@phdthesis{Pfrang2023,
  author    = {Pfrang, Konstantin Johannes},
  title     = {Search for light primordial black holes with VERITAS using gamma γ-ray and optical observations},
  doi       = {10.25932/publishup-58726},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-587266},
  school      = {Universit{\"a}t Potsdam},
  pages     = {139},
  year      = {2023},
  abstract  = {The Very Energetic Radiation Imaging Telescope Array System (VERITAS) is an array of four imaging atmospheric Cherenkov telescopes (IACTs). VERITAS is sensitive to very-high-energy gamma-rays in the range of 100 GeV to >30 TeV. Hypothesized primordial black holes (PBHs) are attractive targets for IACTs. If they exist, their potential cosmological impact reaches beyond the candidacy for constituents of dark matter. The sublunar mass window is the largest unconstrained range of PBH masses. This thesis aims to develop novel concepts searching for light PBHs with VERITAS. PBHs below the sublunar window lose mass due to Hawking radiation. They would evaporate at the end of their lifetime, leading to a short burst of gamma-rays. If PBHs formed at about 10^15 g, the evaporation would occur nowadays. Detecting these signals might not only confirm the existence of PBHs but also prove the theory of Hawking radiation. This thesis probes archival VERITAS data recorded between 2012 and 2021 for possible PBH signals. This work presents a new automatic approach to assess the quality of the VERITAS data. The array-trigger rate and far infrared temperature are well suited to identify periods with poor data quality. These are masked by time cuts to obtain a consistent and clean dataset which contains about 4222 hours. The PBH evaporations could occur at any location in the field of view or time within this data. Only a blind search can be performed to identify these short signals. This thesis implements a data-driven deep learning based method to search for short transient signals with VERITAS. It does not depend on the modelling of the effective area and radial acceptance. This work presents the first application of this method to actual observational IACT data. This thesis develops new concepts dealing with the specifics of the data and the transient detection method. These are reflected in the developed data preparation pipeline and search strategies. After correction for trial factors, no candidate PBH evaporation is found in the data. Thus, new constraints of the local rate of PBH evaporations are derived. At the 99\% confidence limit it is below <1.07 * 10^5 pc^-3 yr^-1. This constraint with the new, independent analysis approach is in the range of existing limits for the evaporation rate. This thesis also investigates an alternative novel approach to searching for PBHs with IACTs. Above the sublunar window, the PBH abundance is constrained by optical microlensing studies. The sampling speed, which is of order of minutes to hours for traditional optical telescopes, is a limiting factor in expanding the limits to lower masses. IACTs are also powerful instruments for fast transient optical astronomy with up to O(ns) sampling. This thesis investigates whether IACTs might constrain the sublunar window with optical microlensing observations. This study confirms that, in principle, the fast sampling speed might allow extending microlensing searches into the sublunar mass window. However, the limiting factor for IACTs is the modest sensitivity to detect changes in optical fluxes. This thesis presents the expected rate of detectable events for VERITAS as well as prospects of possible future next-generation IACTs. For VERITAS, the rate of detectable microlensing events in the sublunar range is ~10^-6 per year of observation time. The future prospects for a 100 times more sensitive instrument are at ~0.05 events per year.},
  language  = {en}
}
@phdthesis{Ksianzou2010,
  author    = {Ksianzou, Viachaslau},
  title     = {Second-order nonlinear optical interactions and cascading effects in thinorganic films},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-49186},
  school      = {Universit{\"a}t Potsdam},
  year      = {2010},
  abstract  = {The presented work describes new concepts of fast switching elements based on principles of photonics. The waveguides working in visible and infra-red ranges are put in a basis of these elements. And as materials for manufacturing of waveguides the transparent polymers, dopped by molecules of the dyes possessing second order nonlinear-optical properties are proposed. The work shows how nonlinear-optical processes in such structures can be implemented by electro-optical and opto-optical control circuit signals. In this paper we consider the complete cycle of fabrication of several types of integral photonic elements. The theoretical analysis of high-intensity beam propagation in media with second-order optical nonlinearity is performed. Quantitative estimations of necessary conditions of occurrence of the nonlinear-optical phenomena of the second order taking into account properties of used materials are made. The paper describes the various stages of manufacture of the basic structure of the integrated photonics: a planar waveguide. Using the finite element method the structure of the electromagnetic field inside the waveguide in different modes was analysed. A separate part of the work deals with the creation of composite organic materials with high optical nonlinearity. Using the methods of quantum chemistry, the dependence of nonlinear properties of dye molecules from its structure were investigated in details. In addition, the paper discusses various methods of inducing of an optical nonlinearity in dye-doping of polymer films. In the work, for the first time is proposed the use of spatial modulation of nonlinear properties of waveguide according Fibonacci law. This allows involving several different nonlinear optical processes simultaneously. The final part of the work describes various designs of integrated optical modulators and switches constructed of organic nonlinear optical waveguides. A practical design of the optical modulator based on Mach-Zehnder interferometer made by a photolithography on polymer film is presented.},
  language  = {en}
}
@phdthesis{Bittermann2015,
  author    = {Bittermann, Klaus},
  title     = {Semi-empirical sea-level modelling},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-93881},
  school      = {Universit{\"a}t Potsdam},
  pages     = {v, 88},
  year      = {2015},
  abstract  = {Semi-empirical sea-level models (SEMs) exploit physically motivated empirical relationships between global sea level and certain drivers, in the following global mean temperature. This model class evolved as a supplement to process-based models (Rahmstorf (2007)) which were unable to fully represent all relevant processes. They thus failed to capture past sea-level change (Rahmstorf et al. (2012)) and were thought likely to underestimate future sea-level rise. Semi-empirical models were found to be a fast and useful tool for exploring the uncertainties in future sea-level rise, consistently giving significantly higher projections than process-based models. In the following different aspects of semi-empirical sea-level modelling have been studied. Models were first validated using various data sets of global sea level and temperature. SEMs were then used on the glacier contribution to sea level, and to infer past global temperature from sea-level data via inverse modelling. Periods studied encompass the instrumental period, covered by tide gauges (starting 1700 CE (Common Era) in Amsterdam) and satellites (first launched in 1992 CE), the era from 1000 BCE (before CE) to present, and the full length of the Holocene (using proxy data). Accordingly different data, model formulations and implementations have been used. It could be shown in Bittermann et al. (2013) that SEMs correctly predict 20th century sea-level when calibrated with data until 1900 CE. SEMs also turned out to give better predictions than the Intergovernmental Panel on Climate Change (IPCC) 4th assessment report (AR4, IPCC (2007)) models, for the period from 1961-2003 CE. With the first multi-proxy reconstruction of global sea-level as input, estimate of the human-induced component of modern sea-level change and projections of future sea-level rise were calculated (Kopp et al. (2016)). It turned out with 90\% confidence that more than 40 \% of the observed 20th century sea-level rise is indeed anthropogenic. With the new semi-empirical and IPCC (2013) 5th assessment report (AR5) projections the gap between SEM and process-based model projections closes, giving higher credibility to both. Combining all scenarios, from strong mitigation to business as usual, a global sea-level rise of 28-131 cm relative to 2000 CE, is projected with 90\% confidence. The decision for a low carbon pathway could halve the expected global sea-level rise by 2100 CE. Present day temperature and thus sea level are driven by the globally acting greenhouse-gas forcing. Unlike that, the Milankovich forcing, acting on Holocene timescales, results mainly in a northern-hemisphere temperature change. Therefore a semi-empirical model can be driven with northernhemisphere temperatures, which makes it possible to model the main subcomponent of sea-level change over this period. It showed that an additional positive constant rate of the order of the estimated Antarctic sea-level contribution is then required to explain the sea-level evolution over the Holocene. Thus the global sea level, following the climatic optimum, can be interpreted as the sum of a temperature induced sea-level drop and a positive long-term contribution, likely an ongoing response to deglaciation coming from Antarctica.},
  language  = {en}
}
@phdthesis{Soriano2016,
  author    = {Soriano, Manuel Flores},
  title     = {Short-term evolution and coexistence of photospheric and chromospheric activity on LQ Hydrae},
  school      = {Universit{\"a}t Potsdam},
  pages     = {90},
  year      = {2016},
  language  = {en}
}
@phdthesis{Mari2012,
  author    = {Mari, Andrea},
  title     = {Signatures of non-classicality in optomechanical systems},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-59814},
  school      = {Universit{\"a}t Potsdam},
  year      = {2012},
  abstract  = {This thesis contains several theoretical studies on optomechanical systems, i.e. physical devices where mechanical degrees of freedom are coupled with optical cavity modes. This optomechanical interaction, mediated by radiation pressure, can be exploited for cooling and controlling mechanical resonators in a quantum regime. The goal of this thesis is to propose several new ideas for preparing meso- scopic mechanical systems (of the order of 10^15 atoms) into highly non-classical states. In particular we have shown new methods for preparing optomechani-cal pure states, squeezed states and entangled states. At the same time, proce-dures for experimentally detecting these quantum effects have been proposed. In particular, a quantitative measure of non classicality has been defined in terms of the negativity of phase space quasi-distributions. An operational al- gorithm for experimentally estimating the non-classicality of quantum states has been proposed and successfully applied in a quantum optics experiment. The research has been performed with relatively advanced mathematical tools related to differential equations with periodic coefficients, classical and quantum Bochner's theorems and semidefinite programming. Nevertheless the physics of the problems and the experimental feasibility of the results have been the main priorities.},
  language  = {en}
}
@phdthesis{Uyaver2004,
  author    = {Uyaver, Sahin},
  title     = {Simulation of annealed polyelectrolytes in poor solvents},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0001488},
  school      = {Universit{\"a}t Potsdam},
  year      = {2004},
