocean over the last four decades. In order to further advance the understanding of the related mechanisms, an additional objective is to separate dynamic and thermodynamic cyclone impacts on sea ice and assess their relative importance. Finally, this thesis aims to quantify recent changes in cyclone impacts on SIC. These research objectives are tackled utilizing various data sets, including atmospheric and oceanic reanalysis data as well as a coupled model simulation and a cyclone tracking algorithm. Results from this thesis demonstrate that cyclones are significantly impacting SIC in the Atlantic Arctic Ocean from autumn to spring, while there are mostly no significant impacts in summer. The strength and the sign (SIC decreasing or SIC increasing) of the cyclone impacts strongly depends on the considered daily time scale and the region of the Atlantic Arctic Ocean. Specifically, an initial decrease in SIC (day -3 to day 0 relative to the cyclone) is found in the Greenland, Barents and Kara Seas, while SIC increases following cyclones (day 0 to day 5 relative to the cyclone) are mostly limited to the Barents and Kara Seas. For the cold season, this results in a pronounced regional difference between overall (day -3 to day 5 relative to the cyclone) SIC-decreasing cyclone impacts in the Greenland Sea and overall SIC-increasing cyclone impacts in the Barents and Kara Seas. A cyclone case study based on a coupled model simulation indicates that both dynamic and thermodynamic mechanisms contribute to cyclone impacts on sea ice in winter. A typical pattern consisting of an initial dominance of dynamic sea-ice changes followed by enhanced thermodynamic ice growth after the cyclone passage was found. This enhanced ice growth after the cyclone passage most likely also explains the (statistical) overall SIC-increasing effects of cyclones in the Barents and Kara Seas in the cold season. Significant changes in cyclone impacts on SIC over the last four decades have emerged throughout the year. These recent changes are strongly varying from region to region and month to month. The strongest trends in cyclone impacts on SIC are found in autumn in the Barents and Kara Seas. Here, the magnitude of destructive cyclone impacts on SIC has approximately doubled over the last four decades. The SIC-increasing effects following the cyclone passage have particularly weakened in the Barents Sea in autumn. As a consequence, previously existing overall SIC-increasing cyclone impacts in this region in autumn have recently disappeared. Generally, results from this thesis show that changes in the state of the sea-ice cover (decrease in mean sea-ice concentration and thickness) and near-surface air temperature are most important for changed cyclone impacts on SIC, while changes in cyclone properties (i.e. intensity) do not play a significant role.}, language = {en} } @phdthesis{Berner2016, author = {Berner, Nadine}, title = {Deciphering multiple changes in complex climate time series using Bayesian inference}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-100065}, school = {Universit{\"a}t Potsdam}, pages = {xvi, 135}, year = {2016}, abstract = {Change points in time series are perceived as heterogeneities in the statistical or dynamical characteristics of the observations. Unraveling such transitions yields essential information for the understanding of the observed system's intrinsic evolution and potential external influences. A precise detection of multiple changes is therefore of great importance for various research disciplines, such as environmental sciences, bioinformatics and economics. The primary purpose of the detection approach introduced in this thesis is the investigation of transitions underlying direct or indirect climate observations. In order to develop a diagnostic approach capable to capture such a variety of natural processes, the generic statistical features in terms of central tendency and dispersion are employed in the light of Bayesian inversion. In contrast to established Bayesian approaches to multiple changes, the generic approach proposed in this thesis is not formulated in the framework of specialized partition models of high dimensionality requiring prior specification, but as a robust kernel-based approach of low dimensionality employing least informative prior distributions. First of all, a local Bayesian inversion approach is developed to robustly infer on the location and the generic patterns of a single transition. The analysis of synthetic time series comprising changes of different observational evidence, data loss and outliers validates the performance, consistency and sensitivity of the inference algorithm. To systematically investigate time series for multiple changes, the Bayesian inversion is extended to a kernel-based inference approach. By introducing basic kernel measures, the weighted kernel inference results are composed into a proxy probability to a posterior distribution of multiple transitions. The detection approach is applied to environmental time series from the Nile river in Aswan and the weather station Tuscaloosa, Alabama comprising documented changes. The method's performance confirms the approach as a powerful diagnostic tool to decipher multiple changes underlying direct climate observations. Finally, the kernel-based Bayesian inference approach is used to investigate a set of complex terrigenous dust records interpreted as climate indicators of the African region of the Plio-Pleistocene period. A detailed inference unravels multiple transitions underlying the indirect climate observations, that are interpreted as conjoint changes. The identified conjoint changes coincide with established global climate events. In particular, the two-step transition associated to the establishment of the modern Walker-Circulation contributes to the current discussion about the influence of paleoclimate changes on the environmental conditions in tropical and subtropical Africa at around two million years ago.}, language = {en} } @phdthesis{Kubowicz2005, author = {Kubowicz, Stephan}, title = {Design and characterization of multicompartment micelles in aqueous solution}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-5752}, school = {Universit{\"a}t Potsdam}, year = {2005}, abstract = {Self-assembly of polymeric building blocks is a powerful tool for the design of novel materials and structures that combine different properties and may respond to external stimuli. In the past decades, most studies were focused on the self-assembly of amphiphilic diblock copolymers in solution. The dissolution of these block copolymers in a solvent selective for one block results mostly in the formation of micelles. The micellar structure of diblock copolymers is inherently limited to a homogeneous core surrounded by a corona, which keeps the micelle in solution. Thus, for drug-delivery applications, such structures only offer a single domain (the hydrophobic inner core) for drug entrapment. Whereas multicompartment micelles composed of a water-soluble shell and a segregated hydrophobic core are novel, interesting morphologies for applications in a variety of fields including medicine, pharmacy and biotechnology. The separated incompatible compartments of the hydrophobic core could enable the selective entrapment and release of various hydrophobic drugs while the hydrophilic shell would permit the stabilization of these nanostructures in physiological media. However, so far, the preparation and control of stable multicompartment micellar systems are in the first stages and the number of morphological studies concerning such micelles is rather low. Thus considerably little is known about their exact inner structures. In the present study, we concentrate on four different approaches for the preparation of multicompartment micelles by self-assembly in aqueous media. A similarity of all approaches was that hydrocarbon and fluorocarbon blocks were selected for all employed copolymers since such segments tend to be strongly incompatible, and thus favor the segregation into distinct domains. Our studies have shown that the self-assembly of the utilized copolymers in aqueous solution leads in three cases to the formation of multicompartment micelles. As expected the shape and size of the micelles depend on the molecular architecture and to some extent also on the way of preparation. These novel structured colloids may serve as models as well as mimics for biological structures such as globular proteins, and may open interesting opportunities for nanotechnology applications.}, subject = {Amphiphile Verbindungen}, language = {en} } @phdthesis{Kappe2006, author = {Kappe, Philip}, title = {Design and investigation of the emission dynamics of a mode-locked SBS-laser oscillator}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-11514}, school = {Universit{\"a}t Potsdam}, year = {2006}, abstract = {The primary objective of this work was to develop a laser source for fundamental investigations in the field of laser - materials interactions. In particular it is supposed to facilitate the study of the influence of the temporal energy distribution such as the interaction between adjacent pulses on ablation processes. Therefore, the aim was to design a laser with a highly flexible and easily controllable temporal energy distribution. The laser to meet these demands is an SBS-laser with optional active mode-locking. The nonlinear reflectivity of the SBS-mirror leads to a passive Q-switching and issues ns-pulse bursts with µs spacing. The pulse train parameters such as pulse duration, pulse spacing, pulse energy and number of pulses within a burst can be individually adjusted by tuning the pump parameters and the starting conditions for the laser. Another feature of the SBS-reflection is phase conjugation, which leads to an excellent beam quality thanks to the compensation of phase distortions. Transverse fundamental mode operation and a beam quality better than 1.4 times diffraction limited can be maintained for average output powers of up to 10 W. In addition to the dynamics on a ns-timescale described above, a defined splitting up of each ns-pulse into a train of ps-pulses can be achieved by additional active mode-locking. This twofold temporal focussing of the intensity leads to single pulse energies of up to 2 mJ at pulse durations of approximately 400 ps which corresponds to a pulse peak power of 5 MW. While the pulse duration is of the same order of magnitude as those of other passively Q-switched lasers with simultaneous mode-locking, the pulse energy and pulse peak power exceeds the values of these systems found in the literature by an order of magnitude. To the best of my knowledge the laser presented here is the first implementation of a self-starting mode-locked SBS-laser oscillator. In order to gain a better understanding and control of the transient output of the laser two complementary numerical models were developed. The first is based on laser rate equations which are solved for each laser mode individually while the mode-locking dynamics are calculated from the resultant transient spectrum. The rate equations consider the mean photon densities in the resonator, therefore the propagation of the light inside the resonator is not properly displayed. The second model, in contrast, introduces a spatial resolution of the resonator and hence the propagation inside the resonator can more accurately be considered. Consequently, a mismatch between the loss modulation frequency and the resonator round trip time can be conceived. The model calculates all dynamics in the time domain and therefore the spectral influences such as the Stokes-shift have to be neglected. Both models achieve an excellent reproduction of the ns-dynamics that are generated by the SBS-Q-switch. Separately, each model fails to reproduce all aspects of the ps-dynamics of the SBS-laser in detail. This can be attributed to the complexity of the numerous physical processes involved in this system. But thanks to their complementary nature they provide a very useful tool for investigating the various influences on the dynamics of the mode-locked SBS-laser individually. These aspects can eventually be recomposed to give a complete picture of the mechanisms which govern the output dynamics. Among the aspects under scrutiny were in particular the start resonator quality which determines the starting condition for the SBS-Q-switch, the modulation depth of the AOM and the phonon lifetime as well as the Brillouin-frequency of the SBS-medium. The numerical simulations and the experiments have opened several doors inviting further investigations and promising a potential for further improvement of the experimental results: The results of the simulations in combination with the experimental results which determined the starting conditions for the simulations leave no doubt that the bandwidth generation can primarily be attributed to the SBS-Stokes-shift during the buildup of the Q-switch pulse. For each resonator round trip, bandwidth is generated by shifting a part of the revolving light in frequency. The magnitude of the frequency shift corresponds to the Brillouin-frequency which is a constant of the SBS material and amounts in the case of SF6 to 240 MHz. The modulation of the AOM merely provides an exchange of population between spectrally adjacent modes and therefore diminishes a modulation in the spectrum. By use of a material with a Brillouin-frequency in the GHz range the bandwidth generation can be considerably accelerated thereby shortening the pulse duration. Also, it was demonstrated that yet another nonlinear effect of the SBS can be exploited: If the phonon lifetime is short compared to the resonator round trip time we obtain a modulation in the SBS-reflectivity that supports the modulation of the AOM. The application of an external optical feedback by a conventional mirror turns out to be an alternative to the AOM in synchronizing the longitudinal resonator modes. The interesting feature about this system is that it is ― although highly complex in the physical processes and the temporal output dynamics ― very simple and inexpensive from a technical point of view. No expensive modulators and no control electronics are necessary. Finally, the numerical models constitute a powerful tool for the investigation of emission dynamics of complex laser systems on arbitrary timescales and can also display the spectral evolution of the laser output. In particular it could be demonstrated that differences in the results of the complementary models vanish for systems of lesser complexity.}, language = {en} } @phdthesis{Nayak2022, author = {Nayak, Abani Shankar}, title = {Design, Characterization and On-sky Testing of an Integrated Optics Device for Stellar Interferometry: from Pupil Remappers to Discrete Beam Combiner}, doi = {10.25932/publishup-55874}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-558743}, school = {Universit{\"a}t Potsdam}, pages = {xl, 175}, year = {2022}, abstract = {Stellar interferometry is the only method in observational astronomy for obtaining the highest resolution images of astronomical targets. This method is based on combining light from two or more separate telescopes to obtain the complex visibility that contains information about the brightness distribution of an astronomical source. The applications of stellar interferometry have made significant contributions in the exciting research areas of astronomy and astrophysics, including the precise measurement of stellar diameters, imaging of stellar surfaces, observations of circumstellar disks around young stellar objects, predictions of Einstein's General relativity at the galactic center, and the direct search for exoplanets to name a few. One important related technique is aperture masking interferometry, pioneered in the 1960s, which uses a mask with holes at the re-imaged pupil of the telescope, where the light from the holes is combined using the principle of stellar interferometry. While this can increase the resolution, it comes with a disadvantage. Due to the finite size of the holes, the majority of the starlight (typically > 80 \%) is lost at the mask, thus limiting the signal-to-noise ratio (SNR) of the output images. This restriction of aperture masking only to the bright targets can be avoided using pupil remapping interferometry - a technique combining aperture masking interferometry and advances in photonic technologies using single-mode fibers. Due to the inherent spatial filtering properties, the single-mode fibers can be placed at the focal plane of the re-imaged pupil, allowing the utilization of the whole pupil of the telescope to produce a high-dynamic range along with high-resolution images. Thus, pupil remapping interferometry is one of the most promising application areas in the emerging field of astrophotonics. At the heart of an interferometric facility, a beam combiner exists whose primary function is to combine light to obtain high-contrast fringes. A beam combiner can be as simple as a beam splitter or an anamorphic lens to combine light from 2 apertures (or telescopes) or as complex as a cascade of beam splitters and lenses to combine light for > 2 apertures. However, with the field of astrophotonics, interferometric facilities across the globe are increasingly employing some form of photonics technologies by using single-mode fibers or integrated optics (IO) chips as an efficient way to combine light from several apertures. The state-of-the-art instrument - GRAVITY at the very large telescope interferometer (VLTI) facility uses an IO-based beam combiner device reaching visibilities accuracy of better than < 0.25 \%, which is roughly 50× as precise as a few decades back. Therefore, in the context of IO-based components for applications in stellar interferometry, this Thesis describes the work towards the development of a 3-dimensional (3-D) IO device - a monolithic astrophotonics component containing both the pupil remappers and a discrete beam combiner (DBC). In this work, the pupil remappers are 3-D single-mode waveguides in a glass substrate collecting light from the re-imaged pupil of the telescope and feeding the light to a DBC, where the combination takes place. The DBC is a lattice of 3-D single-mode waveguides, which interact through evanescent coupling. By observing the output power of single-mode waveguides of the DBC, the visibilities are retrieved by using a calibrated transfer matrix ({U}) of the device. The feasibility of the DBC in retrieving the visibilities theoretically and experimentally had already been studied in the literature but was only limited to laboratory tests with monochromatic light sources. Thus, a part of this work extends these studies by investigating the response of a 4-input DBC to a broad-band light source. Hence, the objectives of this Thesis are the following: 1) Design an IO device for broad-band light operation such that accurate and precise visibilities could be retrieved experimentally at astronomical H-band (1.5-1.65 μm), and 2) Validation of the DBC as a possible beam combination scheme for future interferometric facilities through on-sky testing at the William Herschel Telescope (WHT). This work consisted of designing three different 3-D IO devices. One of the popular methods for fabricating 3-D photonic components in a glass substrate is ultra-fast laser inscription (ULI). Thus, manufacturing of the designed devices was outsourced to Politecnico di Milano as part of an iterative fabrication process using their state-of-the-art ULI facility. The devices were then characterized using a 2-beam Michelson interferometric setup obtaining both the monochromatic and polychromatic visibilities. The retrieved visibilities for all devices were in good agreement as predicted by the simulation results of a DBC, which confirms both the repeatability of the ULI process and the stability of the Michelson setup, thus fulfilling the first objective. The best-performing device was then selected for the pupil-remapping of the WHT using a different optical setup consisting of a deformable mirror and a microlens array. The device successfully collected stellar photons from Vega and Altair. The visibilities were retrieved using a previously calibrated {U} but showed significant deviations from the expected results. Based on the analysis of comparable simulations, it was found that such deviations were primarily caused by the limited SNR of the stellar observations, thus constituting a first step towards the fulfillment of the second objective.}, language = {en} } @phdthesis{Rust2007, author = {Rust, Henning}, title = {Detection of long-range dependence : applications in climatology and hydrology}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-13347}, school = {Universit{\"a}t Potsdam}, year = {2007}, abstract = {It is desirable to reduce the potential threats that result from the variability of nature, such as droughts or heat waves that lead to food shortage, or the other extreme, floods that lead to severe damage. To prevent such catastrophic events, it is necessary to understand, and to be capable of characterising, nature's variability. Typically one aims to describe the underlying dynamics of geophysical records with differential equations. There are, however, situations where this does not support the objectives, or is not feasible, e.g., when little is known about the system, or it is too complex for the model parameters to be identified. In such situations it is beneficial to regard certain influences as random, and describe them with stochastic processes. In this thesis I focus on such a description with linear stochastic processes of the FARIMA type and concentrate on the detection of long-range dependence. Long-range dependent processes show an algebraic (i.e. slow) decay of the autocorrelation function. Detection of the latter is important with respect to, e.g. trend tests and uncertainty analysis. Aiming to provide a reliable and powerful strategy for the detection of long-range dependence, I suggest a way of addressing the problem which is somewhat different from standard approaches. Commonly used methods are based either on investigating the asymptotic behaviour (e.g., log-periodogram regression), or on finding a suitable potentially long-range dependent model (e.g., FARIMA[p,d,q]) and test the fractional difference parameter d for compatibility with zero. Here, I suggest to rephrase the problem as a model selection task, i.e.comparing the most suitable long-range dependent and the most suitable short-range dependent model. Approaching the task this way requires a) a suitable class of long-range and short-range dependent models along with suitable means for parameter estimation and b) a reliable model selection strategy, capable of discriminating also non-nested models. With the flexible FARIMA model class together with the Whittle estimator the first requirement is fulfilled. Standard model selection strategies, e.g., the likelihood-ratio test, is for a comparison of non-nested models frequently not powerful enough. Thus, I suggest to extend this strategy with a simulation based model selection approach suitable for such a direct comparison. The approach follows the procedure of a statistical test, with the likelihood-ratio as the test statistic. Its distribution is obtained via simulations using the two models under consideration. For two simple models and different parameter values, I investigate the reliability of p-value and power estimates obtained from the simulated distributions. The result turned out to be dependent on the model parameters. However, in many cases the estimates allow an adequate model selection to be established. An important feature of this approach is that it immediately reveals the ability or inability to discriminate between the two models under consideration. Two applications, a trend detection problem in temperature records and an uncertainty analysis for flood return level estimation, accentuate the importance of having reliable methods at hand for the detection of long-range dependence. In the case of trend detection, falsely concluding long-range dependence implies an underestimation of a trend and possibly leads to a delay of measures needed to take in order to counteract the trend. Ignoring long-range dependence, although present, leads to an underestimation of confidence intervals and thus to an unjustified belief in safety, as it is the case for the return level uncertainty analysis. A reliable detection of long-range dependence is thus highly relevant in practical applications. Examples related to extreme value analysis are not limited to hydrological applications. The increased uncertainty of return level estimates is a potentially problem for all records from autocorrelated processes, an interesting examples in this respect is the assessment of the maximum strength of wind gusts, which is important for designing wind turbines. The detection of long-range dependence is also a relevant problem in the exploration of financial market volatility. With rephrasing the detection problem as a model selection task and suggesting refined methods for model comparison, this thesis contributes to the discussion on and development of methods for the detection of long-range dependence.}, language = {en} } @phdthesis{vonNordheim2018, author = {von Nordheim, Danny}, title = {Dielectric non-linearities of P(VDF-TrFE) single and multilayers for memory applications}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-421778}, school = {Universit{\"a}t Potsdam}, pages = {XVI, 109}, year = {2018}, abstract = {Poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) ferroelectric thin films of different molar ratio have been studied with regard to data memory applications. Therefore, films with thicknesses of 200 nm and less have been spin coated from solution. Observations gained from single layers have been extended to multilayer capacitors and three terminal transistor devices. Besides conventional hysteresis measurements, the measurement of dielectric non-linearities has been used as a main tool of characterisation. Being a very sensitive and non-destructive method, non-linearity measurements are well suited for polarisation readout and property studies. Samples have been excited using a high quality, single-frequency sinusoidal voltage with an amplitude significantly smaller than the coercive field of the samples. The response was then measured at the excitation frequency and its higher harmonics. Using the measurement results, the linear and non-linear dielectric permittivities ɛ₁, ɛ₂ and ɛ₃ have been determined. The permittivities have been used to derive the temperature-dependent polarisation behaviour as well as the polarisation state and the order of the phase transitions. The coercive field in VDF-TrFE copolymers is high if compared to their ceramic competitors. Therefore, the film thickness had to be reduced significantly. Considering a switching voltage of 5 V and a coercive field of 50 MV/m, the film thickness has to be 100 nm and below. If the thickness becomes substantially smaller than the other dimensions, surface and interface layer effects become more pronounced. For thicker films of P(VDF-TrFE) with a molar fraction of 56/44 a second-order phase transition without a thermal hysteresis for an ɛ₁(T) temperature cycle has been predicted and observed. This however, could not be confirmed by the measurements of thinner films. A shift of transition temperatures as well as a temperature independent, non-switchable polarisation and a thermal hysteresis for P(VDF-TrFE) 56/44 have been observed. The impact of static electric fields on the polarisation and the phase transition has therefore been studied and simulated, showing that all aforementioned phenomena including a linear temperature dependence of the polarisation might originate from intrinsic electric fields. In further experiments the knowledge gained from single layer capacitors has been extended to bilayer copolymer thin films of different molar composition. Bilayers have been deposited by succeeding cycles of spin coating from solution. Single layers and their bilayer combination have been studied individually in order to prove the layers stability. The individual layers have been found to be physically stable. But while the bilayers reproduced the main ɛ₁(T) properties of the single layers qualitatively, quantitative numbers could not be explained by a simple serial connection of capacitors. Furthermore, a linear behaviour of the polarisation throughout the measured temperature range has been observed. This was found to match the behaviour predicted considering a constant electric field. Retention time is an important quantity for memory applications. Hence, the retention behaviour of VDF-TrFE copolymer thin films has been determined using dielectric non-linearities. The polarisation loss in P(VDF-TrFE) poled samples has been found to be less than 20\% if recorded over several days. The loss increases significantly if the samples have been poled with lower amplitudes, causing an unsaturated polarisation. The main loss was attributed to injected charges. Additionally, measurements of dielectric non-linearities have been proven to be a sensitive and non-destructive tool to measure the retention behaviour. Finally, a ferroelectric field effect transistor using mainly organic materials (FerrOFET) has been successfully studied. DiNaphtho[2,3-b:2',3'-f]Thieno[3,2-b]Thiophene (DNTT) has proven to be a stable, suitable organic semiconductor to build up ferroelectric memory devices. Furthermore, an oxidised aluminium bottom electrode and additional dielectric layers, i.e. parylene C, have proven to reduce the leakage current and therefore enhance the performance significantly.