  abstract  = {Polymere sind lange kettenartige Molek{\"u}le. Sie bestehen aus vielen elementaren chemischen Einheiten, den Monomeren, die durch kovalente Bindungen aneinander gekettet sind. Polyelektrolyte sind Polymere, die ionisierbare Monomeren enthalten. Aufgrund ihrer speziellen Eigenschaften sind Polyelektrolyte sowohl in der Molekular- und Zellbiologie von großen Bedeutung als auch in der Chemie großtechnisch relevant. Verglichen mit ungeladenen Polymeren sind Polyelektrolyte theoretisch noch wenig verstanden. Insbesondere gilt dies f{\"u}r Polyelektrolyte in sogenanntem schlechten L{\"o}sungsmittel. Ein schlechtes L{\"o}sungsmittel bewirkt eine effektive Anziehung zwischen den Monomeren. F{\"u}r Polyelektrolyte in schlechtem L{\"o}sungsmittel kommt es daher zu einer Konkurrenz zwischen dieser Anziehung und der elektrostatischen Abstoßung. Geladene Polymere werden im Rahmen der chemischen Klassifikation in starke und schwache Polyelektrolyte unterschieden. Erstere zeigen vollst{\"a}ndige Dissoziation unabh{\"a}ngig vom pH-Wert der L{\"o}sung. Die Position der Ladungen auf der Kette wird ausschließlich w{\"a}hrend der Polymersynthese festgelegt. In der Physik spricht man deshalb von Polyelektrolyten mit eingefrorener Ladungsverteilung (quenched polyelectrolytes). Im Falle von schwachen Polyelektrolyten ist die Ladungsdichte auf der Kette nicht konstant, sondern wird durch der pH-Wert der L{\"o}sung kontrolliert. Durch Rekombinations- und Dissoziationsprozesse sind die Ladungen auf der Kette beweglich. Im allgemeinen stellt sich eine inhomogene Gleichgewichtsverteilung ein, die mit der Struktur der Kette gekoppelt ist. Diese Polymere werden deshalb auch Polyelektrolyte mit Gleichgewichtsladungsverteilung (annealed polyelectrolytes) genannt. Wegen des zus{\"a}tzlichen Freiheitsgrades in der Ladungsverteilung werden eine Reihe ungew{\"o}hnlicher Eigenschaften theoretisch vorhergesagt. Mit Hilfe von Simulationen ist es zum ersten Mal gelungen, zu zeigen daß 'annealed' Polyelektrolyte in relativ schlechtem L{\"o}sungsmittel einen diskontinuierlichen Phasen{\"u}bergang durchlaufen, wenn ein kritischer pH-Werts der L{\"o}sung {\"u}berschritten wird. Bei diesem Phasen{\"u}bergang, gehen die Polyelektolyte von einer schwach geladenen kompakten globul{\"a}ren Struktur zu einer stark geladenen gestreckten Konfiguration {\"u}ber. Aufgrund theoretischer Vorhersagen wird erwartet, daß die globul{\"a}re Struktur in weniger schlechtem L{\"o}sungsmittel instabil wird und sich eine Perlenkettenkonfiguration ausbildet. Diese Vorhersage konnte f{\"u}r 'annealed' Polyelektrolyte mit den durchgef{\"u}hrten Simulationen zum ersten Mal best{\"a}tigt werden - inzwischen auch durch erste experimentelle Ergebnisse. Schließlich zeigen die Simulationen auch, daß annealed Polyelektrolyte bei einer kritischen Salzkonzentration in der L{\"o}sung einen scharfen {\"U}bergang zwischen einem stark geladenen gestreckten Zustand und einem schwach geladenen globul{\"a}ren Zustand aufweisen, wiederum in {\"U}bereinstimmung mit theoretischen Erwartungen.},
  language  = {en}
}
@phdthesis{Stoyanov2011,
  author    = {Stoyanov, Hristiyan},
  title     = {Soft nanocomposites with enhanced electromechanical response for dielectric elastomer actuators},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-51194},
  school      = {Universit{\"a}t Potsdam},
  year      = {2011},
  abstract  = {Electromechanical transducers based on elastomer capacitors are presently considered for many soft actuation applications, due to their large reversible deformation in response to electric field induced electrostatic pressure. The high operating voltage of such devices is currently a large drawback, hindering their use in applications such as biomedical devices and biomimetic robots, however, they could be improved with a careful design of their material properties. The main targets for improving their properties are increasing the relative permittivity of the active material, while maintaining high electric breakdown strength and low stiffness, which would lead to enhanced electrostatic storage ability and hence, reduced operating voltage. Improvement of the functional properties is possible through the use of nanocomposites. These exploit the high surface-to-volume ratio of the nanoscale filler, resulting in large effects on macroscale properties. This thesis explores several strategies for nanomaterials design. The resulting nanocomposites are fully characterized with respect to their electrical and mechanical properties, by use of dielectric spectroscopy, tensile mechanical analysis, and electric breakdown tests. First, nanocomposites consisting of high permittivity rutile TiO2 nanoparticles dispersed in thermoplastic block copolymer SEBS (poly-styrene-coethylene-co-butylene-co-styrene) are shown to exhibit permittivity increases of up to 3.7 times, leading to 5.6 times improvement in electrostatic energy density, but with a trade-off in mechanical properties (an 8-fold increase in stiffness). The variation in both electrical and mechanical properties still allows for electromechanical improvement, such that a 27 \% reduction of the electric field is found compared to the pure elastomer. Second, it is shown that the use of nanofiller conductive particles (carbon black (CB)) can lead to a strong increase of relative permittivity through percolation, however, with detrimental side effects. These are due to localized enhancement of the electric field within the composite, which leads to sharp reductions in electric field strength. Hence, the increase in permittivity does not make up for the reduction in breakdown strength in relation to stored electrical energy, which may prohibit their practical use. Third, a completely new approach for increasing the relative permittivity and electrostatic energy density of a polymer based on 'molecular composites' is presented, relying on chemically grafting soft π-conjugated macromolecules to a flexible elastomer backbone. Polarization caused by charge displacement along the conjugated backbone is found to induce a large and controlled permittivity enhancement (470 \% over the elastomer matrix), while chemical bonding, encapsulates the PANI chains manifesting in hardly any reduction in electric breakdown strength, and hence resulting in a large increase in stored electrostatic energy. This is shown to lead to an improvement in the sensitivity of the measured electromechanical response (83 \% reduction of the driving electric field) as well as in the maximum actuation strain (250 \%). These results represent a large step forward in the understanding of the strategies which can be employed to obtain high permittivity polymer materials with practical use for electro-elastomer actuation.},
  language  = {en}
}
@phdthesis{Maerten2015,
  author    = {Maerten, Lena},
  title     = {Spectroscopic perspectives on ultrafast coupling phenomena in perovskite oxides},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-77623},
  school      = {Universit{\"a}t Potsdam},
  year      = {2015},
  abstract  = {In this thesis, I study ultrafast dynamics in perovskite oxides using time resolved broadband spectroscopy. I focus on the observation of coherent phonon propagation by time resolved Brillouin scattering: following the excition of metal transducer films with a femtosecond infrared pump pulse, coherent phonon dynamics in the GHz frequency range are triggered. Their propagation is monitored using a delayed white light probe pulse. The technique is illustrated on various thin films and multilayered samples. I apply the technique to investigate the linear and nonlinear acoustic response in bulk SrTiO_3, which displays a ferroelastic phase transition from a cubic to a tetragonal structural phase at T_a=105 K. In the linear regime, I observe a coupling of the observed acoustic phonon mode to the softening optic modes describing the phase transition. In the nonlinear regime, I find a giant slowing down of the sound velocity in the low temperature phase that is only observable for a strain amplitude exceeding the tetragonality of the material. It is attributed to a coupling of the high frequency phonons to ferroelastic domain walls in the material. I propose a new mechanism for the coupling of strain waves to the domain walls that is only effective for high amplitude strain. A detailed study of the phonon attenuation across a wide temperature range shows that the phonon attenuation at low temperatures is influenced by the domain configuration, which is determined by interface strain. Preliminary measurements on magnetic-ferroelectric multilayers reveal that the excitation fluence needs to be carefully controlled when dynamics at phase transitions are studied.},
  language  = {en}
}
@phdthesis{Scholz2012,
  author    = {Scholz, Markus Reiner},
  title     = {Spin polarization, circular dichroism, and robustness of topological surface states},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-96686},
  school      = {Universit{\"a}t Potsdam},
  pages     = {153},
  year      = {2012},
  abstract  = {Dreidimensionale topologische Isolatoren sind ein neues Materialsystem, welches dadurch charakterisiert ist, dass es in seinem Inneren isolierend an der Ober {\"a}che jedoch leitend ist. Urs{\"a}chlich f{\"u}r die Leitf{\"a}higkeit an der Ober {\"a}che sind sogenannte topologische Ober- {\"a}chenzust{\"a}nde, welche das Valenzband des Inneren mit dem Leitungsband des Inneren verbinden. An der Ober {\"a}che ist also die Bandl{\"u}cke, welche die isolierende Eigenschaft verursacht, geschlossen. Die vorliegende Arbeit untersucht diese Ober {\"a}chenzust{\"a}nde mittels spin- und winkelauf- gel{\"o}ster Photoemissionsspektroskopie. Es wird gezeigt, dass in den Materialien Bi2Se3 und Bi2Te3, in {\"u}bereinstimmung mit der Literatur, die entscheidenden Charakteristika eines topologischen Ober {\"a}chenzustands vorzu nden sind: Die Ober {\"a}chenzust{\"a}nde dieser Sys- teme durchqueren die Bandl{\"u}cke in ungerader Anzahl, sie sind nicht entartet und weisen folgerichtig eine hohe Spinpolarisation auf. Weiterhin wird durch Aufdampfen diverser Adsorbate gezeigt, dass der Ober {\"a}chenzust{\"a}n- de von Bi2Se3 und Bi2Te3, wie erwartet, extrem robust ist. Ober {\"a}chenzust{\"a}nde topologisch trivialer Systeme erf{\"u}llen diese Eigenschaft nicht; bereits kleine Verunreinigungen k{\"o}n- nen diese Zust{\"a}nde zerst{\"o}ren, bzw. die Ober {\"a}che isolierend machen. Die topologischen Ober {\"a}chenzust{\"a}nde k{\"o}nnen in der vorliegenden Arbeit noch bis zur Detektionsgrenze der experimentellen Messmethode nachgewiesen werden und die Ober {\"a}che bleibt Leitf{\"a}hig. Unter den Adsorbaten be ndet sich auch Eisen, ein bekanntermaßen magnetisches Materi- al. Eine der Grundvoraussetzungen f{\"u}r topologische Isolatoren ist die Zeitumkehrsymme- trie, die Elektronen, welche den topologischen Ober {\"a}chenzustand besetzen, vorschreibt, dass sie eine bestimmte Spinrichtung haben m{\"u}ssen, wenn sie sich beispielsweise nach links bewegen und den entgegengesetzten Spin wenn sie sich nach rechts bewegen. In magnetischen Materialien ist die Zeitumkehrsymmetrie jedoch explizit gebrochen und die gezeigte Robustheit des Ober {\"a}chenzustands gegen magnetische Materialien daher uner- wartet. Die Zeitumkehrsymmetrie sorgt auch daf{\"u}r, dass eine Streuung der Elektronen um 180°, beispielsweise an einem Gitterdefekt oder an einem Phonon strikt verboten ist. Bei einem solchen Streuprozess bleibt die Spinrichtung erhalten, da aber in der Gegenrichtung nur Zust{\"a}nde mit entgegengesetztem Spin vorhanden sind kann das Elektron nicht in diese Richtung gestreut werden. Dieses Prinzip wird anhand der Lebensdauer der durch Pho- toemission angeregten Zust{\"a}nde untersucht. Hierbei wird gezeigt, dass die Kopplung der Elektronen des Ober {\"a}chenzustands von Bi2Te3 an Phononen unerwartet hoch ist und dass sich eine Anisotropie in der Bandstruktur des Selbigen auch in den Lebensdauern der ange- regten Zust{\"a}nde widerspiegelt. Weiterhin wird gezeigt, dass sich die Ein {\"u}sse von magne- tischen und nicht-magnetischen Verunreinigungen auf die Lebensdauern stark voneinander unterscheiden. Im letzten Teil der vorliegenden Arbeit wird untersucht, ob eine Asymmetrie in der Inten- sit{\"a}tsverteilung der winkelaufgel{\"o}sten Photoemissionsspektren, bei Anregung mit zirku- lar polarisiertem Licht, in Bi2Te3 R{\"u}ckschl{\"u}sse auf die Spinpolarisation der Elektronen erlaubt. Bei Variation der Energie des eingestrahlten Lichts wird ein Vorzeichenwechsel der Asymmetrie beobachtet. Daraus l{\"a}sst sich schlussfolgern, dass die Asymmetrie keine R{\"u}ckschl{\"u}sse auf die Spinpolarisation erlaubt.},
  language  = {en}
}
@phdthesis{Krivenkov2020,
  author    = {Krivenkov, Maxim},
  title     = {Spin textures and electron scattering in nanopatterned monolayer graphene},
  doi       = {10.25932/publishup-48701},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-487017},
  school      = {Universit{\"a}t Potsdam},
  pages     = {x, 176},
  year      = {2020},