}, language = {en} } @phdthesis{Berger2012, author = {Berger, Florian}, title = {Different modes of cooperative transport by molecular motors}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-60319}, school = {Universit{\"a}t Potsdam}, year = {2012}, abstract = {Cargo transport by molecular motors is ubiquitous in all eukaryotic cells and is typically driven cooperatively by several molecular motors, which may belong to one or several motor species like kinesin, dynein or myosin. These motor proteins transport cargos such as RNAs, protein complexes or organelles along filaments, from which they unbind after a finite run length. Understanding how these motors interact and how their movements are coordinated and regulated is a central and challenging problem in studies of intracellular transport. In this thesis, we describe a general theoretical framework for the analysis of such transport processes, which enables us to explain the behavior of intracellular cargos based on the transport properties of individual motors and their interactions. Motivated by recent in vitro experiments, we address two different modes of transport: unidirectional transport by two identical motors and cooperative transport by actively walking and passively diffusing motors. The case of cargo transport by two identical motors involves an elastic coupling between the motors that can reduce the motors' velocity and/or the binding time to the filament. We show that this elastic coupling leads, in general, to four distinct transport regimes. In addition to a weak coupling regime, kinesin and dynein motors are found to exhibit a strong coupling and an enhanced unbinding regime, whereas myosin motors are predicted to attain a reduced velocity regime. All of these regimes, which we derive both by analytical calculations and by general time scale arguments, can be explored experimentally by varying the elastic coupling strength. In addition, using the time scale arguments, we explain why previous studies came to different conclusions about the effect and relevance of motor-motor interference. In this way, our theory provides a general and unifying framework for understanding the dynamical behavior of two elastically coupled molecular motors. The second mode of transport studied in this thesis is cargo transport by actively pulling and passively diffusing motors. Although these passive motors do not participate in active transport, they strongly enhance the overall cargo run length. When an active motor unbinds, the cargo is still tethered to the filament by the passive motors, giving the unbound motor the chance to rebind and continue its active walk. We develop a stochastic description for such cooperative behavior and explicitly derive the enhanced run length for a cargo transported by one actively pulling and one passively diffusing motor. We generalize our description to the case of several pulling and diffusing motors and find an exponential increase of the run length with the number of involved motors.}, language = {en} } @phdthesis{Gengel2021, author = {Gengel, Erik}, title = {Direct and inverse problems of network analysis}, doi = {10.25932/publishup-51236}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-512367}, school = {Universit{\"a}t Potsdam}, pages = {VIII, 102}, year = {2021}, abstract = {Selfsustained oscillations are some of the most commonly observed phenomena in biological systems. They emanate from non-linear systems in a heterogeneous environment and can be described by the theory of dynamical systems. Part of this theory considers reduced models of the oscillator dynamics by means of amplitudes and a phase variable. Such variables are highly attractive for theoretical and experimental studies. Theoretically these variables correspond to an integrable linearization of the generally non-linear system. Experimentally, there exist well established approaches to extract phases from oscillator signals. Notably, one can define phase models also for networks of oscillators. One highly active field examines effects of non-local coupling among oscillators, which is thought to play a key role in networks with strong coupling. The dissertation introduces and expands the knowledge about high-order phase coupling in networks of oscillators. Mathematical calculations consider the Stuart-Landau oscillator. A novel phase estimation scheme for direct observations of an oscillator dynamics is introduced based on numerics. A numerical study of high-order phase coupling applies a Fourier fit for the Stuart-Landau and for the van-der-Pol oscillator. The numerical approach is finally tested on observation-based phase estimates of the Morris-Lecar neuron. A popular approach for the construction of phases from signals is based on phase demodulation by means of the Hilbert transform. Generally, observations of oscillations contain a small and generic variation of their amplitude. The work presents a way to quantify how much the variations of signal amplitude spoil a phase demodulation procedure. For the ideal case of phase modulated signals, amplitude modulations vanish. However, the Hilbert transform produces artificial variations of the reconstructed amplitude even in this case. The work proposes a novel procedure called Iterative Hilbert Transform Embedding to obtain an optimal demodulation of signals. The text presents numerous examples and tests of application for the method, covering multicomponent signals, observables of highly stable limit cycle oscillations and noisy phase dynamics. The numerical results are supported by a spectral theory of convergence for weak phase modulations.}, language = {en} } @phdthesis{Kretschmer2017, author = {Kretschmer, Marlene}, title = {Disentangling causal pathways of the stratospheric polar vortex}, school = {Universit{\"a}t Potsdam}, pages = {171}, year = {2017}, language = {en} } @phdthesis{Guber2017, author = {Guber, Christoph Rudolf}, title = {Dust depletion of Ca and Ti in quasar absorption-line systems}, school = {Universit{\"a}t Potsdam}, pages = {178}, year = {2017}, language = {en} } @phdthesis{SaadHassanin2018, author = {Saad Hassanin, Alshaimaa}, title = {Dynamic coronal mass ejection process and magnetic reconnection}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-419626}, school = {Universit{\"a}t Potsdam}, pages = {xix, 113}, year = {2018}, abstract = {The Sun is the nearest star to the Earth. It consists of an interior and an atmosphere. The convection zone is the outermost layer of the solar interior. A flux rope may emerge as a coherent structure from the convection zone into the solar atmosphere or be formed by magnetic reconnection in the atmosphere. A flux rope is a bundle of magnetic field lines twisting around an axis field line, creating a helical shape by which dense filament material can be supported against gravity. The flux rope is also considered as the key structure of the most energetic phenomena in the solar system, such as coronal mass ejections (CMEs) and flares. These magnetic flux ropes can produce severe geomagnetic storms. In particular, to improve the ability to forecast space weather, it is important to enrich our knowledge about the dynamic formation of flux ropes and the underlying physical mechanisms that initiate their eruption, such as a CME. A confined eruption consists of a filament eruption and usually an associated are, but does not evolve into a CME; rather, the moving plasma is halted in the solar corona and usually seen to fall back. The first detailed observations of a confined filament eruption were obtained on 2002 May 27by the TRACE satellite in the 195 A band. So, in the Chapter 3, we focus on a flux rope instability model. A twisted flux rope can become unstable by entering the kink instability regime. We show that the kink instability, which occurs if the twist of a flux rope exceeds a critical value, is capable of initiating of an eruption. This model is tested against the well observed confined eruption on 2002 May 27 in a parametric magnetohydrodynamic (MHD) simulation study that comprises all phases of the event. Very good agreement with the essential observed properties is obtained, only except for a relatively poor matching of the initial filament height. Therefore, in Chapter 4, we submerge the center point of the flux rope deeper below the photosphere to obtain a flatter coronal rope section and a better matching with the initial height profile of the erupting filament. This implies a more realistic inclusion of the photospheric line tying. All basic assumptions and the other parameter settings are kept the same as in Chapter 3. This complement of the parametric study shows that the flux rope instability model can yield an even better match with the observational data. We also focus in Chapters 3 and 4 on the magnetic reconnection during the confined eruption, demonstrating that it occurs in two distinct locations and phases that correspond to the observed brightenings and changes of topology, and consider the fate of the erupting flux, which can reform a (less twisted) flux rope. The Sun also produces series of homologous eruptions, i.e. eruptions which occur repetitively in the same active region and are of similar morphology. Therefore, in Chapter 5, we employ the reformed flux rope as a new initial condition, to investigate the possibility of subsequent homologous eruptions. Free magnetic energy is built up by imposing motions in the bottom boundary, such as converging motions, leading to flux cancellation. We apply converging motions in the sunspot area, such that a small part of the flux from the sunspots with different polarities is transported toward the polarity inversion line (PIL) and cancels with each other. The reconnection associated with the cancellation process forms more helical magnetic flux around the reformed flux rope, which leads to a second and a third eruption. In this study, we obtain the first MHD simulation results of a homologous sequence of eruptions that show a transition from a confined to two ejective eruptions, based on the reformation of a flux rope after each eruption.}, language = {en} } @phdthesis{Schneider2004, author = {Schneider, Judith}, title = {Dynamical structures and manifold detection in 2D and 3D chaotic flows}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0001696}, school = {Universit{\"a}t Potsdam}, year = {2004}, abstract = {In dieser Arbeit werden die dynamischen Strukturen und Mannigfaltigkeiten in geschlossenen chaotischen Systemen untersucht. Das Wissen um diese dynamischen Strukturen (und Mannigfaltigkeiten) ist von Bedeutung, da sie uns einen ersten {\"U}berblick {\"u}ber die Dynamik des Systems geben, dass heisst, mit ihrer Hilfe sind wir in der Lage, das System zu charakterisieren und eventuell sogar seine Dynamik vorherzusagen. Die Visualisierung der dynamischen Strukturen, speziell in geschlossenen chaotischen Systemen, ist ein schwieriger und oft langer Prozess. Hier werden wir die sogenannte 'Leaking-Methode' (an Beispielen einfacher mathematischer Modelle wie der B{\"a}cker- oder der Sinus Abbildung) vorstellen, mit deren Hilfe wir die M{\"o}glichkeit haben, Teile der Mannigfaltigkeiten des chaotischen Sattels des Systems zu visualisieren. Vergleiche zwischen den gewonnenen Strukturen und Strukturen die durch chemische oder biologische Reaktionen hervorgerufen werden, werden anhand eines kinematischen Modells des Golfstroms durchgef{\"u}hrt. Es wird gezeigt, dass mittels der Leaking-Methode dynamische Strukturen auch in Umweltsystemen sichtbar gemacht werden k{\"o}nnen. Am Beispiel eines realistischen Modells des Mittelmeeres erweitern wir die Leaking-Methode zur sogenannten 'Exchange-Methode'. Diese erlaubt es den Transport zwischen zwei Regionen zu charakterisieren, die Transport-Routen und Austausch-Bassins sichtbar zu machen und die Austausch-Zeiten zu berechnen. Austausch-Bassins und Zeiten werden f{\"u}r die n{\"o}rdliche und s{\"u}dliche Region des westlichen Mittelmeeres pr{\"a}sentiert. Weiterhin werden Mischungseigenschaften im Erdmantel charakterisiert und die geometrischen Eigenschaften von Mannigfaltigkeiten in einem 3dimensionalen mathematischen Modell (ABC-Abbildung) untersucht.}, language = {en} } @phdthesis{Sachse2017, author = {Sachse, Manuel}, title = {Dynamics and distribution of dust ejected from the Galilean moons of Jupiter}, school = {Universit{\"a}t Potsdam}, pages = {105}, year = {2017}, language = {en} } @phdthesis{Washuettl2004, author = {Wash{\"u}ttl, Albert}, title = {EI Eridani and the art of doppler imaging : a long-term study}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0001714}, school = {Universit{\"a}t Potsdam}, year = {2004}, abstract = {Das Verst{\"a}ndnis magnetisch verursachter Aktivit{\"a}t auf Sternen sowie der zugrundeliegenden Dynamoprozesse ist von fundamentaler Bedeutung f{\"u}r das Verst{\"a}ndnis von Entstehung und Entwicklung von Sternen sowie des Lebens im Universum. Sichtbare Erscheinungen dieser stellaren Aktivit{\"a}t sind u.a. Sternflecken, welche als Indikatoren des zugrundeliegenden Magnetfeldes dienen. Solche Flecken k{\"o}nnen auf anderen Sternen als der Sonne nicht direkt beobachtet werden, zumal mit den heutigen technischen Mitteln eine Aufl{\"o}sung der Oberfl{\"a}che selbst der benachbarten Sterne unm{\"o}glich ist. Eine indirekte Rekonstruktionsmethode namens 'Doppler Imaging' erlaubt es jedoch, auf die Temperaturverteilung auf der Sternoberfl{\"a}che zu schließen. F{\"u}r diese Arbeit wurden elf Jahre kontinuierlicher spektroskopischer Beobachtungen des aktiven Doppelsterns EI Eridani herangezogen, um insgesamt 34 Dopplerkarten zu erstellen. In der Folge wird versucht, eine Grundlage zu schaffen f{\"u}r die Analyse des zweidimensionalen Informationsgehalts dieser Karten. Drei Oberfl{\"a}chenkartenparameter werden vorgeschlagen: gemittelte Temperatur, getrennt f{\"u}r verschiedenen stellare Breitenb{\"a}nder; relative Fleckenh{\"a}ufigkeit; und, zum Zwecke der Auswertung der strukturellen Temperaturverteilung, L{\"a}ngen- und Breiten-Ortsfunktion der Sternfleckenh{\"a}ufung. Die resultierenden Werte zeigen deutlich, daß kein zeitlicher Zusammenhang mit dem photometrischen Aktivit{\"a}tszyklus besteht. Die Morphologie der Fleckenverteilung bleibt w{\"a}hrend des kompletten Beobachtungszeitraums im wesentlichen konstant. Im Gegensatz zur Sonne gibt es also, im beobachteten Zeitraum und innerhalb der bestehenden Genauigkeit, keinen Fleckenzyklus auf dem aktiven Stern EI Eri. Dar{\"u}berhinaus wurde eine ausf{\"u}hrliche Studie der stellaren Parameter von EI Eri und eine vorl{\"a}ufige Absch{\"a}tzung der differentiellen Rotation auf EI Eri durchgef{\"u}hrt, die eine anti-solare Ausrichtung aufzuweisen scheint, d.h. der Pol rotiert schneller als der {\"A}quator.}, language = {en} } @phdthesis{CanUcar2017, author = {Can Ucar, Mehmet}, title = {Elastic interactions between antagonistic molecular motors}, school = {Universit{\"a}t Potsdam}, pages = {106}, year = {2017}, language = {en} } @phdthesis{Bischofs2004, author = {Bischofs, Ilka Bettina}, title = {Elastic interactions of cellular force patterns}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0001767}, school = {Universit{\"a}t Potsdam}, year = {2004}, abstract = {Gewebezellen sammeln st{\"a}ndig Informationen {\"u}ber die mechanischen Eigenschaften ihrer Umgebung, indem sie aktiv an dieser ziehen. Diese Kr{\"a}fte werden an Zell-Matrix-Kontakten {\"u}bertragen, die als Mechanosensoren fungieren. J{\"u}ngste Experimente mit Zellen auf elastischen Substraten zeigen, dass Zellen sehr empfindlich auf Ver{\"a}nderungen der effektiven Steifigkeit ihrer Umgebung reagieren, die zu einer Reorganisation des Zytoskeletts f{\"u}hren k{\"o}nnen. In dieser Arbeit wird ein theoretisches Model entwickelt, um die Selbstorganisation von Zellen in weichen Materialien vorherzusagen. Obwohl das Zellverhalten durch komplexe regulatorische Vorg{\"a}nge in der Zelle gesteuert wird, scheint die typische Antwort von Zellen auf mechanische Reize eine einfache Pr{\"a}ferenz f{\"u}r große effektive Steifigkeit der Umgebung zu sein, m{\"o}glicherweise weil in einer steiferen Umgebung Kr{\"a}fte an den Kontakten effektiver aufgebaut werden k{\"o}nnen. Der Begriff Steifigkeit umfasst dabei sowohl Effekte, die durch gr{\"o}ßere H{\"a}rte als auch durch elastische Verzerrungsfelder in der Umgebung verursacht werden. Diese Beobachtung kann man als ein Extremalprinzip in der Elastizit{\"a}tstheorie formulieren. Indem man das zellul{\"a}re Kraftmuster spezifiziert, mit dem Zellen mit ihrer Umgebung wechselwirken, und die Umgebung selbst als linear elastisches Material modelliert, kann damit die optimale Orientierung und Position von Zellen vorhergesagt werden. Es werden mehrere praktisch relevante Beispiele f{\"u}r Zellorganisation theoretisch betrachtet: Zellen in externen Spannungsfeldern und Zellen in der N{\"a}he von Grenzfl{\"a}chen f{\"u}r verschiedene Geometrien und Randbedingungen des elastischen Mediums. Daf{\"u}r werden die entsprechenden elastischen Randwertprobleme in Vollraum, Halbraum und Kugel exakt gel{\"o}st. Die Vorhersagen des Models stimmen hervorragend mit experimentellen Befunden f{\"u}r Fibroblastzellen {\"u}berein, sowohl auf elastischen Substraten als auch in physiologischen Hydrogelen. Mechanisch aktive Zellen wie Fibroblasten k{\"o}nnen auch elastisch miteinander wechselwirken. Es werden daher optimale Strukturen als Funktion von Materialeigenschaften und Zelldichte bzw. der Geometrie der Zellpositionen berechnet. Schließlich wird mit Hilfe von Monte Carlo Simulationen der Einfluss stochastischer St{\"o}rungen auf die Strukturbildung untersucht. Das vorliegende Model tr{\"a}gt nicht nur zu einem besseren Verst{\"a}ndnis von vielen physiologischen Situationen bei, sondern k{\"o}nnte in Zukunft auch f{\"u}r biomedizinische Anwendungen benutzt werden, um zum Beispiel Protokolle f{\"u}r k{\"u}nstliche Gewebe im Bezug auf Substratgeometrie, Randbedingungen, Materialeigenschaften oder Zelldichte zu optimieren.}, language = {en} } @phdthesis{Wang2020, author = {Wang, Jingwen}, title = {Electret properties of polypropylene with surface chemical modification and crystalline reconstruction}, doi = {10.25932/publishup-47027}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-470271}, school = {Universit{\"a}t Potsdam}, pages = {vi, 121}, year = {2020}, abstract = {As one of the most-produced commodity polymers, polypropylene draws considerable scientific and commercial interest as an electret material. In the present thesis, the influence of the surface chemical modification and crystalline reconstruction on the electret properties of the polypropylene thin films will be discussed. The chemical treatment with orthophosphoric acid can significantly improve the surface charge stability of the polypropylene electrets by introducing phosphorus- and oxygen-containing structures onto the modified surface. The thermally stimulated discharge measurement and charge profiling by means of piezoelectrically generated pressure steps are used to investigate the electret behaviour. It is concluded that deep traps of limited number density are created during the treatment with inorganic chemicals. Hence, the improvement dramatically decreases when the surface-charge density is substantially higher than ±1.2×10^(-3) C·m^(-2). The newly formed traps also show a higher trapping energy for negative charges. The energetic distributions of the traps in the non-treated and chemically treated samples offer an insight regarding the surface and foreign-chemical dominance on the charge storage and transport in the polypropylene electrets. Additionally, different electret properties are observed on the polypropylene films with the spherulitic and transcrystalline structures. It indicates the dependence of the charge storage and transport on the crystallite and molecular orientations in the crystalline phase. In general, a more diverse crystalline growth in the spherulitic samples can result in a more complex energetic trap distribution, in comparison to that in a transcrystalline polypropylene. The double-layer transcrystalline polypropylene film with a crystalline interface in the middle can be obtained by crystallising the film in contact with rough moulding surfaces on both sides. A layer of heterocharges appears on each side of the interface in the double-layer transcrystalline polypropylene electrets after the thermal poling. However, there is no charge captured within the transcrystalline layers. The phenomenon reveals the importance of the crystalline interface in terms of creating traps with the higher activation energy in polypropylene. The present studies highlight the fact that even slight variations in the polypropylene film may lead to dramatic differences in its electret properties.}, language = {en} } @phdthesis{RodriguezZuluaga2020, author = {Rodriguez Zuluaga, Juan}, title = {Electric and magnetic characteristics of equatorial plasma depletions}, doi = {10.25932/publishup-44587}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-445873}, school = {Universit{\"a}t Potsdam}, pages = {xvi, 87}, year = {2020}, abstract = {Near-Earth space represents a significant scientific and technological challenge. Particularly at magnetic low-latitudes, the horizontal magnetic field geometry at the dip equator and its closed field-lines support the existence of a distinct electric current system, abrupt electric field variations and the development of plasma irregularities. Of particular interest are small-scale irregularities associated with equatorial plasma depletions (EPDs). They are responsible for the disruption of trans-ionospheric radio waves used for navigation, communication, and Earth observation. The fast increase of satellite missions makes it imperative to study the near-Earth space, especially the phenomena known to harm space technology or disrupt their signals. EPDs correspond to the large-scale structure (i.e., tens to hundreds of kilometers) of topside F region irregularities commonly known as Spread F. They are observed as depleted-plasma density channels aligned with the ambient magnetic field in the post-sunset low-latitude ionosphere. Although the climatological variability of their occurrence in terms of season, longitude, local time and solar flux is well-known, their day to day variability is not. The sparse observations from ground-based instruments like radars and the few simultaneous measurements of ionospheric parameters by space-based instruments have left gaps in the knowledge of EPDs essential to comprehend their variability. In this dissertation, I profited from the unique observations of the ESA's Swarm constellation mission launched in November 2013 to tackle three issues that revealed novel and significant results on the current knowledge of EPDs. I used Swarm's measurements of the electron density, magnetic, and electric fields to answer, (1.) what is the direction of propagation of the electromagnetic energy associated with EPDs?, (2.) what are the spatial and temporal characteristics of the electric currents (field-aligned and diamagnetic currents) related to EPDs, i.e., seasonal/geographical, and local time dependencies?, and (3.) under what conditions does the balance between magnetic and plasma pressure across EPDs occur? The results indicate that: (1.) The electromagnetic energy associated with EPDs presents a preference for interhemispheric flows; that is, the related Poynting flux directs from one magnetic hemisphere to the other and varies with longitude and season. (2.) The field-aligned currents at the edges of EPDs are interhemispheric. They generally close in the hemisphere with the highest Pedersen conductance. Such hemispherical preference presents a seasonal/longitudinal dependence. The diamagnetic currents increase or decrease the magnetic pressure inside EPDs. These two effects rely on variations of the plasma temperature inside the EPDs that depend on longitude and local time. (3.) EPDs present lower or higher plasma pressure than the ambient. For low-pressure EPDs the plasma pressure gradients are mostly dominated by variations of the plasma density so that variations of the temperature are negligible. High-pressure EPDs suggest significant temperature variations with magnitudes of approximately twice the ambient. Since their occurrence is more frequent in the vicinity of the South Atlantic magnetic anomaly, such high temperatures are suggested to be due to particle precipitation. In a broader context, this dissertation shows how dedicated satellite missions with high-resolution capabilities improve the specification of the low-latitude ionospheric electrodynamics and expand knowledge on EPDs which is valuable for current and future communication, navigation, and Earth-observing missions. The contributions of this investigation represent several 'firsts' in the study of EPDs: (1.) The first observational evidence of interhemispheric electromagnetic energy flux and field-aligned currents. (2.) The first spatial and temporal characterization of EPDs based on their associated field-aligned and diamagnetic currents. (3.) The first evidence of high plasma pressure in regions of depleted plasma density in the ionosphere. These findings provide new insights that promise to advance our current knowledge of not only EPDs but the low-latitude post-sunset ionosphere environment.}, language = {en} } @phdthesis{Nguyen2019, author = {Nguyen, Quyet Doan}, title = {Electro-acoustical probing of space-charge and dipole-polarization profiles in polymer dielectrics for electret and electrical-insulation applications}, doi = {10.25932/publishup-44562}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-445629}, school = {Universit{\"a}t Potsdam}, pages = {105}, year = {2019}, abstract = {Electrets are dielectrics with quasi-permanent electric charge and/or dipoles, sometimes can be regarded as an electric analogy to a magnet. Since the discovery of the excellent charge retention capacity of poly(tetrafluoro ethylene) and the invention of the electret microphone, electrets have grown out of a scientific curiosity to an important application both in science and technology. The history of electret research goes hand in hand with the quest for new materials with better capacity at charge and/or dipole retention. To be useful, electrets normally have to be charged/poled to render them electro-active. This process involves electric-charge deposition and/or electric dipole orientation within the dielectrics ` surfaces and bulk. Knowledge of the spatial distribution of electric charge and/or dipole polarization after their deposition and subsequent decay is crucial in the task to improve their stability in the dielectrics. Likewise, for dielectrics used in electrical insulation applications, there are also needs for accumulated space-charge and polarization spatial profiling. Traditionally, space-charge accumulation and large dipole polarization within insulating dielectrics is considered undesirable and harmful to the insulating dielectrics as they might cause dielectric loss and could lead to internal electric field distortion and local field enhancement. High local electric field could trigger several aging processes and reduce the insulating dielectrics' lifetime. However, with the advent of high-voltage DC transmission and high-voltage capacitor for energy storage, these are no longer the case. There are some overlapped between the two fields of electrets and electric insulation. While quasi-permanently trapped electric-charge and/or large remanent dipole polarization are the requisites for electret operation, stably trapped electric charge in electric insulation helps reduce electric charge transport and overall reduced electric conductivity. Controlled charge trapping can help in preventing further charge injection and accumulation as well as serving as field grading purpose in insulating dielectrics whereas large dipole polarization can be utilized in energy storage applications. In this thesis, the Piezoelectrically-generated Pressure Steps (PPSs) were employed as a nondestructive method to probe the electric-charge and dipole polarization distribution in a range of thin film (several hundred micron) polymer-based materials, namely polypropylene (PP), low-density polyethylene/magnesium oxide (LDPE/MgO) nanocomposites and poly(vinylidene fluoride-co- trifluoro ethylene) (P(VDF-TrFE)) copolymer. PP film surface-treated with phosphoric acid to introduce surfacial isolated nanostructures serves as example of 2-dimensional nano-composites whereas LDPE/MgO serves as the case of 3-dimensional nano-composites with MgO nano-particles dispersed in LDPE polymer matrix. It is evidenced that the nanoparticles on the surface of acid-treated PP and in the bulk of LDPE/MgO nanocomposites improve charge trapping capacity of the respective material and prevent further charge injection and transport and that the enhanced charge trapping capacity makes PP and LDPE/MgO nanocomposites potential materials for both electret and electrical insulation applications. As for PVDF and VDF-based copolymers, the remanent spatial polarization distribution depends critically on poling method as well as specific parameters used in the respective poling method. In this work, homogeneous polarization poling of P(VDF-TrFE) copolymers with different VDF-contents have been attempted with hysteresis cyclical poling. The behaviour of remanent polarization growth and spatial polarization distribution are reported and discussed. The Piezoelectrically-generated Pressure Steps (PPSs) method has proven as a powerful method for the charge storage and transport characterization of a wide range of polymer material from nonpolar, to polar, to polymer nanocomposites category.