  abstract  = {The current thesis is focused on the properties of graphene supported by metallic substrates and specifically on the behaviour of electrons in such systems. Methods of scanning tunneling microscopy, electron diffraction and photoemission spectroscopy were applied to study the structural and electronic properties of graphene. The purpose of the first part of this work is to introduce the most relevant aspects of graphene physics and the methodical background of experimental techniques used in the current thesis. The scientific part of this work starts with the extensive study by means of scanning tunneling microscopy of the nanostructures that appear in Au intercalated graphene on Ni(111). This study was aimed to explore the possible structural explanations of the Rashba-type spin splitting of ~100 meV experimentally observed in this system — much larger than predicted by theory. It was demonstrated that gold can be intercalated under graphene not only as a dense monolayer, but also in the form of well-periodic arrays of nanoclusters, a structure previously not reported. Such nanocluster arrays are able to decouple graphene from the strongly interacting Ni substrate and render it quasi-free-standing, as demonstrated by our DFT study. At the same time calculations confirm strong enhancement of the proximity-induced SOI in graphene supported by such nanoclusters in comparison to monolayer gold. This effect, attributed to the reduced graphene-Au distance in the case of clusters, provides a large Rashba-type spin splitting of ~60 meV. The obtained results not only provide a possible mechanism of SOI enhancement in this particular system, but they can be also generalized for graphene on other strongly interacting substrates intercalated by nanostructures of heavy noble d metals. Even more intriguing is the proximity of graphene to heavy sp-metals that were predicted to induce an intrinsic SOI and realize a spin Hall effect in graphene. Bismuth is the heaviest stable sp-metal and its compounds demonstrate a plethora of exciting physical phenomena. This was the motivation behind the next part of the current thesis, where structural and electronic properties of a previously unreported phase of Bi-intercalated graphene on Ir(111) were studied by means of scanning tunneling microscopy, spin- and angle-resolved photoemission spectroscopy and electron diffraction. Photoemission experiments revealed a remarkable, nearly ideal graphene band structure with strongly suppressed signatures of interaction between graphene and the Ir(111) substrate, moreover, the characteristic moir{\´e} pattern observed in graphene on Ir(111) by electron diffraction and scanning tunneling microscopy was strongly suppressed after intercalation. The whole set of experimental data evidences that Bi forms a dense intercalated layer that efficiently decouples graphene from the substrate. The interaction manifests itself only in the n-type charge doping (~0.4 eV) and a relatively small band gap at the Dirac point (~190 meV). The origin of this minor band gap is quite intriguing and in this work it was possible to exclude a wide range of mechanisms that could be responsible for it, such as induced intrinsic spin-orbit interaction, hybridization with the substrate states and corrugation of the graphene lattice. The main origin of the band gap was attributed to the A-B symmetry breaking and this conclusion found support in the careful analysis of the interference effects in photoemission that provided the band gap estimate of ~140 meV. While the previous chapters were focused on adjusting the properties of graphene by proximity to heavy metals, graphene on its own is a great object to study various physical effects at crystal surfaces. The final part of this work is devoted to a study of surface scattering resonances by means of photoemission spectroscopy, where this effect manifests itself as a distinct modulation of photoemission intensity. Though scattering resonances were widely studied in the past by means of electron diffraction, studies about their observation in photoemission experiments started to appear only recently and they are very scarce. For a comprehensive study of scattering resonances graphene was selected as a versatile model system with adjustable properties. After the theoretical and historical introduction to the topic of scattering resonances follows a detailed description of the unusual features observed in the photoemission spectra obtained in this work and finally the equivalence between these features and scattering resonances is proven. The obtained photoemission results are in a good qualitative agreement with the existing theory, as verified by our calculations in the framework of the interference model. This simple model gives a suitable explanation for the general experimental observations. The possibilities of engineering the scattering resonances were also explored. A systematic study of graphene on a wide range of substrates revealed that the energy position of the resonances is in a direct relation to the magnitude of charge transfer between graphene and the substrate. Moreover, it was demonstrated that the scattering resonances in graphene on Ir(111) can be suppressed by nanopatterning either by a superlattice of Ir nanoclusters or by atomic hydrogen. These effects were attributed to strong local variations of tork function and/or destruction of long-range order of thephene lattice. The tunability of scattering resonances can be applied for optoelectronic devices based on graphene. Moreover, the results of this study expand the general understanding of the phenomenon of scattering resonances and are applicable to many other materials besides graphene.},
  language  = {en}
}
@phdthesis{Kuhlbrodt2002,
  author    = {Kuhlbrodt, Till},
  title     = {Stability and variability of open-ocean deep convection in deterministic and stochastic simple models},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0000622},
  school      = {Universit{\"a}t Potsdam},
  year      = {2002},
  abstract  = {Die Tiefenkonvektion ist ein wesentlicher Bestandteil der Zirkulation im Nordatlantik. Sie beeinflusst den nordw{\"a}rtigen W{\"a}rmetransport der thermohalinen Zirkulation. Ein Verst{\"a}ndnis ihrer Stabilit{\"a}t und Variabilit{\"a}t ist daher n{\"o}tig, um Klimaver{\"a}nderungen im Bereich des Nordatlantiks einsch{\"a}tzen zu k{\"o}nnen. Diese Arbeit hat zum Ziel, das konzeptionelle Verst{\"a}ndnis der Stabilit{\"a}t und der Variabilit{\"a}t der Tiefenkonvektion zu verbessern. Beobachtungsdaten aus der Labradorsee zeigen Phasen mit und ohne Tiefenkonvektion. Ein einfaches Modell mit zwei Boxen wird an diese Daten angepasst. Das Ergebnis legt nahe, dass die Labradorsee zwei koexistierende stabile Zust{\"a}nde hat, einen mit regelm{\"a}ßiger Tiefenkonvektion und einen ohne Tiefenkonvektion. Diese Bistabilit{\"a}t ergibt sich aus einer positiven Salzgehalts-R{\"u}ckkopplung, deren Ursache ein Netto-S{\"u}ßwassereintrag in die Deckschicht ist. Der konvektive Zustand kann schnell instabil werden, wenn der mittlere Antrieb sich hin zu w{\"a}rmeren oder weniger salzhaltigen Bedingungen {\"a}ndert. Die wetterbedingte Variabilit{\"a}t des externen Antriebs wird durch die Addition eines stochastischen Antriebsterms in das Modell eingebaut. Es zeigt sich, dass dann die Tiefenkonvektion h{\"a}ufig an- und wieder ausgeschaltet wird. Die mittlere Aufenthaltszeit in beiden Zust{\"a}nden ist ein Maß ihrer stochastischen Stabilit{\"a}t. Die stochastische Stabilit{\"a}t h{\"a}ngt in glatter Weise von den Parametern des Antriebs ab, im Gegensatz zu der deterministischen (nichtstochastischen) Stabilit{\"a}t, die sich abrupt {\"a}ndern kann. Sowohl das Mittel als auch die Varianz des stochastischen Antriebs beeinflussen die H{\"a}ufigkeit von Tiefenkonvektion. Eine Abnahme der Konvektionsh{\"a}ufigkeit, als Reaktion auf eine Abnahme des Salzgehalts an der Oberfl{\"a}che, kann zum Beispiel durch eine Zunahme der Variabilit{\"a}t in den W{\"a}rmefl{\"u}ssen kompensiert werden. Mit einem weiter vereinfachten Box-Modell werden einige Eigenschaften der stochastischen Stabilit{\"a}t analytisch untersucht. Es wird ein neuer Effekt beschrieben, die wandernde Monostabilit{\"a}t: Auch wenn die Tiefenkonvektion aufgrund ge{\"a}nderter Parameter des Antriebs kein stabiler Zustand mehr ist, kann der stochastische Antrieb immer noch h{\"a}ufig Konvektionsereignisse ausl{\"o}sen. Die analytischen Gleichungen zeigen explizit, wie die wandernde Monostabilit{\"a}t sowie andere Effekte von den Modellparametern abh{\"a}ngen. Diese Abh{\"a}ngigkeit ist f{\"u}r die mittleren Aufenthaltszeiten immer exponentiell, f{\"u}r die Wahrscheinlichkeit langer nichtkonvektiver Phasen dagegen nur dann, wenn diese Wahrscheinlichkeit gering ist. Es ist zu erwarten, dass wandernde Monostabilit{\"a}t auch in anderen Teilen des Klimasystems eine Rolle spielt. Insgesamt zeigen die Ergebnisse, dass die Stabilit{\"a}t der Tiefenkonvektion in der Labradorsee sehr empfindlich auf den Antrieb reagiert. Die Rolle der Variabilit{\"a}t ist entscheidend f{\"u}r ein Verst{\"a}ndnis dieser Empfindlichkeit. Kleine {\"A}nderungen im Antrieb k{\"o}nnen bereits die H{\"a}ufigkeit von Tiefenkonvektionsereignissen deutlich mindern, was sich vermutlich stark auf das regionale Klima auswirkt.},
  subject      = {Labradorsee ; Thermohaline Konvektion ; Stochastisches Modell},
  language  = {en}
}
@phdthesis{Kalbitz2011,
  author    = {Kalbitz, Ren{\´e}},
  title     = {Stability of polarization in organic ferroelectric metal-insulator-semiconductor structures},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-57276},
  school      = {Universit{\"a}t Potsdam},
  year      = {2011},
  abstract  = {Organic thin film transistors (TFT) are an attractive option for low cost electronic applications and may be used for active matrix displays and for RFID applications. To extend the range of applications there is a need to develop and optimise the performance of non-volatile memory devices that are compatible with the solution-processing fabrication procedures used in plastic electronics. A possible candidate is an organic TFT incorporating the ferroelectric co-polymer poly(vinylidenefluoride-trifluoroethylene)(P(VDF-TrFE)) as the gate insulator. Dielectric measurements have been carried out on all-organic metal-insulator-semiconductor structures with the ferroelectric polymer poly(vinylidenefluoride-trifluoroethylene) (P(VDF-TrFE)) as the gate insu-lator. The capacitance spectra of MIS devices, were measured under different biases, showing the effect of charge accumulation and depletion on the Maxwell-Wagner peak. The position and height of this peak clearly indicates the lack of stable depletion behavior and the decrease of mobility when increasing the depletion zone width, i.e. upon moving into the P3HT bulk. The lack of stable depletion was further investigated with capacitance-voltage (C-V) measurements. When the structure was driven into depletion, C-V plots showed a positive flat-band voltage shift, arising from the change in polarization state of the ferroelectric insulator. When biased into accumulation, the polarization was reversed. It is shown that the two polarization states are stable i.e. no depolarization occurs below the coercive field. However, negative charge trapped at the semiconductor-insulator interface during the depletion cycle masks the negative shift in flat-band voltage expected during the sweep to accumulation voltages. The measured output characteristics of the studied ferroelectric-field-effect transistors confirmed the results of the C-V plots. Furthermore, the results indicated a trapping of electrons at the positively charged surfaces of the ferroelectrically polarized P(VDF-TrFE) crystallites near the insulator/semiconductor in-terface during the first poling cycles. The study of the MIS structure by means of thermally stimulated current (TSC) revealed further evidence for the stability of the polarization under depletion voltages. It was shown, that the lack of stable depletion behavior is caused by the compensation of the orientational polarization by fixed electrons at the interface and not by the depolarization of the insulator, as proposed in several publications. The above results suggest a performance improvement of non-volatile memory devices by the optimization of the interface.},
  language  = {en}
}
@phdthesis{Koelsch2005,
  author    = {K{\"o}lsch, Patrick},
  title     = {Static and dynamic properties of soluble surfactants at the air/water interface},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-5716},
  school      = {Universit{\"a}t Potsdam},
  year      = {2005},