}, language = {en} } @phdthesis{Miteva2007, author = {Miteva, Rositsa Stoycheva}, title = {Electron acceleration at localized wave structures in the solar corona}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-14775}, school = {Universit{\"a}t Potsdam}, year = {2007}, abstract = {Our dynamic Sun manifests its activity by different phenomena: from the 11-year cyclic sunspot pattern to the unpredictable and violent explosions in the case of solar flares. During flares, a huge amount of the stored magnetic energy is suddenly released and a substantial part of this energy is carried by the energetic electrons, considered to be the source of the nonthermal radio and X-ray radiation. One of the most important and still open question in solar physics is how the electrons are accelerated up to high energies within (the observed in the radio emission) short time scales. Because the acceleration site is extremely small in spatial extent as well (compared to the solar radius), the electron acceleration is regarded as a local process. The search for localized wave structures in the solar corona that are able to accelerate electrons together with the theoretical and numerical description of the conditions and requirements for this process, is the aim of the dissertation. Two models of electron acceleration in the solar corona are proposed in the dissertation: I. Electron acceleration due to the solar jet interaction with the background coronal plasma (the jet--plasma interaction) A jet is formed when the newly reconnected and highly curved magnetic field lines are relaxed by shooting plasma away from the reconnection site. Such jets, as observed in soft X-rays with the Yohkoh satellite, are spatially and temporally associated with beams of nonthermal electrons (in terms of the so-called type III metric radio bursts) propagating through the corona. A model that attempts to give an explanation for such observational facts is developed here. Initially, the interaction of such jets with the background plasma leads to an (ion-acoustic) instability associated with growing of electrostatic fluctuations in time for certain range of the jet initial velocity. During this process, any test electron that happen to feel this electrostatic wave field is drawn to co-move with the wave, gaining energy from it. When the jet speed has a value greater or lower than the one, required by the instability range, such wave excitation cannot be sustained and the process of electron energization (acceleration and/or heating) ceases. Hence, the electrons can propagate further in the corona and be detected as type III radio burst, for example. II. Electron acceleration due to attached whistler waves in the upstream region of coronal shocks (the electron--whistler--shock interaction) Coronal shocks are also able to accelerate electrons, as observed by the so-called type II metric radio bursts (the radio signature of a shock wave in the corona). From in-situ observations in space, e.g., at shocks related to co-rotating interaction regions, it is known that nonthermal electrons are produced preferably at shocks with attached whistler wave packets in their upstream regions. Motivated by these observations and assuming that the physical processes at shocks are the same in the corona as in the interplanetary medium, a new model of electron acceleration at coronal shocks is presented in the dissertation, where the electrons are accelerated by their interaction with such whistlers. The protons inflowing toward the shock are reflected there by nearly conserving their magnetic moment, so that they get a substantial velocity gain in the case of a quasi-perpendicular shock geometry, i.e, the angle between the shock normal and the upstream magnetic field is in the range 50--80 degrees. The so-accelerated protons are able to excite whistler waves in a certain frequency range in the upstream region. When these whistlers (comprising the localized wave structure in this case) are formed, only the incoming electrons are now able to interact resonantly with them. But only a part of these electrons fulfill the the electron--whistler wave resonance condition. Due to such resonant interaction (i.e., of these electrons with the whistlers), the electrons are accelerated in the electric and magnetic wave field within just several whistler periods. While gaining energy from the whistler wave field, the electrons reach the shock front and, subsequently, a major part of them are reflected back into the upstream region, since the shock accompanied with a jump of the magnetic field acts as a magnetic mirror. Co-moving with the whistlers now, the reflected electrons are out of resonance and hence can propagate undisturbed into the far upstream region, where they are detected in terms of type II metric radio bursts. In summary, the kinetic energy of protons is transfered into electrons by the action of localized wave structures in both cases, i.e., at jets outflowing from the magnetic reconnection site and at shock waves in the corona.}, language = {en} } @phdthesis{Born2021, author = {Born, Artur}, title = {Electronic structure, quasi-particle interaction and relaxation in 3d-elements from X-ray spectroscopy}, school = {Universit{\"a}t Potsdam}, pages = {123}, year = {2021}, abstract = {Any physical system can be described on the level of interacting particles, thus it is of fundamental importance to improve the scientific understanding of interacting many-body systems. This thesis experimentally addresses specific quasi-particle interactions, namely interactions be- tween electrons and between electrons and phonons. It describes the consequential effects of those processes on the electronic structure and the core-hole relaxation pathways in 3d metals. Despite the great amount of experimental and theoretical studies of these interactions and their impact on the behavior of solid-state matter, there are still open questions concerning the cor- responding physical, chemical and mechanical properties of solid-state matter. Especially, the study of 3d metals and their compounds is a great experimental challenge, since those exhibit a variety of spectral features originating from many-body effects such as multiplet splitting, shake up/off satellites, vibrationally excited states or more complex effects like superconductivity and ultrafast demagnetization. In X-ray spectroscopy, these effects often produce overlapping fea- tures, complicating the analysis and limiting the understanding. In this thesis, to overcome the limitations set by conventional X-ray spectroscopy, two different experimental approaches were successfully refined, namely Auger electron photoelectron coincidence spectroscopy (APECS) and temperature-dependent X-ray emission spectroscopy (tXES), which enabled the separation of different core-hole relaxation pathways and the isolation of the impact of specific many-body interactions in the experimental spectra. APECS was utilized at the new Coincidence electron spectroscopy for chemical analysis (Co- ESCA) station at BESSY II to study the core-hole decay and electron-correlation effects in single- crystal Ni, Cu and Co. The observation of photoelectrons in coincidence with Auger electrons allows for the separation of the initial and final state effects in the Auger electron spectra. The results show that a Cu LV V Auger spectrum can be represented by broadened atomic multiplets confirming the localized nature of the intermediate core-hole states. In contrast, the Co LV V Auger spectrum is band-like and can be represented by the self-convolution of the valence band. Ni behaves mixed, localized and itinerant. Thus, the Ni Auger spectrum can only be represented by a mixture of atomic multiplet peaks and the self-convoluted valence band. In the case of Ni, the LV V Auger electrons in coincidence with the 6 eV satellite photoelectrons were also stud- ied. Utilizing the core-hole clock method, the lifetime of the localized double-hole intermediate 2 p53d9 states of 1.8 fs could be determined. However, a fraction of these states delocalizes before the Auger decay contributing to the main peak. A similar delocalization was observed for the double-hole states produced by the L2L3M4,5 Coster-Kronig process. Additionally, the influence of surface oxidation on the Ni(111) 3p levels was studied with APECS. The Ni 3p PES spectrum is broad and featureless, due to overlapping many-body effects and gives little chance for exact analysis using conventional photoelectron spectroscopy. Utilizing APECS or precisely the final state selectivity of the method, the spectral width of the 3p levels could be narrowed and their positions and the spin-orbit splitting were determined. Moreover, due to the surface sensitivity of the method, the chemically shifted 3p photoelectron peaks originating from the oxidized surface and the bulk Ni were disentangled. For the study of the atomic electron-phonon spin-flip scattering in 3d metals as a spin-relaxation channel, the tXES method at the SolidFlexRIXS station was developed. The atomic spin-flip scat- tering was studied in single-crystal Ni, Cu, Co and in FeNi alloys, which show considerable dif- ferences in their behavior. The scattering rate in Ni increases with temperature, whereas the rate in Cu and Co remains constant within the measured temperature range up to 1000 K. In FeNi alloys, our results reveal that the spin-flip scattering is restricted by sublattice exchange energies J. The electron-phonon scattering driven spin-flips only appear in the case where the thermal energy ex- ceeds the exchange energy kT > J. This thresholding is an important microscopic process for the description of the sublattice dynamics in alloys, but as shown also relevant for elemental magnetic systems. Overall, the results strongly indicate that the spin-flip probability is correlated with the exchange energy, which might become an important parameter in the ultrafast demagnetization debate. Taken together, the applied experimental approaches allowed to study complex many-body effects in 3d metals. The results show that utilizing APECS enabled the distinction and clear assignment of otherwise overlapping features in AES or PES spectra of Ni, Cu, Co and NiO. This is of fundamental importance for the basic understanding of photoionization and core-hole decay processes but also for the chemical analysis in applied science. The measurement of the atomic electron-phonon spin-flip scattering rate utilizing tXES shows that the electron-phonon spin-flip scattering is a relevant atomic process for the macroscopic demagnetization process. Additionally, a temperature-dependent thresholding mechanism was discovered, which introduces an important dynamic factor into the electron-phonon spin-flip model.}, language = {en} } @phdthesis{Eschenlohr2012, author = {Eschenlohr, Andrea}, title = {Element-resolved ultrafast magnetization dynamics in ferromagnetic alloys and multilayers}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-62846}, school = {Universit{\"a}t Potsdam}, year = {2012}, abstract = {The microscopic origin of ultrafast demagnetization, i.e. the quenching of the magnetization of a ferromagnetic metal on a sub-picosecond timescale after laser excitation, is still only incompletely understood, despite a large body of experimental and theoretical work performed since the discovery of the effect more than 15 years ago. Time- and element-resolved x-ray magnetic circular dichroism measurements can provide insight into the microscopic processes behind ultrafast demagnetization as well as its dependence on materials properties. Using the BESSY II Femtoslicing facility, a storage ring based source of 100 fs short soft x-ray pulses, ultrafast magnetization dynamics of ferromagnetic NiFe and GdTb alloys as well as a Au/Ni layered structure were investigated in laser pump - x-ray probe experiments. After laser excitation, the constituents of Ni50Fe50 and Ni80Fe20 exhibit distinctly different time constants of demagnetization, leading to decoupled dynamics, despite the strong exchange interaction that couples the Ni and Fe sublattices under equilibrium conditions. Furthermore, the time constants of demagnetization for Ni and Fe are different in Ni50Fe50 and Ni80Fe20, and also different from the values for the respective pure elements. These variations are explained by taking the magnetic moments of the Ni and Fe sublattices, which are changed from the pure element values due to alloying, as well as the strength of the intersublattice exchange interaction into account. GdTb exhibits demagnetization in two steps, typical for rare earths. The time constant of the second, slower magnetization decay was previously linked to the strength of spin-lattice coupling in pure Gd and Tb, with the stronger, direct spin-lattice coupling in Tb leading to a faster demagnetization. In GdTb, the demagnetization of Gd follows Tb on all timescales. This is due to the opening of an additional channel for the dissipation of spin angular momentum to the lattice, since Gd magnetic moments in the alloy are coupled via indirect exchange interaction to neighboring Tb magnetic moments, which are in turn strongly coupled to the lattice. Time-resolved measurements of the ultrafast demagnetization of a Ni layer buried under a Au cap layer, thick enough to absorb nearly all of the incident pump laser light, showed a somewhat slower but still sub-picosecond demagnetization of the buried Ni layer in Au/Ni compared to a Ni reference sample. Supported by simulations, I conclude that demagnetization can thus be induced by transport of hot electrons excited in the Au layer into the Ni layer, without the need for direct interaction between photons and spins.}, language = {en} } @phdthesis{Schubert2014, author = {Schubert, Marcel}, title = {Elementary processes in layers of electron transporting Donor-acceptor copolymers : investigation of charge transport and application to organic solar cells}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-70791}, school = {Universit{\"a}t Potsdam}, year = {2014}, abstract = {Donor-acceptor (D-A) copolymers have revolutionized the field of organic electronics over the last decade. Comprised of a electron rich and an electron deficient molecular unit, these copolymers facilitate the systematic modification of the material's optoelectronic properties. The ability to tune the optical band gap and to optimize the molecular frontier orbitals as well as the manifold of structural sites that enable chemical modifications has created a tremendous variety of copolymer structures. Today, these materials reach or even exceed the performance of amorphous inorganic semiconductors. Most impressively, the charge carrier mobility of D-A copolymers has been pushed to the technologically important value of 10 cm^{2}V^{-1}s^{-1}. Furthermore, owed to their enormous variability they are the material of choice for the donor component in organic solar cells, which have recently surpassed the efficiency threshold of 10\%. Because of the great number of available D-A copolymers and due to their fast chemical evolution, there is a significant lack of understanding of the fundamental physical properties of these materials. Furthermore, the complex chemical and electronic structure of D-A copolymers in combination with their semi-crystalline morphology impede a straightforward identification of the microscopic origin of their superior performance. In this thesis, two aspects of prototype D-A copolymers were analysed. These are the investigation of electron transport in several copolymers and the application of low band gap copolymers as acceptor component in organic solar cells. In the first part, the investigation of a series of chemically modified fluorene-based copolymers is presented. The charge carrier mobility varies strongly between the different derivatives, although only moderate structural changes on the copolymers structure were made. Furthermore, rather unusual photocurrent transients were observed for one of the copolymers. Numerical simulations of the experimental results reveal that this behavior arises from a severe trapping of electrons in an exponential distribution of trap states. Based on the comparison of simulation and experiment, the general impact of charge carrier trapping on the shape of photo-CELIV and time-of-flight transients is discussed. In addition, the high performance naphthalenediimide (NDI)-based copolymer P(NDI2OD-T2) was characterized. It is shown that the copolymer posses one of the highest electron mobilities reported so far, which makes it attractive to be used as the electron accepting component in organic photovoltaic cells.\par Solar cells were prepared from two NDI-containing copolymers, blended with the hole transporting polymer P3HT. I demonstrate that the use of appropriate, high boiling point solvents can significantly increase the power conversion efficiency of these devices. Spectroscopic studies reveal that the pre-aggregation of the copolymers is suppressed in these solvents, which has a strong impact on the blend morphology. Finally, a systematic study of P3HT:P(NDI2OD-T2) blends is presented, which quantifies the processes that limit the efficiency of devices. The major loss channel for excited states was determined by transient and steady state spectroscopic investigations: the majority of initially generated electron-hole pairs is annihilated by an ultrafast geminate recombination process. Furthermore, exciton self-trapping in P(NDI2OD-T2) domains account for an additional reduction of the efficiency. The correlation of the photocurrent to microscopic morphology parameters was used to disclose the factors that limit the charge generation efficiency. Our results suggest that the orientation of the donor and acceptor crystallites relative to each other represents the main factor that determines the free charge carrier yield in this material system. This provides an explanation for the overall low efficiencies that are generally observed in all-polymer solar cells.}, language = {en} } @phdthesis{Marwan2003, author = {Marwan, Norbert}, title = {Encounters with neighbours}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0000856}, school = {Universit{\"a}t Potsdam}, year = {2003}, abstract = {Diese Arbeit besch{\"a}ftigt sich mit verschiedenen Aspekten und Anwendungen von Recurrence Plots. Nach einer {\"U}bersicht {\"u}ber Methoden, die auf Recurrence Plots basieren, werden neue Komplexit{\"a}tsmaße eingef{\"u}hrt, die geometrische Strukturen in den Recurrence Plots beschreiben. Diese neuen Maße erlauben die Identifikation von Chaos-Chaos-{\"U}berg{\"a}ngen in dynamischen Prozessen. In einem weiteren Schritt werden Cross Recurrence Plots eingef{\"u}hrt, mit denen zwei verschiedene Prozesse untersucht werden. Diese bivariate Analyse erm{\"o}glicht die Bewertung von Unterschieden zwischen zwei Prozessen oder das Anpassen der Zeitskalen von zwei Zeitreihen. Diese Technik kann auch genutzt werden, um {\"a}hnliche Abschnitte in zwei verschiedenen Datenreihen zu finden. Im Anschluß werden diese neuen Entwicklungen auf Daten verschiedener Art angewendet. Methoden, die auf Recurrence Plots basieren, k{\"o}nnen an die speziellen Probleme angepaßt werden, so daß viele weitere Anwendungen m{\"o}glich sind. Durch die Anwendung der neu eingef{\"u}hrten Komplexit{\"a}tsmaße k{\"o}nnen Chaos-Chaos-{\"U}berg{\"a}nge in Herzschlagdaten vor dem Auftreten einer lebensbedrohlichen Herzrhythmusst{\"o}rung festgestellt werden, was f{\"u}r die Entwicklung neuer Therapien dieser Herzrhythmusst{\"o}rungen von Bedeutung sein k{\"o}nnte. In einem weiteren Beispiel, in dem EEG-Daten aus einem kognitiv orientierten Experiment untersucht werden, erm{\"o}glichen diese Komplexit{\"a}tsmaße das Erkennen von spezifischen Reaktionen im Gehirn bereits in Einzeltests. Normalerweise k{\"o}nnen diese Reaktionen erst durch die Auswertung von vielen Einzeltests erkannt werden. Mit der Hilfe von Cross Recurrence Plots wird die Existenz einer klimatischen Zirkulation, die der heutigen El Ni{\~n}o/ Southern Oscillation sehr {\"a}hnlich ist, im Nordwesten Argentiniens vor etwa 34000 Jahren nachgewiesen. Außerdem k{\"o}nnen mit Cross Recurrence Plots die Zeitskalen verschiedener Bohrlochdaten aufeinander abgeglichen werden. Diese Methode kann auch dazu genutzt werden, ein geologisches Profil mit Hilfe eines Referenzprofiles mit bekannter Zeitskala zu datieren. Weitere Beispiele aus den Gebieten der Molekularbiologie und der Spracherkennung unterstreichen das Potential dieser Methode.}, language = {en} } @phdthesis{Sun2024, author = {Sun, Bowen}, title = {Energy losses in low-offset organic solar cells}, doi = {10.25932/publishup-62143}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-621430}, school = {Universit{\"a}t Potsdam}, pages = {xxi, 190}, year = {2024}, abstract = {Organic solar cells (OSCs) represent a new generation of solar cells with a range of captivating attributes including low-cost, light-weight, aesthetically pleasing appearance, and flexibility. Different from traditional silicon solar cells, the photon-electron conversion in OSCs is usually accomplished in an active layer formed by blending two kinds of organic molecules (donor and acceptor) with different energy levels together. The first part of this thesis focuses on a better understanding of the role of the energetic offset and each recombination channel on the performance of these low-offset OSCs. By combining advanced experimental techniques with optical and electrical simulation, the energetic offsets between CT and excitons, several important insights were achieved: 1. The short circuit current density and fill-factor of low-offset systems are largely determined by field-dependent charge generation in such low-offset OSCs. Interestingly, it is strongly evident that such field-dependent charge generation originates from a field-dependent exciton dissociation yield. 2. The reduced energetic offset was found to be accompanied by strongly enhanced bimolecular recombination coefficient, which cannot be explained solely by exciton repopulation from CT states. This implies the existence of another dark decay channel apart from CT. The second focus of the thesis was on the technical perspective. In this thesis, the influence of optical artifacts in differential absorption spectroscopy upon the change of sample configuration and active layer thickness was studied. It is exemplified and discussed thoroughly and systematically in terms of optical simulations and experiments, how optical artifacts originated from non-uniform carrier profile and interference can manipulate not only the measured spectra, but also the decay dynamics in various measurement conditions. In the end of this study, a generalized methodology based on an inverse optical transfer matrix formalism was provided to correct the spectra and decay dynamics manipulated by optical artifacts. Overall, this thesis paves the way for a deeper understanding of the keys toward higher PCEs in low-offset OSC devices, from the perspectives of both device physics and characterization techniques.}, language = {en} } @phdthesis{Lepro2021, author = {Lepro, Valentino}, title = {Experimental and theoretical study on amoeboid cell-cargo active motion}, doi = {10.25932/publishup-49089}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-490890}, school = {Universit{\"a}t Potsdam}, pages = {xx, 114}, year = {2021}, abstract = {As society paves its way towards device miniaturization and precision medicine, micro-scale actuation and guided transport become increasingly prominent research fields, with high potential impact in both technological and clinical contexts. In order to accomplish directed motion of micron-sized objects, as biosensors and drug-releasing microparticles, towards specific target sites, a promising strategy is the use of living cells as smart biochemically-powered carriers, building the so-called bio-hybrid systems. Inspired by leukocytes, native cells of living organisms efficiently migrating to critical targets as tumor tissue, an emerging concept is to exploit the amoeboid crawling motility of such cells as mean of transport for drug delivery applications. In the research work described in this thesis, I synergistically applied experimental, computational and theoretical modeling approaches to investigate the behaviour and transport mechanism of a novel kind of bio-hybrid system for active transport at the micro-scale, referred to as cellular truck. This system consists of an amoeboid crawling cell, the carrier, attached to a microparticle, the cargo, which may ideally be drug-loaded for specific therapeutic treatments. For the purposes of experimental investigation, I employed the amoeba Dictyostelium discoideum as crawling cellular carrier, being a renowned model organism for leukocyte migration and, in general, for eukaryotic cell motility. The performed experiments revealed a complex recurrent cell-cargo relative motion, together with an intermittent motility of the cellular truck as a whole. The evidence suggests the presence of cargoes on amoeboid cells to act as mechanical stimulus leading cell polarization, thus promoting cell motility and giving rise to the observed intermittent dynamics of the truck. Particularly, bursts in cytoskeletal polarity along the cell-cargo axis have been found to occur in time with a rate dependent on cargo geometrical features, as particle diameter. Overall, the collected experimental evidence pointed out a pivotal role of cell-cargo interactions in the emergent cellular truck motion dynamics. Especially, they can determine the transport capabilities of amoeboid cells, as the cargo size significantly impacts the cytoskeletal activity and repolarization dynamics along the cell-cargo axis, the latter responsible for truck displacement and reorientation. Furthermore, I developed a modeling framework, built upon the experimental evidence on cellular truck behaviour, that connects the relative dynamics and interactions arising at the truck scale with the actual particle transport dynamics. In fact, numerical simulations of the proposed model successfully reproduced the phenomenology of the cell-cargo system, while enabling the prediction of the transport properties of cellular trucks over larger spatial and temporal scales. The theoretical analysis provided a deeper understanding of the role of cell-cargo interaction on mass transport, unveiling in particular how the long-time transport efficiency is governed by the interplay between the persistence time of cell polarity and time scales of the relative dynamics stemming from cell-cargo interaction. Interestingly, the model predicts the existence of an optimal cargo size, enhancing the diffusivity of cellular trucks; this is in line with previous independent experimental data, which appeared rather counterintuitive and had no explanation prior to this study. In conclusion, my research work shed light on the importance of cargo-carrier interactions in the context of crawling cell-mediated particle transport, and provides a prototypical, multifaceted framework for the analysis and modelling of such complex bio-hybrid systems and their perspective optimization.}, language = {en} } @phdthesis{Eickelmann2018, author = {Eickelmann, Stephan Felix}, title = {Experimental Study of Liquid Interfaces with Compositional Gradients}, doi = {10.17617/2.3010222}, school = {Universit{\"a}t Potsdam}, pages = {153}, year = {2018}, abstract = {Der Inhalt dieser Arbeit ist die experimentelle Untersuchung von verdunstenen d{\"u}nnen Filmen auf glatten Oberfl{\"a}chen, und die Anreicherung, das Kristallwachstum so wie Marangoni-Fluss in der N{\"a}he der Dreiphasenlinie bei partiell benetzenden Mischungen aus fl{\"u}chtigen und nichtfl{\"u}chtigen Fl{\"u}ssigkeiten. Im Detail werden die Eigenschaften von planaren Fl{\"u}ssigkeitsfilmen und d{\"u}nnen Fl{\"u}ssigkeitsabschnitten in der N{\"a}he der dreiphasigen Kontaktlinie behandelt. In beiden F{\"a}llen verliert die Fl{\"u}ssigkeit kontinuierlich eine Komponente durch Verdampfung. Ein Thema ist das ntnetzungsveralten ultrad{\"u}nner Filme aus bin{\"a}ren Mischungen eines fl{\"u}chtigen L{\"o}sungsmittels und eines nichtfl{\"u}chtigen Stoffes. Dabei wird analysiert wie die Dicke, bei der der Film reißt, mit der Kristallisation des gel{\"o}sten Stoffes an der Grenzfl{\"a}che zwischen Fl{\"u}ssigkeit und Substrat in Verbindung steht, sobald der gel{\"o}ste Stoff seine {\"U}bers{\"a}ttigung erreicht. Die Resultate dieses Projektes zeigen eine universelle Beziehung zwischen der Entnetzungdicke und dem S{\"a}ttigungsverhalten. Das zweite Forschungsgebiet sind einzelne Nanopartikel, die in molekular d{\"u}nne Filme auf planaren Substraten eingebettet sind. Es zeigt sich, dass die Nanopartikel eine unerwartet große Filmoberfl{\"a}chenverzerrung (Meniskus) verursachen. Diese Verzerrung kann durch herk{\"o}mmliche Reflexionsmikroskopie quantitativ gemessen werden, obwohl die Nanopartikel viel kleiner als die Rayleigh-Beugungsgrenze sind. Untersuchungen mit bin{\"a}ren Mischungen fl{\"u}chtiger L{\"o}sungsmittel und nichtfl{\"u}chtiger Stoffe (Polymere) zielen auf ein besseres Verst{\"a}ndnis/Vorhersage der finalen Schichtdicke, zeitaufgel{\"o}sten Verd{\"u}nnung, zeitaufgel{\"o}sten Verdunstung und der Entwicklung der Konzentration des gel{\"o}sten Stoffes innerhalb des verd{\"u}nnenden Filmes. Eine quantitative theoretische Beschreibung der experimentellen Ergebnisse wird hergeleitet. Unerwarteterweise zeigen die Experimente, mit vollst{\"a}ndig mischbarer bin{\"a}rer Mischungen fl{\"u}chtiger Fl{\"u}ssigkeiten, die einzeln glatte Filme bilden, dass Filme dieser Mischungen nicht notwendigerweise kontinuierlich und glatt sind. Vielmehr k{\"o}nnen sie Oberfl{\"a}chenwellen bilden oder sogar aufreißen. Dies wird mit Oberfl{\"a}chen-Marangoni-Str{\"o}mungen erkl{\"a}rt. Es wird eine neue Methode f{\"u}r die schnelle Herstellung von ultralang gerichtetn Diphenylalanin-Einkristallen (Dip- Casting) (mm/min) vorgestellt. Dabei viii wird gezeigt, wie die spezifischen Verdunstungsbedingen an der Dreiphasenlinie f{\"u}r einen kontrollierten Peptidkristallwachstumsprozess verwendet werden k{\"o}nnen. Abschließend wird gezeigt, wie die Beschr{\"a}nkung innerhalb einer kleinen Kapillare die Peptidkristallisation beeinflusst, diese verstanden und verwendet werden kann.