  abstract  = {Amphiphilic molecules contain a hydrophilic headgroup and a hydrophobic tail. The headgroup is polar or ionic and likes water, the tail is typically an aliphatic chain that cannot be accommodated in a polar environment. The prevailing molecular asymmetry leads to a spontaneous adsorption of amphiphiles at the air/water or oil/water interfaces. As a result, the surface tension and the surface rheology is changed. Amphiphiles are important tools to deliberately modify the interfacial properties of liquid interfaces and enable new phenomena such as foams which cannot be formed in a pure liquid. In this thesis we investigate the static and dynamic properties of adsorption layers of soluble amphiphiles at the air/water interface, the so called Gibbs monolayers. The classical way for an investigation of these systems is based on a thermodynamic analysis of the equilibrium surface tension as a function of the bulk composition in the framework of Gibbs theory. However, thermodynamics does not provide any structural information and several recent publications challenge even fundamental text book concepts. The experimental investigation faces difficulties imposed by the low surface coverage and the presence of dissolved amphiphiles in the adjacent bulk phase. In this thesis we used a suite of techniques with the sensitivity to detect less than a monolayer of molecules at the air-water interface. Some of these techniques are extremely complex such as infrared visible sum frequency generation (IR-VIS SFG) spectroscopy or second harmonic generation (SHG). Others are traditional techniques, such as ellipsometry employed in new ways and pushed to new limits. Each technique probes selectively different parts of the interface and the combination provides a profound picture of the interfacial architecture. The first part of the thesis is dedicated to the distribution of ions at interfaces. Adsorption layers of ionic amphiphiles serve as model systems allowing to produce a defined surface charge. The charge of the monolayer is compensated by the counterions. As a result of a complex zoo of interactions there will be a defined distribution of ions at the interface, however, its experimental determination is a big scientific challenge. We could demonstrate that a combination of linear and nonlinear techniques gives direct insights in the prevailing ion distribution. Our investigations reveal specific ion effects which cannot be described by classical Poisson-Boltzmann mean field type theories. Adsorption layer and bulk phase are in thermodynamic equilibrium, however, it is important to stress that there is a constant molecular exchange between adsorbed and dissolved species. This exchange process is a key element for the understanding of some of the thermodynamic properties. An excellent way to study Gibbs monolayers is to follow the relaxation from a non-equilibrium to an equilibrium state. Upon compression amphiphiles must leave the adsorption layer and dissolve in the adjacent bulk phase. Upon expansion amphiphiles must adsorb at the interface to restore the equilibrium coverage. Obviously the frequency of the expansion and compression cycles must match the molecular exchange processes. At too low frequencies the equilibrium is maintained at all times. If the frequency is too fast the system behaves as a monolayer of insoluble surfactants. In this thesis we describe an unique variant of an oscillating bubble technique that measures precisely the real and imaginary part of the complex dilational modulus E in a frequency range up to 500 Hz. The extension of about two decades in the time domain in comparison to the conventional method of an oscillating drop is a tremendous achievement. The imaginary part of the complex dilational modulus E is a consequence of a dissipative process which is interpreted as an intrinsic surface dilational viscosity. The IR-VIS SFG spectra of the interfacial water provide a molecular interpretation of the underlying dissipative process.},
  subject      = {Nichtlineare Optik},
  language  = {en}
}
@phdthesis{Kiel2012,
  author    = {Kiel, Mareike},
  title     = {Static and ultrafast optical properties of nanolayered composites : gold nanoparticles embedded in polyelectrolytes},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-61823},
  school      = {Universit{\"a}t Potsdam},
  year      = {2012},
  abstract  = {In the course of this thesis gold nanoparticle/polyelectrolyte multilayer structures were prepared, characterized, and investigated according to their static and ultrafast optical properties. Using the dip-coating or spin-coating layer-by-layer deposition method, gold-nanoparticle layers were embedded in a polyelectrolyte environment with high structural perfection. Typical structures exhibit four repetition units, each consisting of one gold-particle layer and ten double layers of polyelectrolyte (cationic+anionic polyelectrolyte). The structures were characterized by X-ray reflectivity measurements, which reveal Bragg peaks up to the seventh order, evidencing the high stratication of the particle layers. In the same measurements pronounced Kiessig fringes were observed, which indicate a low global roughness of the samples. Atomic force microscopy (AFM) images veried this low roughness, which results from the high smoothing capabilities of polyelectrolyte layers. This smoothing effect facilitates the fabrication of stratified nanoparticle/polyelectrolyte multilayer structures, which were nicely illustrated in a transmission electron microscopy image. The samples' optical properties were investigated by static spectroscopic measurements in the visible and UV range. The measurements revealed a frequency shift of the reflectance and of the plasmon absorption band, depending on the thickness of the polyelectrolyte layers that cover a nanoparticle layer. When the covering layer becomes thicker than the particle interaction range, the absorption spectrum becomes independent of the polymer thickness. However, the reflectance spectrum continues shifting to lower frequencies (even for large thicknesses). The range of plasmon interaction was determined to be in the order of the particle diameter for 10 nm, 20 nm, and 150 nm particles. The transient broadband complex dielectric function of a multilayer structure was determined experimentally by ultrafast pump-probe spectroscopy. This was achieved by simultaneous measurements of the changes in the reflectance and transmittance of the excited sample over a broad spectral range. The changes in the real and imaginary parts of the dielectric function were directly deduced from the measured data by using a recursive formalism based on the Fresnel equations. This method can be applied to a broad range of nanoparticle systems where experimental data on the transient dielectric response are rare. This complete experimental approach serves as a test ground for modeling the dielectric function of a nanoparticle compound structure upon laser excitation.},
  language  = {en}
}
@phdthesis{Zillmer2003,
  author    = {Zillmer, R{\"u}diger},
  title     = {Statistical properties and scaling of the Lyapunov exponents in stochastic systems},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0001147},
  school      = {Universit{\"a}t Potsdam},
  year      = {2003},
  abstract  = {Die vorliegende Arbeit umfaßt drei Abhandlungen, welche allgemein mit einer stochastischen Theorie f{\"u}r die Lyapunov-Exponenten befaßt sind. Mit Hilfe dieser Theorie werden universelle Skalengesetze untersucht, die in gekoppelten chaotischen und ungeordneten Systemen auftreten. Zun{\"a}chst werden zwei zeitkontinuierliche stochastische Modelle f{\"u}r schwach gekoppelte chaotische Systeme eingef{\"u}hrt, um die Skalierung der Lyapunov-Exponenten mit der Kopplungsst{\"a}rke ('coupling sensitivity of chaos') zu untersuchen. Mit Hilfe des Fokker-Planck-Formalismus werden Skalengesetze hergeleitet, die von Ergebnissen numerischer Simulationen best{\"a}tigt werden. Anschließend wird gezeigt, daß 'coupling sensitivity' im Fall gekoppelter ungeordneter Ketten auftritt, wobei der Effekt sich durch ein singul{\"a}res Anwachsen der Lokalisierungsl{\"a}nge {\"a}ußert. Numerische Ergebnisse f{\"u}r gekoppelte Anderson-Modelle werden bekr{\"a}ftigt durch analytische Resultate f{\"u}r gekoppelte raumkontinuierliche Schr{\"o}dinger-Gleichungen. Das resultierende Skalengesetz f{\"u}r die Lokalisierungsl{\"a}nge {\"a}hnelt der Skalierung der Lyapunov-Exponenten gekoppelter chaotischer Systeme. Schließlich wird die Statistik der exponentiellen Wachstumsrate des linearen Oszillators mit parametrischem Rauschen studiert. Es wird gezeigt, daß die Verteilung des zeitabh{\"a}ngigen Lyapunov-Exponenten von der Normalverteilung abweicht. Mittels der verallgemeinerten Lyapunov-Exponenten wird der Parameterbereich bestimmt, in welchem die Abweichungen von der Normalverteilung signifikant sind und Multiskalierung wesentlich wird.},
  language  = {en}
}
@phdthesis{Hantschmann2023,
  author    = {Hantschmann, Markus},
  title     = {Stimulated resonant inelastic X-ray scattering in transition metal systems},
  doi       = {10.25932/publishup-58476},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-584761},
  school      = {Universit{\"a}t Potsdam},
  pages     = {xv, 160},
  year      = {2023},
  abstract  = {With the implementation of intense, short pulsed light sources throughout the last years, the powerful technique of resonant inelastic X-ray scattering (RIXS) became feasible for a wide range of experiments within femtosecond dynamics in correlated materials and molecules. In this thesis I investigate the potential to bring RIXS into the fluence regime of nonlinear X-ray-matter interactions, especially focusing on the impact of stimulated scattering on RIXS in transition metal systems in a transmission spectroscopy geometry around transition metal L-edges. After presenting the RIXS toolbox and the capabilities of free electron laser light sources for ultrafast intense X-ray experiments, the thesis explores an experiment designed to understand the impact of stimulated scattering on diffraction and direct beam transmission spectroscopy on a CoPd multilayer system. The experiments require short X-ray pulses that can only be generated at free electron lasers (FEL). Here the pulses are not only short, but also very intense, which opens the door to nonlinear X-ray-matter interactions. In the second part of this thesis, we investigate observations in the nonlinear interaction regime, look at potential difficulties for classic spectroscopy and investigate possibilities to enhance the RIXS through stimulated scattering. Here, a study on stimulated RIXS is presented, where we investigate the light field intensity dependent CoPd demagnetization in transmission as well as scattering geometry. Thereby we show the first direct observation of stimulated RIXS as well as light field induced nonlinear effects, namely the breakdown of scattering intensity and the increase in sample transmittance. The topic is of ongoing interest and will just increase in relevance as more free electron lasers are planned and the number of experiments at such light sources will continue to increase in the near future. Finally we present a discussion on the accessibility of small DOS shifts in the absorption-band of transition metal complexes through stimulated resonant X-ray scattering. As these shifts occur for example in surface states this finding could expand the experimental selectivity of NEXAFS and RIXS to the detectability of surface states. We show how stimulation can indeed enhance the visibility of DOS shifts through the detection of stimulated spectral shifts and enhancements in this theoretical study. We also forecast the observation of stimulated enhancements in resonant excitation experiments at FEL sources in systems with a high density of states just below the Fermi edge and in systems with an occupied to unoccupied DOS ratio in the valence band above 1.},
  language  = {en}
}
@phdthesis{Erdmann2005,
  author    = {Erdmann, Thorsten},
  title     = {Stochastic dynamics of adhesion clusters under force},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-5564},
  school      = {Universit{\"a}t Potsdam},
  year      = {2005},
  abstract  = {Adhesion of biological cells to their environment is mediated by two-dimensional clusters of specific adhesion molecules which are assembled in the plasma membrane of the cells. Due to the activity of the cells or external influences, these adhesion sites are usually subject to physical forces. In recent years, the influence of such forces on the stability of cellular adhesion clusters was increasingly investigated. In particular, experimental methods that were originally designed for the investigation of single bond rupture under force have been applied to investigate the rupture of adhesion clusters. The transition from single to multiple bonds, however, is not trivial and requires theoretical modelling. Rupture of biological adhesion molecules is a thermally activated, stochastic process. In this work, a stochastic model for the rupture and rebinding dynamics of clusters of parallel adhesion molecules under force is presented. In particular, the influence of (i) a constant force as it may be assumed for cellular adhesion clusters is investigated and (ii) the influence of a linearly increasing force as commonly used in experiments is considered. Special attention is paid to the force-mediated cooperativity of parallel adhesion bonds. Finally, the influence of a finite distance between receptors and ligands on the binding dynamics is investigated. Thereby, the distance can be bridged by polymeric linker molecules which tether the ligands to a substrate.},