}, language = {en} } @phdthesis{Zou2007, author = {Zou, Yong}, title = {Exploring recurrences in quasiperiodic systems}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-16497}, school = {Universit{\"a}t Potsdam}, year = {2007}, abstract = {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.}, language = {en} } @phdthesis{Laquai2022, author = {Laquai, Ren{\´e}}, title = {Extending synchrotron X-ray refraction techniques to the quantitative analysis of metallic materials}, doi = {10.25932/publishup-54183}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-541835}, school = {Universit{\"a}t Potsdam}, pages = {vii, 71, XIV}, year = {2022}, abstract = {In this work, two X-ray refraction based imaging methods, namely, synchrotron X-ray refraction radiography (SXRR) and synchrotron X-ray refraction computed tomography (SXRCT), are applied to analyze quantitatively cracks and porosity in metallic materials. SXRR and SXRCT make use of the refraction of X-rays at inner surfaces of the material, e.g., the surfaces of cracks and pores, for image contrast. Both methods are, therefore, sensitive to smaller defects than their absorption based counterparts X-ray radiography and computed tomography. They can detect defects of nanometric size. So far the methods have been applied to the analysis of ceramic materials and fiber reinforced plastics. The analysis of metallic materials requires higher photon energies to achieve sufficient X-ray transmission due to their higher density. This causes smaller refraction angles and, thus, lower image contrast because the refraction index depends on the photon energy. Here, for the first time, a conclusive study is presented exploring the possibility to apply SXRR and SXRCT to metallic materials. It is shown that both methods can be optimized to overcome the reduced contrast due to smaller refraction angles. Hence, the only remaining limitation is the achievable X-ray transmission which is common to all X-ray imaging methods. Further, a model for the quantitative analysis of the inner surfaces is presented and verified. For this purpose four case studies are conducted each posing a specific challenge to the imaging task. Case study A investigates cracks in a coupon taken from an aluminum weld seam. This case study primarily serves to verify the model for quantitative analysis and prove the sensitivity to sub-resolution features. In case study B, the damage evolution in an aluminum-based particle reinforced metal-matrix composite is analyzed. Here, the accuracy and repeatability of subsequent SXRR measurements is investigated showing that measurement errors of less than 3 \% can be achieved. Further, case study B marks the fist application of SXRR in combination with in-situ tensile loading. Case study C is out of the highly topical field of additive manufacturing. Here, porosity in additively manufactured Ti-Al6-V4 is analyzed with a special interest in the pore morphology. A classification scheme based on SXRR measurements is devised which allows to distinguish binding defects from keyhole pores even if the defects cannot be spatially resolved. In case study D, SXRCT is applied to the analysis of hydrogen assisted cracking in steel. Due to the high X-ray attenuation of steel a comparatively high photonenergy of 50 keV is required here. This causes increased noise and lower contrast in the data compared to the other case studies. However, despite the lower data quality a quantitative analysis of the occurance of cracks in dependence of hydrogen content and applied mechanical load is possible.}, language = {en} } @phdthesis{Tukhlina2008, author = {Tukhlina, Natalia}, title = {Feedback control of complex oscillatory systems}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-18546}, school = {Universit{\"a}t Potsdam}, year = {2008}, abstract = {In the present dissertation paper an approach which ensures an efficient control of such diverse systems as noisy or chaotic oscillators and neural ensembles is developed. This approach is implemented by a simple linear feedback loop. The dissertation paper consists of two main parts. One part of the work is dedicated to the application of the suggested technique to a population of neurons with a goal to suppress their synchronous collective dynamics. The other part is aimed at investigating linear feedback control of coherence of a noisy or chaotic self-sustained oscillator. First we start with a problem of suppressing synchronization in a large population of interacting neurons. The importance of this task is based on the hypothesis that emergence of pathological brain activity in the case of Parkinson's disease and other neurological disorders is caused by synchrony of many thousands of neurons. The established therapy for the patients with such disorders is a permanent high-frequency electrical stimulation via the depth microelectrodes, called Deep Brain Stimulation (DBS). In spite of efficiency of such stimulation, it has several side effects and mechanisms underlying DBS remain unclear. In the present work an efficient and simple control technique is suggested. It is designed to ensure suppression of synchrony in a neural ensemble by a minimized stimulation that vanishes as soon as the tremor is suppressed. This vanishing-stimulation technique would be a useful tool of experimental neuroscience; on the other hand, control of collective dynamics in a large population of units represents an interesting physical problem. The main idea of suggested approach is related to the classical problem of oscillation theory, namely the interaction between a self-sustained (active) oscillator and a passive load (resonator). It is known that under certain conditions the passive oscillator can suppress the oscillations of an active one. In this thesis a much more complicated case of active medium, which itself consists of thousands of oscillators is considered. Coupling this medium to a specially designed passive oscillator, one can control the collective motion of the ensemble, specifically can enhance or suppress it. Having in mind a possible application in neuroscience, the problem of suppression is concentrated upon. Second, the efficiency of suggested suppression scheme is illustrated by considering more complex case, i.e. when the population of neurons generating the undesired rhythm consists of two non-overlapping subpopulations: the first one is affected by the stimulation, while the collective activity is registered from the second one. Generally speaking, the second population can be by itself both active and passive; both cases are considered here. The possible applications of suggested technique are discussed. Third, the influence of the external linear feedback on coherence of a noisy or chaotic self-sustained oscillator is considered. Coherence is one of the main properties of self-oscillating systems and plays a key role in the construction of clocks, electronic generators, lasers, etc. The coherence of a noisy limit cycle oscillator in the context of phase dynamics is evaluated by the phase diffusion constant, which is in its turn proportional to the width of the spectral peak of oscillations. Many chaotic oscillators can be described within the framework of phase dynamics, and, therefore, their coherence can be also quantified by the way of the phase diffusion constant. The analytical theory for a general linear feedback, considering noisy systems in the linear and Gaussian approximation is developed and validated by numerical results.}, language = {en} } @phdthesis{Chirvasa2010, author = {Chirvasa, Mihaela}, title = {Finite difference methods for 1st Order in time, 2nd order in space, hyperbolic systems used in numerical relativity}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-42135}, school = {Universit{\"a}t Potsdam}, year = {2010}, abstract = {This thesis is concerned with the development of numerical methods using finite difference techniques for the discretization of initial value problems (IVPs) and initial boundary value problems (IBVPs) of certain hyperbolic systems which are first order in time and second order in space. This type of system appears in some formulations of Einstein equations, such as ADM, BSSN, NOR, and the generalized harmonic formulation. For IVP, the stability method proposed in [14] is extended from second and fourth order centered schemes, to 2n-order accuracy, including also the case when some first order derivatives are approximated with off-centered finite difference operators (FDO) and dissipation is added to the right-hand sides of the equations. For the model problem of the wave equation, special attention is paid to the analysis of Courant limits and numerical speeds. Although off-centered FDOs have larger truncation errors than centered FDOs, it is shown that in certain situations, off-centering by just one point can be beneficial for the overall accuracy of the numerical scheme. The wave equation is also analyzed in respect to its initial boundary value problem. All three types of boundaries - outflow, inflow and completely inflow that can appear in this case, are investigated. Using the ghost-point method, 2n-accurate (n = 1, 4) numerical prescriptions are prescribed for each type of boundary. The inflow boundary is also approached using the SAT-SBP method. In the end of the thesis, a 1-D variant of BSSN formulation is derived and some of its IBVPs are considered. The boundary procedures, based on the ghost-point method, are intended to preserve the interior 2n-accuracy. Numerical tests show that this is the case if sufficient dissipation is added to the rhs of the equations.}, language = {en} } @phdthesis{Kruesemann2016, author = {Kr{\"u}semann, Henning}, title = {First passage phenomena and single-file motion in ageing continuous time random walks and quenched energy landscapes}, school = {Universit{\"a}t Potsdam}, pages = {122}, year = {2016}, abstract = {In der Physik gibt es viele Prozesse, die auf Grund ihrer Komplexit{\"a}t nicht durch physikalische Gleichungen beschrieben werden k{\"o}nnen, beispielsweise die Bewegung eines Staubkorns in der Luft. Durch die vielen St{\"o}ße mit Luftmolek{\"u}len f{\"u}hrt es eine Zufallsbewegung aus, die so genannte Diffusion. Auch Molek{\"u}le in biologischen Zellen diffundieren, jedoch befinden sich in einer solchen Zelle im selben Volumen viel mehr oder viel gr{\"o}ßere Molek{\"u}le. Das beobachtete Teilchen st{\"o}ßt dementsprechend {\"o}fter mit anderen zusammen und die Diffusion wird langsamer, sie wird subdiffusiv. Mit der Zeit kann sich die Charakteristik der Subdiffusion {\"a}ndern; dies wird als (mikroskopisches) Altern bezeichnet. Ich untersuche in der vorliegenden Arbeit zwei mathematische Modelle f{\"u}r eindimensionale Subdiffusion, einmal den continuous time random walk (CTRW) und einmal die Zufallsbewegung in einer eingefrorenen Energielandschaft (QEL=quenched energy landscape). Beide sind Sprungprozesse, das heißt, sie sind Abfolgen von r{\"a}umlichen Spr{\"u}ngen, die durch zufallsverteilte Wartezeiten getrennt sind. Die Wartezeiten in der QEL sind r{\"a}umlich korrelliert, w{\"a}hrend sie im CTRW unkorrelliert sind. Ich untersuche in der vorliegenden Arbeit verschiedene statistische Gr{\"o}ßen in beiden Modellen. Zun{\"a}chst untersuche ich den Einfluss des Alters und den Einfluss der Korrellationen einer QEL auf die Verteilung der Zeiten, die das diffundierendes Teilchen ben{\"o}tigt, um eine (r{\"a}umliche) Schwelle zu {\"u}berqueren. Ausserdem bestimme ich den Effekt des Alters auf Str{\"o}me von (sub)diffundierenden Partikeln, die sich auf eine absorbierende Barriere zubewegen. Zuletzt besch{\"a}ftige ich mich mit der Diffusion einer eindimensionalen Anordnung von Teilchen in einer QEL, in der diese als harte Kugeln miteinander wechselwirken. Dabei vergleiche ich die gemeinsame Bewegung in einer QEL und als individuelle CTRWs miteinander {\"u}ber die Standartabweichung von der Startposition, f{\"u}r die ich das Mittel {\"u}ber mehrere QELs untersuche. Meine Arbeit setzt sich zusammen aus theoretischen {\"U}berlegungen und Berechnungen sowie der Simulation der Zufallsprozesse. Die Ergebnisse der Simulation und, soweit vorhanden, experimentelle Daten werden mit der Theorie verglichen.}, language = {en} } @phdthesis{Haakh2012, author = {Haakh, Harald Richard}, title = {Fluctuation-mediated interactions of atoms and surfaces on a mesoscopic scale}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-61819}, school = {Universit{\"a}t Potsdam}, year = {2012}, abstract = {Thermal and quantum fluctuations of the electromagnetic near field of atoms and macroscopic bodies play a key role in quantum electrodynamics (QED), as in the Lamb shift. They lead, e.g., to atomic level shifts, dispersion interactions (Van der Waals-Casimir-Polder interactions), and state broadening (Purcell effect) because the field is subject to boundary conditions. Such effects can be observed with high precision on the mesoscopic scale which can be accessed in micro-electro-mechanical systems (MEMS) and solid-state-based magnetic microtraps for cold atoms ('atom chips'). A quantum field theory of atoms (molecules) and photons is adapted to nonequilibrium situations. Atoms and photons are described as fully quantized while macroscopic bodies can be included in terms of classical reflection amplitudes, similar to the scattering approach of cavity QED. The formalism is applied to the study of nonequilibrium two-body potentials. We then investigate the impact of the material properties of metals on the electromagnetic surface noise, with applications to atomic trapping in atom-chip setups and quantum computing, and on the magnetic dipole contribution to the Van der Waals-Casimir-Polder potential in and out of thermal equilibrium. In both cases, the particular properties of superconductors are of high interest. Surface-mode contributions, which dominate the near-field fluctuations, are discussed in the context of the (partial) dynamic atomic dressing after a rapid change of a system parameter and in the Casimir interaction between two conducting plates, where nonequilibrium configurations can give rise to repulsion.