  subject      = {Biophysik},
  language  = {en}
}
@phdthesis{Korn2007,
  author    = {Korn, Christian},
  title     = {Stochastic dynamics of cell adhesion in hydrodynamic flow},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-12997},
  school      = {Universit{\"a}t Potsdam},
  year      = {2007},
  abstract  = {In this thesis the interplay between hydrodynamic transport and specific adhesion is theoretically investigated. An important biological motivation for this work is the rolling adhesion of white blood cells experimentally investigated in flow chambers. There, specific adhesion is mediated by weak bonds between complementary molecular building blocks which are either located on the cell surface (receptors) or attached to the bottom plate of the flow chamber (ligands). The model system under consideration is a hard sphere covered with receptors moving above a planar ligand-bearing wall. The motion of the sphere is influenced by a simple shear flow, deterministic forces, and Brownian motion. An algorithm is given that allows to numerically simulate this motion as well as the formation and rupture of bonds between receptors and ligands. The presented algorithm spatially resolves receptors and ligands. This opens up the perspective to apply the results also to flow chamber experiments done with patterned substrates based on modern nanotechnological developments. In the first part the influence of flow rate, as well as of the number and geometry of receptors and ligands, on the probability for initial binding is studied. This is done by determining the mean time that elapses until the first encounter between a receptor and a ligand occurs. It turns out that besides the number of receptors, especially the height by which the receptors are elevated above the surface of the sphere plays an important role. These findings are in good agreement with observations of actual biological systems like white blood cells or malaria-infected red blood cells. Then, the influence of bonds which have formed between receptors and ligands, but easily rupture in response to force, on the motion of the sphere is studied. It is demonstrated that different states of motion-for example rolling-can be distinguished. The appearance of these states depending on important model parameters is then systematically investigated. Furthermore, it is shown by which bond property the ability of cells to stably roll in a large range of applied flow rates is increased. Finally, the model is applied to another biological process, the transport of spherical cargo particles by molecular motors. In analogy to the so far described systems molecular motors can be considered as bonds that are able to actively move. In this part of the thesis the mean distance the cargo particles are transported is determined.},
  language  = {en}
}
@phdthesis{Kleinen2005,
  author    = {Kleinen, Thomas Christopher},
  title     = {Stochastic information in the assessment of climate change},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-5382},
  school      = {Universit{\"a}t Potsdam},
  year      = {2005},
  abstract  = {Stochastic information, to be understood as \&quot;information gained by the application of stochastic methods\&quot;, is proposed as a tool in the assessment of changes in climate. This thesis aims at demonstrating that stochastic information can improve the consideration and reduction of uncertainty in the assessment of changes in climate. The thesis consists of three parts. In part one, an indicator is developed that allows the determination of the proximity to a critical threshold. In part two, the tolerable windows approach (TWA) is extended to a probabilistic TWA. In part three, an integrated assessment of changes in flooding probability due to climate change is conducted within the TWA. The thermohaline circulation (THC) is a circulation system in the North Atlantic, where the circulation may break down in a saddle-node bifurcation under the influence of climate change. Due to uncertainty in ocean models, it is currently very difficult to determine the distance of the THC to the bifurcation point. We propose a new indicator to determine the system's proximity to the bifurcation point by considering the THC as a stochastic system and using the information contained in the fluctuations of the circulation around the mean state. As the system is moved closer to the bifurcation point, the power spectrum of the overturning becomes \&quot;redder\&quot;, i.e. more energy is contained in the low frequencies. Since the spectral changes are a generic property of the saddle-node bifurcation, the method is not limited to the THC, but it could also be applicable to other systems, e.g. transitions in ecosystems. In part two, a probabilistic extension to the tolerable windows approach (TWA) is developed. In the TWA, the aim is to determine the complete set of emission strategies that are compatible with so-called guardrails. Guardrails are limits to impacts of climate change or to climate change itself. Therefore, the TWA determines the \&quot;maneuvering space\&quot; humanity has, if certain impacts of climate change are to be avoided. Due to uncertainty it is not possible to definitely exclude the impacts of climate change considered, but there will always be a certain probability of violating a guardrail. Therefore the TWA is extended to a probabilistic TWA that is able to consider \&quot;probabilistic uncertainty\&quot;, i.e. uncertainty that can be expressed as a probability distribution or uncertainty that arises through natural variability. As a first application, temperature guardrails are imposed, and the dependence of emission reduction strategies on probability distributions for climate sensitivities is investigated. The analysis suggests that it will be difficult to observe a temperature guardrail of 2\&deg;C with high probabilities of actually meeting the target. In part three, an integrated assessment of changes in flooding probability due to climate change is conducted. A simple hydrological model is presented, as well as a downscaling scheme that allows the reconstruction of the spatio-temporal natural variability of temperature and precipitation. These are used to determine a probabilistic climate impact response function (CIRF), a function that allows the assessment of changes in probability of certain flood events under conditions of a changed climate. The assessment of changes in flooding probability is conducted in 83 major river basins. Not all floods can be considered: Events that either happen very fast, or affect only a very small area can not be considered, but large-scale flooding due to strong longer-lasting precipitation events can be considered. Finally, the probabilistic CIRFs obtained are used to determine emission corridors, where the guardrail is a limit to the fraction of world population that is affected by a predefined shift in probability of the 50-year flood event. This latter analysis has two main results. The uncertainty about regional changes in climate is still very high, and even small amounts of further climate change may lead to large changes in flooding probability in some river systems.},
  subject      = {Anthropogene Klima{\"a}nderung},
  language  = {en}
}
@phdthesis{FrancoGonzalez2002,
  author    = {Franco Gonz{\´a}lez, Olga},
  title     = {Structural and spectroscopical study of crystals of 1,3,4-oxadiazole derivatives at high pressure},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0000572},
  school      = {Universit{\"a}t Potsdam},
  year      = {2002},
  abstract  = {Die Suche nach neuen Materialien von technischem Interesse hat in den letzten Jahren neue Antriebe zu der Untersuchung organischer Verbindungen gegeben. Organische Substanzen haben viele Vorteile wie z.B. die M{\"o}glichkeit, ihre Eigenschaften durch verschiedene chemische und physikalische Techniken im Herstellung-Prozess f{\"u}r ein bestimmtes Ziel zu modifizieren. Oxadiazolverbindungen sind interessant aufgrund ihrer Nutzung als Material f{\"u}r Licht emittierende Dioden und Scintillatoren. Die physikalischen Eigenschaften eines Festk{\"o}rpers h{\"a}ngen von seiner Struktur ab. Unterschiedliche Strukturen entwickeln unterschiedliche intra- und intermolek{\"u}lare Wechselwirkungen. Eine ausgezeichnete Weise, um sowohl die intra- als auch die intermolekularen Wechselwirkungen eines bestimmtes Stoffes zu beeinflussen, ohne seine chemischen Charakteristiken zu {\"a}ndern, ist die Verwendung von hohem Druck. Wir haben den Einfluss von hohem Druck und hoher Temperatur auf die super-molekulare Struktur einiger Oxadiazolverbindungen im kristallinem Zustand untersucht. Aus diesen Untersuchungsergebnissen wurde eine Zustandsgleichung f{\"u}r diese Kristalle bestimmt. {\"U}berdies wurden die spektroskopischen Eigenschaften dieser Materialien unter hohem Druck charakterisiert.},
  subject      = {Oxadiazolderivate ; Kristallstruktur ; Hochdruck ; UV-VIS-Spektroskopie ; Raman-Spektroskopie},
  language  = {en}
}
@phdthesis{RodriguezLoureiro2018,
  author    = {Rodriguez Loureiro, Ignacio},
  title     = {Structural characterization of single and interacting soft interfaces displaying brushes of synthetic or biomolecular polymers},
  doi       = {10.25932/publishup-42367},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-423675},
  school      = {Universit{\"a}t Potsdam},
  pages     = {132},
  year      = {2018},
  abstract  = {The interaction between surfaces displaying end-grafted hydrophilic polymer brushes plays important roles in biology and in many wet-technological applications. The outer surfaces of Gram-negative bacteria, for example, are composed of lipopolysaccharide (LPS) molecules exposing oligo- and polysaccharides to the aqueous environment. This unique, structurally complex biological interface is of great scientific interest as it mediates the interaction of bacteria with neighboring bacteria in colonies and biofilms. The interaction between polymer-decorated surfaces is generally coupled to the distance-dependent conformation of the polymer chains. Therefore, structural insight into the interacting surfaces is a prerequisite to understand the interaction characteristics as well as the underlying physical mechanisms. This problem has been addressed by theory, but accurate experimental data on polymer conformations under confinement are rare, because obtaining perturbation-free structural insight into buried soft interfaces is inherently difficult. In this thesis, lipid membrane surfaces decorated with hydrophilic polymers of technological and biological relevance are investigated under controlled interaction conditions, i.e., at defined surface separations. For this purpose, dedicated sample architectures and experimental tools are developed. Via ellipsometry and neutron reflectometry pressure-distance curves and distance-dependent polymer conformations in terms of brush compression and reciprocative interpenetration are determined. Additional element-specific structural insight into the end-point distribution of interacting brushes is obtained by standing-wave x-ray fluorescence (SWXF). The methodology is first established for poly[ethylene glycol] (PEG) brushes of defined length and grafting density. For this system, neutron reflectometry revealed pronounced brush interpenetration, which is not captured in common brush theories and therefore motivates rigorous simulation-based treatments. In the second step the same approach is applied to realistic mimics of the outer surfaces of Gram-negative bacteria: monolayers of wild type LPSs extracted from E. Coli O55:B5 displaying strain-specific O-side chains. The neutron reflectometry experiments yield unprecedented structural insight into bacterial interactions, which are of great relevance for the properties of biofilms.},
  language  = {en}
}
@phdthesis{Herzog2012,
  author    = {Herzog, Marc},
  title     = {Structural dynamics of photoexcited nanolayered perovskites studied by ultrafast x-ray diffraction},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-62632},
  school      = {Universit{\"a}t Potsdam},
  year      = {2012},
  abstract  = {This publication-based thesis represents a contribution to the active research field of ultrafast structural dynamics in laser-excited nanostructures. The investigation of such dynamics is mandatory for the understanding of the various physical processes on microscopic scales in complex materials which have great potentials for advances in many technological applications. I theoretically and experimentally examine the coherent, incoherent and anharmonic lattice dynamics of epitaxial metal-insulator heterostructures on timescales ranging from femtoseconds up to nanoseconds. To infer information on the transient dynamics in the photoexcited crystal lattices experimental techniques using ultrashort optical and x-ray pulses are employed. The experimental setups include table-top sources as well as large-scale facilities such as synchrotron sources. At the core of my work lies the development of a linear-chain model to simulate and analyze the photoexcited atomic-scale dynamics. The calculated strain fields are then used to simulate the optical and x-ray response of the considered thin films and multilayers in order to relate the experimental signatures to particular structural processes. This way one obtains insight into the rich lattice dynamics exhibiting coherent transport of vibrational energy from local excitations via delocalized phonon modes of the samples. The complex deformations in tailored multilayers are identified to give rise to highly nonlinear x-ray diffraction responses due to transient interference effects. The understanding of such effects and the ability to precisely calculate those are exploited for the design of novel ultrafast x-ray optics. In particular, I present several Phonon Bragg Switch concepts to efficiently generate ultrashort x-ray pulses for time-resolved structural investigations. By extension of the numerical models to include incoherent phonon propagation and anharmonic lattice potentials I present a new view on the fundamental research topics of nanoscale thermal transport and anharmonic phonon-phonon interactions such as nonlinear sound propagation and phonon damping. The former issue is exemplified by the time-resolved heat conduction from thin SrRuO3 films into a SrTiO3 substrate which exhibits an unexpectedly slow heat conductivity. Furthermore, I discuss various experiments which can be well reproduced by the versatile numerical models and thus evidence strong lattice anharmonicities in the perovskite oxide SrTiO3. The thesis also presents several advances of experimental techniques such as time-resolved phonon spectroscopy with optical and x-ray photons as well as concepts for the implementation of x-ray diffraction setups at standard synchrotron beamlines with largely improved time-resolution for investigations of ultrafast structural processes. This work forms the basis for ongoing research topics in complex oxide materials including electronic correlations and phase transitions related to the elastic, magnetic and polarization degrees of freedom.},
  language  = {en}
}
@phdthesis{Richter2007,
  author    = {Richter, Andreas},
  title     = {Structure formation and fractionation in systems of colloidal rods},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-13090},
  school      = {Universit{\"a}t Potsdam},
  year      = {2007},
  abstract  = {Nowadays, colloidal rods can be synthesized in large amounts. The rods are typically cylindrically and their length ranges from several nanometers to a few micrometers. In solution, systems of colloidal rodlike molecules or aggregates can form liquid-crystalline phases with long-range orientational and spatial order. In the present work, we investigate structure formation and fractionation in systems of rodlike colloids with the help of Monte Carlo simulations in the NPT ensemble. Repulsive interactions can successfully be mimicked by the hard rod model, which has been studied extensively in the past. In many cases, attractive interactions like van der Waals or depletion forces cannot be neglected, however. In the first part of this work, the phase behavior of monodisperse attractive rods is characterized for different interaction strengths. Phase diagrams as a function of rod length and pressure are presented. Most systems of synthesized mesoscopic rods have a polydisperse length distribution as a consequence of the longitudinal growth process of the rods. For many technical and research applications, a rather small polydispersity is desired in order to have well defined material properties. The polydispersity can be reduced by a spatial demixing (fractionation) of long and short rods. Fractionation and structure formation is studied in a tridisperse and a polydisperse bulk suspension of rods. We observe that the resulting structures depend distinctly on the interaction strength. The fractionation in the system is strongly enhanced with increasing interaction strength. Suspensions are typically confined in a container. We also examine the influence of adjacent substrates in systems of tridisperse and polydisperse rod suspensions. Three different substrate types are studied in detail: a planar wall, a corrugated substrate, and a substrate with rectangular cavities. We analyze the fluid structure close to the substrate and substrate controlled fractionation. The spatial arrangement of long and short rods in front of the substrate depends sensitively on the substrate structure and the pressure. Rods with a predefined length are segregated at substrates with rectangular cavities.},
  language  = {en}
}
@phdthesis{Zemanova2007,
  author    = {Zemanov{\´a}, Lucia},
  title     = {Structure-function relationship in hierarchical model of brain networks},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-18400},
  school      = {Universit{\"a}t Potsdam},
  year      = {2007},
  abstract  = {The mammalian brain is, with its numerous neural elements and structured complex connectivity, one of the most complex systems in nature. Recently, large-scale corticocortical connectivities, both structural and functional, have received a great deal of research attention, especially using the approach of complex networks. Here, we try to shed some light on the relationship between structural and functional connectivities by studying synchronization dynamics in a realistic anatomical network of cat cortical connectivity. We model the cortical areas by a subnetwork of interacting excitable neurons (multilevel model) and by a neural mass model (population model). With weak couplings, the multilevel model displays biologically plausible dynamics and the synchronization patterns reveal a hierarchical cluster organization in the network structure. We can identify a group of brain areas involved in multifunctional tasks by comparing the dynamical clusters to the topological communities of the network. With strong couplings of multilevel model and by using neural mass model, the dynamics are characterized by well-defined oscillations. The synchronization patterns are mainly determined by the node intensity (total input strengths of a node); the detailed network topology is of secondary importance. The biologically improved multilevel model exhibits similar dynamical patterns in the two regimes. Thus, the study of synchronization in a multilevel complex network model of cortex can provide insights into the relationship between network topology and functional organization of complex brain networks.},
  language  = {en}
}
@phdthesis{Kairaliyeva2023,
  author    = {Kairaliyeva, Talmira},
  title     = {Surfactant adorption at liquid interfaces measured by drop and bubble experiments},
  school      = {Universit{\"a}t Potsdam},
  pages     = {VII, 117},
  year      = {2023},
  language  = {en}
}
@phdthesis{Quade2018,
  author    = {Quade, Markus},
  title     = {Symbolic regression for identification, prediction, and control of dynamical systems},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-419790},
  school      = {Universit{\"a}t Potsdam},
  pages     = {xiii, 134},
  year      = {2018},
  abstract  = {In the present work, we use symbolic regression for automated modeling of dynamical systems. Symbolic regression is a powerful and general method suitable for data-driven identification of mathematical expressions. In particular, the structure and parameters of those expressions are identified simultaneously. We consider two main variants of symbolic regression: sparse regression-based and genetic programming-based symbolic regression. Both are applied to identification, prediction and control of dynamical systems. We introduce a new methodology for the data-driven identification of nonlinear dynamics for systems undergoing abrupt changes. Building on a sparse regression algorithm derived earlier, the model after the change is defined as a minimum update with respect to a reference model of the system identified prior to the change. The technique is successfully exemplified on the chaotic Lorenz system and the van der Pol oscillator. Issues such as computational complexity, robustness against noise and requirements with respect to data volume are investigated. We show how symbolic regression can be used for time series prediction. Again, issues such as robustness against noise and convergence rate are investigated us- ing the harmonic oscillator as a toy problem. In combination with embedding, we demonstrate the prediction of a propagating front in coupled FitzHugh-Nagumo oscillators. Additionally, we show how we can enhance numerical weather predictions to commercially forecast power production of green energy power plants. We employ symbolic regression for synchronization control in coupled van der Pol oscillators. Different coupling topologies are investigated. We address issues such as plausibility and stability of the control laws found. The toolkit has been made open source and is used in turbulence control applications. Genetic programming based symbolic regression is very versatile and can be adapted to many optimization problems. The heuristic-based algorithm allows for cost efficient optimization of complex tasks. We emphasize the ability of symbolic regression to yield white-box models. In contrast to black-box models, such models are accessible and interpretable which allows the usage of established tool chains.},
  language  = {en}
}
@phdthesis{RomanoBlasco2004,
  author    = {Romano Blasco, M. Carmen},
  title     = {Synchronization analysis by means of recurrences in phase space},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0001756},
  school      = {Universit{\"a}t Potsdam},
  year      = {2004},
  abstract  = {Die t{\"a}gliche Erfahrung zeigt uns, daß bei vielen physikalischen Systemen kleine {\"A}nderungen in den Anfangsbedingungen auch zu kleinen {\"A}nderungen im Verhalten des Systems f{\"u}hren. Wenn man z.B. das Steuerrad beim Auto fahren nur ein wenig zur Seite dreht, unterscheidet sich die Richtung des Wagens auch nur wenig von der urspr{\"u}nglichen Richtung. Aber es gibt auch Situationen, f{\"u}r die das Gegenteil dieser Regel zutrifft. Die Folge von Kopf und Zahl, die wir erhalten, wenn wir eine M{\"u}nze werfen, zeigt ein irregul{\"a}res oder chaotisches Zeitverhalten, da winzig kleine {\"A}nderungen in den Anfangsbedingungen, die z.B. durch leichte Drehung der Hand hervorgebracht werden, zu vollkommen verschiedenen Resultaten f{\"u}hren. In den letzten Jahren hat man sehr viele nichtlineare Systeme mit schnellen Rechnern untersucht und festgestellt, daß eine sensitive Abh{\"a}ngigkeit von den Anfangsbedingungen, die zu einem chaotischen Verhalten f{\"u}hrt, keinesfalls die Ausnahme darstellt, sondern eine typische Eigenschaft vieler Systeme ist. Obwohl chaotische Systeme kleinen {\"A}nderungen in den Anfangsbedingungen gegen{\"u}ber sehr empfindlich reagieren, k{\"o}nnen sie synchronisieren wenn sie durch eine gemeinsame {\"a}ußere Kraft getrieben werden, oder wenn sie miteinander gekoppelt sind. Das heißt, sie vergessen ihre Anfangsbedingungen und passen ihre Rhythmen aneinander. Diese Eigenschaft chaotischer Systeme hat viele Anwendungen, wie z.B. das Design von Kommunikationsger{\"a}te und die verschl{\"u}sselte {\"U}bertragung von Mitteilungen. Abgesehen davon, findet man Synchronisation in nat{\"u}rlichen Systemen, wie z.B. das Herz-Atmungssystem, raumverteilte {\"o}kologische Systeme, die Magnetoenzephalographische Aktivit{\"a}t von Parkinson Patienten, etc. In solchen komplexen Systemen ist es nicht trivial Synchronisation zu detektieren und zu quantifizieren. Daher ist es notwendig, besondere mathematische Methoden zu entwickeln, die diese Aufgabe erledigen. Das ist das Ziel dieser Arbeit. Basierend auf dergrundlegenden Idee von Rekurrenzen (Wiederkehr) von Trajektorien dynamischer Systeme, sind verschiedene Maße entwickelt worden, die Synchronisation in chaotischen und komplexen Systemen detektieren. Das Wiederkehr von Trajektorien erlaubt uns Vorhersagen {\"u}ber den zuk{\"u}nftigen Zustand eines Systems zu treffen. Wenn man diese Eigenschaft der Wiederkehr von zwei interagierenden Systemen vergleicht, kann man Schl{\"u}sse {\"u}ber ihre dynamische Anpassung oder Synchronisation ziehen. Ein wichtiger Vorteil der Rekurrenzmaße f{\"u}r Synchronisation ist die Robustheit gegen Rauschen und Instationari{\"a}t. Das erlaubt eine Synchronisationsanalyse in Systemen durchzuf{\"u}hren, die bisher nicht darauf untersucht werden konnten.},