}, language = {en} } @phdthesis{Faber2014, author = {Faber, Michael}, title = {Folding dynamics of RNA secondary structures}, pages = {85}, year = {2014}, language = {en} } @phdthesis{Goldshteyn2014, author = {Goldshteyn, Jewgenij}, title = {Frequency-resolved ultrafast dynamics of phonon polariton wavepackets in the ferroelectric crystals LiNbO₃ and LiTaO₃}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-71623}, school = {Universit{\"a}t Potsdam}, year = {2014}, abstract = {During this work I built a four wave mixing setup for the time-resolved femtosecond spectroscopy of Raman-active lattice modes. This setup enables to study the selective excitation of phonon polaritons. These quasi-particles arise from the coupling of electro-magnetic waves and transverse optical lattice modes, the so-called phonons. The phonon polaritons were investigated in the optically non-linear, ferroelectric crystals LiNbO₃ and LiTaO₃. The direct observation of the frequency shift of the scattered narrow bandwidth probe pulses proofs the role of the Raman interaction during the probe and excitation process of phonon polaritons. I compare this experimental method with the measurement where ultra-short laser pulses are used. The frequency shift remains obscured by the relative broad bandwidth of these laser pulses. In an experiment with narrow bandwidth probe pulses, the Stokes and anti-Stokes intensities are spectrally separated. They are assigned to the corresponding counter-propagating wavepackets of phonon polaritons. Thus, the dynamics of these wavepackets was separately studied. Based on these findings, I develop the mathematical description of the so-called homodyne detection of light for the case of light scattering from counter propagating phonon polaritons. Further, I modified the broad bandwidth of the ultra-short pump pulses using bandpass filters to generate two pump pulses with non-overlapping spectra. This enables the frequency-selective excitation of polariton modes in the sample, which allows me to observe even very weak polariton modes in LiNbO₃ or LiTaO₃ that belong to the higher branches of the dispersion relation of phonon polaritons. The experimentally determined dispersion relation of the phonon polaritons could therefore be extended and compared to theoretical models. In addition, I determined the frequency-dependent damping of phonon polaritons.}, language = {en} } @phdthesis{Brose2020, author = {Brose, Robert}, title = {From dawn till dusk}, doi = {10.25932/publishup-47086}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-470865}, school = {Universit{\"a}t Potsdam}, pages = {xiii, 146}, year = {2020}, abstract = {Supernova remnants are believed to be the source of cosmic rays with energies up to 10^15 eV that are produced within our Galaxy. The acceleration mechanism associated with the collision-less shocks in supernova remnants - diffusive shock acceleration - predicts a spectral index of the accelerated non-thermal particles of s = 2. However, measurements of non-thermal emission in radio, X-rays and gamma-rays reveal significant deviations of the particles spectral index from the canonical value of s = 2. The youngest Galactic supernova remnant G1.9+0.3 is an interesting target for next-generation gamma-ray observatories. So far, the remnant is only detected in the radio and the X-ray bands, but its young age of ≈100 yrs and inferred shock speed of ≈ 14, 000 km/s could make it an efficient particle accelerator. I performed spherical symmetric 1D simulations with the RATPaC code, in which I simultaneously solved the transport equation for cosmic rays, the transport equation for magnetic turbulence, and the hydro-dynamical equations for the gas flow. Separately computed distributions of the particles accelerated at the forward and the reverse shock were then used to calculate the spectra of synchrotron, inverse Compton, and Pion-decay radiation from the source. The emission from G1.9+0.3 can be self-consistently explained within the test-particle limit. I find that the X-ray flux is dominated by emission from the forward shock while most of the radio emission originates near the reverse shock, which makes G1.9+0.3 the first remnant with non-thermal radiation detected from the reverse shock. The flux of very-high-energy gamma-ray emission from G1.9+0.3 is expected to be close to the sensitivity threshold of the Cherenkov Telescope Array. The limited time available to grow large-scale turbulence limits the maximum energy of particles to values below 100 TeV, hence G1.9+0.3 is not a PeVatron. Although there are many models for the acceleration of cosmic rays in Supernova remnants, the escape of cosmic rays from these sources is yet understudied. I use our time-dependent acceleration code RATPaC to study the acceleration of cosmic rays and their escape in post-adiabatic Supernova remnants and calculate the subsequent gamma-ray emission from inverse-Compton scattering and Pion decay. My simulations span 100,000 years, thus covering the free-expansion, the Sedov-Taylor, and the beginning of the post-adiabatic phase of the remnant's evolution. At later stages of the evolution cosmic rays over a wide range of energy can reside outside of the remnant, creating spectra that are softer than predicted by standard diffusive shock acceleration and feature breaks in the 10 - 100 GeV-range. The total spectrum of cosmic rays released into the interstellar medium has a spectral index of s ≈ 2.4 above roughly 10 GeV which is close to that required by Galactic propagation models. I further find the gamma-ray luminosity to peak around an age of 4,000 years for inverse-Compton-dominated high-energy emission. Remnants expanding in low-density media emit generally more inverse-Compton radiation matching the fact that the brightest known supernova remnants - RCW86, Vela Jr, HESSJ1721-347 and RXJ1713.7-3946 - are all expanding in low density environments. The importance of feedback from the cosmic-rays on the hydrodynamical evolution of the remnants is debated as a possibility to obtain soft cosmic-ray spectra at low energies. I performed spherically symmetric 1-D simulations with a modified version of the RATPaC code, in which I simultaneously solve the transport equation for cosmic rays and the hydrodynamical equations, including the back-reaction of the cosmic-ray pressure on the flow profiles. Besides the known modification of the flow profiles and the consequently curved cosmic-ray spectra, steady-state models for non-linear diffusive shock acceleration overpredict the total compression ratio that can be reached with cosmic-ray feedback, as there is limited time for building these modifications. Further, I find modifications to the downstream flow structure that change the evolutionary behavior of the remnant and trigger a cosmic-ray-induced instability close to the contact discontinuity, if and when the cosmic-ray pressure becomes dominant there.}, language = {en} } @phdthesis{Sauter2015, author = {Sauter, Tilman}, title = {Function by structure}, school = {Universit{\"a}t Potsdam}, pages = {166}, year = {2015}, language = {en} } @phdthesis{Schroeder2024, author = {Schr{\"o}der, Jakob}, title = {Fundamentals of diffraction-based residual stress and texture analysis of laser powder bed fused Inconel 718}, doi = {10.25932/publishup-62197}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-621972}, school = {Universit{\"a}t Potsdam}, pages = {XVII, 135}, year = {2024}, abstract = {Additive manufacturing (AM) processes enable the production of metal structures with exceptional design freedom, of which laser powder bed fusion (PBF-LB) is one of the most common. In this process, a laser melts a bed of loose feedstock powder particles layer-by-layer to build a structure with the desired geometry. During fabrication, the repeated melting and rapid, directional solidification create large temperature gradients that generate large thermal stress. This thermal stress can itself lead to cracking or delamination during fabrication. More often, large residual stresses remain in the final part as a footprint of the thermal stress. This residual stress can cause premature distortion or even failure of the part in service. Hence, knowledge of the residual stress field is critical for both process optimization and structural integrity. Diffraction-based techniques allow the non-destructive characterization of the residual stress fields. However, such methods require a good knowledge of the material of interest, as certain assumptions must be made to accurately determine residual stress. First, the measured lattice plane spacings must be converted to lattice strains with the knowledge of a strain-free material state. Second, the measured lattice strains must be related to the macroscopic stress using Hooke's law, which requires knowledge of the stiffness of the material. Since most crystal structures exhibit anisotropic material behavior, the elastic behavior is specific to each lattice plane of the single crystal. Thus, the use of individual lattice planes in monochromatic diffraction residual stress analysis requires knowledge of the lattice plane-specific elastic properties. In addition, knowledge of the microstructure of the material is required for a reliable assessment of residual stress. This work presents a toolbox for reliable diffraction-based residual stress analysis. This is presented for a nickel-based superalloy produced by PBF-LB. First, this work reviews the existing literature in the field of residual stress analysis of laser-based AM using diffraction-based techniques. Second, the elastic and plastic anisotropy of the nickel-based superalloy Inconel 718 produced by PBF-LB is studied using in situ energy dispersive synchrotron X-ray and neutron diffraction techniques. These experiments are complemented by ex situ material characterization techniques. These methods establish the relationship between the microstructure and texture of the material and its elastic and plastic anisotropy. Finally, surface, sub-surface, and bulk residual stress are determined using a texture-based approach. Uncertainties of different methods for obtaining stress-free reference values are discussed. The tensile behavior in the as-built condition is shown to be controlled by texture and cellular sub-grain structure, while in the heat-treated condition the precipitation of strengthening phases and grain morphology dictate the behavior. In fact, the results of this thesis show that the diffraction elastic constants depend on the underlying microstructure, including texture and grain morphology. For columnar microstructures in both as-built and heat-treated conditions, the diffraction elastic constants are best described by the Reuss iso-stress model. Furthermore, the low accumulation of intergranular strains during deformation demonstrates the robustness of using the 311 reflection for the diffraction-based residual stress analysis with columnar textured microstructures. The differences between texture-based and quasi-isotropic approaches for the residual stress analysis are shown to be insignificant in the observed case. However, the analysis of the sub-surface residual stress distributions show, that different scanning strategies result in a change in the orientation of the residual stress tensor. Furthermore, the location of the critical sub-surface tensile residual stress is related to the surface roughness and the microstructure. Finally, recommendations are given for the diffraction-based determination and evaluation of residual stress in textured additively manufactured alloys.}, language = {en} } @phdthesis{Federici2014, author = {Federici, Simone}, title = {Gamma-ray studies of the young shell-type SNR RX J1713.7-3946}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-71734}, school = {Universit{\"a}t Potsdam}, year = {2014}, abstract = {One of the most significant current discussions in Astrophysics relates to the origin of high-energy cosmic rays. According to our current knowledge, the abundance distribution of the elements in cosmic rays at their point of origin indicates, within plausible error limits, that they were initially formed by nuclear processes in the interiors of stars. It is also believed that their energy distribution up to 1018 eV has Galactic origins. But even though the knowledge about potential sources of cosmic rays is quite poor above „ 1015 eV, that is the "knee" of the cosmic-ray spectrum, up to the knee there seems to be a wide consensus that supernova remnants are the most likely candidates. Evidence of this comes from observations of non-thermal X-ray radiation, requiring synchrotron electrons with energies up to 1014 eV, exactly in the remnant of supernovae. To date, however, there is not conclusive evidence that they produce nuclei, the dominant component of cosmic rays, in addition to electrons. In light of this dearth of evidence, γ-ray observations from supernova remnants can offer the most promising direct way to confirm whether or not these astrophysical objects are indeed the main source of cosmic-ray nuclei below the knee. Recent observations with space- and ground-based observatories have established shell-type supernova remnants as GeV-to- TeV γ-ray sources. The interpretation of these observations is however complicated by the different radiation processes, leptonic and hadronic, that can produce similar fluxes in this energy band rendering ambiguous the nature of the emission itself. The aim of this work is to develop a deeper understanding of these radiation processes from a particular shell-type supernova remnant, namely RX J1713.7-3946, using observations of the LAT instrument onboard the Fermi Gamma-Ray Space Telescope. Furthermore, to obtain accurate spectra and morphology maps of the emission associated with this supernova remnant, an improved model of the diffuse Galactic γ-ray emission background is developed. The analyses of RX J1713.7-3946 carried out with this improved background show that the hard Fermi-LAT spectrum cannot be ascribed to the hadronic emission, leading thus to the conclusion that the leptonic scenario is instead the most natural picture for the high-energy γ-ray emission of RX J1713.7-3946. The leptonic scenario however does not rule out the possibility that cosmic-ray nuclei are accelerated in this supernova remnant, but it suggests that the ambient density may not be high enough to produce a significant hadronic γ-ray emission. Further investigations involving other supernova remnants using the improved back- ground developed in this work could allow compelling population studies, and hence prove or disprove the origin of Galactic cosmic-ray nuclei in these astrophysical objects. A break- through regarding the identification of the radiation mechanisms could be lastly achieved with a new generation of instruments such as CTA.}, language = {en} }