  language  = {en}
}
@phdthesis{Bergner2011,
  author    = {Bergner, Andr{\´e}},
  title     = {Synchronization in complex systems with multiple time scales},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-53407},
  school      = {Universit{\"a}t Potsdam},
  year      = {2011},
  abstract  = {In the present work synchronization phenomena in complex dynamical systems exhibiting multiple time scales have been analyzed. Multiple time scales can be active in different manners. Three different systems have been analyzed with different methods from data analysis. The first system studied is a large heterogenous network of bursting neurons, that is a system with two predominant time scales, the fast firing of action potentials (spikes) and the burst of repetitive spikes followed by a quiescent phase. This system has been integrated numerically and analyzed with methods based on recurrence in phase space. An interesting result are the different transitions to synchrony found in the two distinct time scales. Moreover, an anomalous synchronization effect can be observed in the fast time scale, i.e. there is range of the coupling strength where desynchronization occurs. The second system analyzed, numerically as well as experimentally, is a pair of coupled CO₂ lasers in a chaotic bursting regime. This system is interesting due to its similarity with epidemic models. We explain the bursts by different time scales generated from unstable periodic orbits embedded in the chaotic attractor and perform a synchronization analysis of these different orbits utilizing the continuous wavelet transform. We find a diverse route to synchrony of these different observed time scales. The last system studied is a small network motif of limit cycle oscillators. Precisely, we have studied a hub motif, which serves as elementary building block for scale-free networks, a type of network found in many real world applications. These hubs are of special importance for communication and information transfer in complex networks. Here, a detailed study on the mechanism of synchronization in oscillatory networks with a broad frequency distribution has been carried out. In particular, we find a remote synchronization of nodes in the network which are not directly coupled. We also explain the responsible mechanism and its limitations and constraints. Further we derive an analytic expression for it and show that information transmission in pure phase oscillators, such as the Kuramoto type, is limited. In addition to the numerical and analytic analysis an experiment consisting of electrical circuits has been designed. The obtained results confirm the former findings.},
  language  = {en}
}
@phdthesis{MontbrioiFairen2004,
  author    = {Montbri{\´o} i Fairen, Ernest},
  title     = {Synchronization in ensembles of nonisochronous oscillators},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0001492},
  school      = {Universit{\"a}t Potsdam},
  year      = {2004},
  abstract  = {Diese Arbeit analysiert Synchronisationsphaenomene, die in grossen Ensembles von interagierenden Oszillatoren auftauchen. Im speziellen werden die Effekte von Nicht-Isochronizitaet (die Abhaengigkeit der Frequenz von der Amplitude des Oszillators) auf den makroskopischen Uebergang zur Synchronisation im Detail studiert. Die neu gefundenen Phaenomene (Anomale Synchronisation) werden sowohl in Populationen von Oszillatoren als auch zwischen Oszillator-Ensembles untersucht.},
  language  = {en}
}
@phdthesis{Vlasov2015,
  author    = {Vlasov, Vladimir},
  title     = {Synchronization of oscillatory networks in terms of global variables},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-78182},
  school      = {Universit{\"a}t Potsdam},
  pages     = {82},
  year      = {2015},
  abstract  = {Synchronization of large ensembles of oscillators is an omnipresent phenomenon observed in different fields of science like physics, engineering, life sciences, etc. The most simple setup is that of globally coupled phase oscillators, where all the oscillators contribute to a global field which acts on all oscillators. This formulation of the problem was pioneered by Winfree and Kuramoto. Such a setup gives a possibility for the analysis of these systems in terms of global variables. In this work we describe nontrivial collective dynamics in oscillator populations coupled via mean fields in terms of global variables. We consider problems which cannot be directly reduced to standard Kuramoto and Winfree models. In the first part of the thesis we adopt a method introduced by Watanabe and Strogatz. The main idea is that the system of identical oscillators of particular type can be described by a low-dimensional system of global equations. This approach enables us to perform a complete analytical analysis for a special but vast set of initial conditions. Furthermore, we show how the approach can be expanded for some nonidentical systems. We apply the Watanabe-Strogatz approach to arrays of Josephson junctions and systems of identical phase oscillators with leader-type coupling. In the next parts of the thesis we consider the self-consistent mean-field theory method that can be applied to general nonidentical globally coupled systems of oscillators both with or without noise. For considered systems a regime, where the global field rotates uniformly, is the most important one. With the help of this approach such solutions of the self-consistency equation for an arbitrary distribution of frequencies and coupling parameters can be found analytically in the parametric form, both for noise-free and noisy cases. We apply this method to deterministic Kuramoto-type model with generic coupling and an ensemble of spatially distributed oscillators with leader-type coupling. Furthermore, with the proposed self-consistent approach we fully characterize rotating wave solutions of noisy Kuramoto-type model with generic coupling and an ensemble of noisy oscillators with bi-harmonic coupling. Whenever possible, a complete analysis of global dynamics is performed and compared with direct numerical simulations of large populations.},
  language  = {en}
}
@phdthesis{Topaj2001,
  author    = {Topaj, Dmitri},
  title     = {Synchronization transitions in complex systems},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0000367},
  school      = {Universit{\"a}t Potsdam},
  year      = {2001},
  abstract  = {Gegenstand dieser Arbeit ist die Untersuchung generischer Synchronisierungsph{\"a}nomene in interagierenden komplexen Systemen. Diese Ph{\"a}nomene werden u.a. in gekoppelten deterministischen chaotischen Systemen beobachtet. Bei sehr schwachen Interaktionen zwischen individuellen Systemen kann ein {\"U}bergang zum schwach koh{\"a}renten Verhalten der Systeme stattfinden. In gekoppelten zeitkontinuierlichen chaotischen Systemen manifestiert sich dieser {\"U}bergang durch den Effekt der Phasensynchronisierung, in gekoppelten chaotischen zeitdiskreten Systemen durch den Effekt eines nichtverschwindenden makroskopischen Feldes. Der {\"U}bergang zur Koh{\"a}renz in einer Kette lokal gekoppelter Oszillatoren, beschrieben durch Phasengleichungen, wird im Bezug auf die Symmetrien des Systems untersucht. Es wird gezeigt, daß die durch die Symmetrien verursachte Reversibilit{\"a}t des Systems nichttriviale topologische Eigenschaften der Trajektorien bedingt, so daß das als dissipativ konstruierte System in einem ganzen Parameterbereich quasi-Hamiltonische Z{\"u}ge aufweist, d.h. das Phasenvolumen ist im Schnitt erhalten, und die Lyapunov-Exponenten sind paarweise symmetrisch. Der {\"U}bergang zur Koh{\"a}renz in einem Ensemble global gekoppelter chaotischer Abbildungen wird durch den Verlust der Stabilit{\"a}t des entkoppelten Zustandes beschrieben. Die entwickelte Methode besteht darin, die Selbstkonsistenz des makroskopischen Feldes aufzuheben, und das Ensemble in Analogie mit einem Verst{\"a}rkerschaltkreis mit R{\"u}ckkopplung durch eine komplexe lineare {\"U}bertragungssfunktion zu charakterisieren. Diese Theorie wird anschließend f{\"u}r einige theoretisch interessanten F{\"a}lle verallgemeinert.},
  language  = {en}
}
@phdthesis{Kucklaender2006,
  author    = {Kuckl{\"a}nder, Nina},
  title     = {Synchronization via correlated noise and automatic control in ecological systems},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-10826},
  school      = {Universit{\"a}t Potsdam},
  year      = {2006},
  abstract  = {<img src="http://vg00.met.vgwort.de/na/806c85cec18906a64e06" width="1" height="1" alt=""> Subject of this work is the possibility to synchronize nonlinear systems via correlated noise and automatic control. The thesis is divided into two parts. The first part is motivated by field studies on feral sheep populations on two islands of the St. Kilda archipelago, which revealed strong correlations due to environmental noise. For a linear system the population correlation equals the noise correlation (Moran effect). But there exists no systematic examination of the properties of nonlinear maps under the influence of correlated noise. Therefore, in the first part of this thesis the noise-induced correlation of logistic maps is systematically examined. For small noise intensities it can be shown analytically that the correlation of quadratic maps in the fixed-point regime is always smaller than or equal to the noise correlation. In the period-2 regime a Markov model explains qualitatively the main dynamical characteristics. Furthermore, two different mechanisms are introduced which lead to a higher correlation of the systems than the environmental correlation. The new effect of "correlation resonance" is described, i. e. the correlation yields a maximum depending on the noise intensity. In the second part of the thesis an automatic control method is presented which synchronizes different systems in a robust way. This method is inspired by phase-locked loops and is based on a feedback loop with a differential control scheme, which allows to change the phases of the controlled systems. The effectiveness of the approach is demonstrated for controlled phase synchronization of regular oscillators and foodweb models.},
  subject      = {Markov-Prozess},
  language  = {en}
}
@phdthesis{Kuehn2018,
  author    = {K{\"u}hn, Danilo},
  title     = {Synchrotron-based angle-resolved time-of-flight electron spectroscopy for dynamics in dichalogenides},
  school      = {Universit{\"a}t Potsdam},
  pages     = {147},
  year      = {2018},
  language  = {en}
}
@phdthesis{RamanVenkatesan2022,
  author    = {Raman Venkatesan, Thulasinath},
  title     = {Tailoring applications-relevant properties in poly(vinylidene fluoride)-based homo-, co- and ter-polymers through modification of their three-phase structure},
  doi       = {10.25932/publishup-54966},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-549667},
  school      = {Universit{\"a}t Potsdam},
  pages     = {xx, 218},
  year      = {2022},
  abstract  = {Poly(vinylidene fluoride) (PVDF)-based homo-, co- and ter-polymers are well-known for their ferroelectric and relaxor-ferroelectric properties. Their semi-crystalline morphology consists of crystalline and amorphous phases, plus interface regions in between, and governs the relevant electro-active properties. In this work, the influence of chemical, thermal and mechanical treatments on the structure and morphology of PVDF-based polymers and on the related ferroelectric/relaxor-ferroelectric properties is investigated. Polymer films were prepared in different ways and subjected to various treatments such as annealing, quenching and stretching. The resulting changes in the transitions and relaxations of the polymer samples were studied by means of dielectric, thermal, mechanical and optical techniques. In particular, the origin(s) behind the mysterious mid-temperature transition (T_{mid}) that is observed in all PVDF-based polymers was assessed. A new hypothesis is proposed to describe the T_{mid} transition as a result of multiple processes taking place within the temperature range of the transition. The contribution of the individual processes to the observed overall transition depends on both the chemical structure of the monomer units and the processing conditions which also affect the melting transition. Quenching results in a decrease of the overall crystallinity and in smaller crystallites. On samples quenched after annealing, notable differences in the fractions of different crystalline phases have been observed when compared to samples that had been slowly cooled. Stretching of poly(vinylidene fluoride-tetrafluoroethylene) (P(VDF-TFE)) films causes an increase in the fraction of the ferroelectric β-phase with simultaneous increments in the melting point (T_m) and the crystallinity (\chi_c) of the copolymer. While an increase in the stretching temperature does not have a profound effect on the amount of the ferroelectric phase, its stability appears to improve. Measurements of the non-linear dielectric permittivity \varepsilon_2^\prime in a poly(vinylidenefluoride-trifluoroethylene-chlorofluoroethylene) (P(VDF-TrFE- CFE)) relaxor-ferroelectric (R-F) terpolymer reveal peaks at 30 and 80 °C that cannot be identified in conventional dielectric spectroscopy. The former peak is associated with T_{mid}\ and may help to understand the non-zero \varepsilon_2^\prime values that are found for the paraelectric terpolymer phase. The latter peak can also be observed during cooling of P(VDF-TrFE) copolymer samples at 100 °C and is due to conduction processes and space-charge polarization as a result of the accumulation of real charges at the electrode-sample interface. Annealing lowers the Curie-transition temperature of the terpolymer as a consequence of its smaller ferroelectric-phase fraction, which by default exists even in terpolymers with relatively high CFE content. Changes in the transition temperatures are in turn related to the behavior of the hysteresis curves observed on differently heat-treated samples. Upon heating, the hysteresis curves evolve from those known for a ferroelectric to those of a typical relaxor-ferroelectric material. Comparing dielectric-hysteresis loops obtained at various temperatures, we find that annealed terpolymer films show higher electric-displacement values and lower coercive fields than the non-annealed samples - irrespective of the measurement temperature - and also exhibit ideal relaxor-ferroelectric behavior at ambient temperatures, which makes them excellent candidates for related applications at or near room temperature. However, non-annealed films - by virtue of their higher ferroelectric activity - show a larger and more stable remanent polarization at room temperature, while annealed samples need to be poled below 0 °C to induce a well-defined polarization. Overall, by modifying the three phases in PVDF-based polymers, it has been demonstrated how the preparation steps and processing conditions can be tailored to achieve the desired properties that are optimal for specific applications.},
  language  = {en}
}
@phdthesis{Jechow2009,
  author    = {Jechow, Andreas},
  title     = {Tailoring the emission of stripe-array diode lasers with external cavities to enable nonlinear frequency conversion},
  publisher = {Universit{\"a}tsverlag Potsdam},
  address   = {Potsdam},
  isbn      = {978-3-86956-031-1},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-39653},
  school      = {Universit{\"a}t Potsdam},
  pages     = {ii, 139},
  year      = {2009},
  abstract  = {A huge number of applications require coherent radiation in the visible spectral range. Since diode lasers are very compact and efficient light sources, there exists a great interest to cover these applications with diode laser emission. Despite modern band gap engineering not all wavelengths can be accessed with diode laser radiation. Especially in the visible spectral range between 480 nm and 630 nm no emission from diode lasers is available, yet. Nonlinear frequency conversion of near-infrared radiation is a common way to generate coherent emission in the visible spectral range. However, radiation with extraordinary spatial temporal and spectral quality is required to pump frequency conversion. Broad area (BA) diode lasers are reliable high power light sources in the near-infrared spectral range. They belong to the most efficient coherent light sources with electro-optical efficiencies of more than 70\%. Standard BA lasers are not suitable as pump lasers for frequency conversion because of their poor beam quality and spectral properties. For this purpose, tapered lasers and diode lasers with Bragg gratings are utilized. However, these new diode laser structures demand for additional manufacturing and assembling steps that makes their processing challenging and expensive. An alternative to BA diode lasers is the stripe-array architecture. The emitting area of a stripe-array diode laser is comparable to a BA device and the manufacturing of these arrays requires only one additional process step. Such a stripe-array consists of several narrow striped emitters realized with close proximity. Due to the overlap of the fields of neighboring emitters or the presence of leaky waves, a strong coupling between the emitters exists. As a consequence, the emission of such an array is characterized by a so called supermode. However, for the free running stripe-array mode competition between several supermodes occurs because of the lack of wavelength stabilization. This leads to power fluctuations, spectral instabilities and poor beam quality. Thus, it was necessary to study the emission properties of those stripe-arrays to find new concepts to realize an external synchronization of the emitters. The aim was to achieve stable longitudinal and transversal single mode operation with high output powers giving a brightness sufficient for efficient nonlinear frequency conversion. For this purpose a comprehensive analysis of the stripe-array devices was done here. The physical effects that are the origin of the emission characteristics were investigated theoretically and experimentally. In this context numerical models could be verified and extended. A good agreement between simulation and experiment was observed. One way to stabilize a specific supermode of an array is to operate it in an external cavity. Based on mathematical simulations and experimental work, it was possible to design novel external cavities to select a specific supermode and stabilize all emitters of the array at the same wavelength. This resulted in stable emission with 1 W output power, a narrow bandwidth in the range of 2 MHz and a very good beam quality with M²<1.5. This is a new level of brightness and brilliance compared to other BA and stripe-array diode laser systems. The emission from this external cavity diode laser (ECDL) satisfied the requirements for nonlinear frequency conversion. Furthermore, a huge improvement to existing concepts was made. In the next step newly available periodically poled crystals were used for second harmonic generation (SHG) in single pass setups. With the stripe-array ECDL as pump source, more than 140 mW of coherent radiation at 488 nm could be generated with a very high opto-optical conversion efficiency. The generated blue light had very good transversal and longitudinal properties and could be used to generate biphotons by parametric down-conversion. This was feasible because of the improvement made with the infrared stripe-array diode lasers due to the development of new physical concepts.},
  language  = {en}
}
@phdthesis{Dixit2023,
  author    = {Dixit, Sneha},
  title     = {Tension-induced conformational changes of the Piezo protein-membrane nano-dome},
  school      = {Universit{\"a}t Potsdam},
  pages     = {94},
  year      = {2023},
  abstract  = {Mechanosensation is a fundamental biological process that provides the basis for sensing touch and pain as well as for hearing and proprioception. A special class of ion-channel proteins known as mechanosensitive proteins convert the mechanical stimuli into electrochemical signals to mediate this process. Mechanosensitive proteins undergo conformational changes in response to mechanical force, which eventually leads to the opening of the proteins' ion channel. Mammalian mechanosensitive proteins remained a long sought-after mystery until 2010 when a family of two proteins - Piezo1 and Piezo2 - was identifed as mechanosensors [1]. The cryo-EM structures of Piezo1 and Piezo2 protein were resolved in the last years and reveal a propeller-shaped homotrimer with 114 transmembrane helices [2, 3, 4, 5]. The protein structures are curved and have been suggested to deform the surrounding membrane into a nano-dome, which mechanically responds to membrane tension resulting from external forces [2]. In this thesis, the conformations of membrane-embedded Piezo1 and Piezo2 proteins and their tension-induced conformational changes are investigated using molecular dynamics simulations. Our coarse-grained molecular dynamics simulations show that the Piezo proteins induce curvature in the surrounding membrane and form a stable protein-membrane nano-dome in the tensionless membrane. These membrane-embedded Piezo proteins, however, adopt substantially less curved conformations in our simulations compared to the cryo-EM structures solved in detergent micelles, which agrees with recent experimental investigations of the overall Piezo nano-dome shape in membrane vesicles [6, 7, 8]. At high membrane tension, the Piezo proteins attain nearly planar conformations in our simulations. Our systematic investigation of Piezo proteins under different membrane tensions indicates a half-maximal conformational response at membrane tension values rather close to the experimentally suggested values of Piezo activation [9, 10]. In addition, our simulations indicate a widening of the Piezo1 ion channel at high membrane tension, which agrees with the channel widening observed in recent nearly flattened cryo-EM structures of Piezo1 in small membrane vesicles [11]. In contrast, the Piezo2 ion channel does not respond to membrane tension in our simulations. These different responses of the Piezo1 and Piezo2 ion channels in our simulations are in line with patch-clamp experiments, in which Piezo1, but not Piezo2, was shown to be activated by membrane tension alone [12].},
  language  = {en}
}
@phdthesis{Raetzel2013,
  author    = {R{\"a}tzel, Dennis},
  title     = {Tensorial spacetime geometries and background-independent quantum field theory},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-65731},
  school      = {Universit{\"a}t Potsdam},
  year      = {2013},
  abstract  = {Famously, Einstein read off the geometry of spacetime from Maxwell's equations. Today, we take this geometry that serious that our fundamental theory of matter, the standard model of particle physics, is based on it. However, it seems that there is a gap in our understanding if it comes to the physics outside of the solar system. Independent surveys show that we need concepts like dark matter and dark energy to make our models fit with the observations. But these concepts do not fit in the standard model of particle physics. To overcome this problem, at least, we have to be open to matter fields with kinematics and dynamics beyond the standard model. But these matter fields might then very well correspond to different spacetime geometries. This is the basis of this thesis: it studies the underlying spacetime geometries and ventures into the quantization of those matter fields independently of any background geometry. In the first part of this thesis, conditions are identified that a general tensorial geometry must fulfill to serve as a viable spacetime structure. Kinematics of massless and massive point particles on such geometries are introduced and the physical implications are investigated. Additionally, field equations for massive matter fields are constructed like for example a modified Dirac equation. In the second part, a background independent formulation of quantum field theory, the general boundary formulation, is reviewed. The general boundary formulation is then applied to the Unruh effect as a testing ground and first attempts are made to quantize massive matter fields on tensorial spacetimes.},
  language  = {en}
}
@phdthesis{RiveraHernandez2012,
  author    = {Rivera Hern{\´a}ndez, Sergio},
  title     = {Tensorial spacetime geometries carrying predictive, interpretable and quantizable matter dynamics},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-61869},
  school      = {Universit{\"a}t Potsdam},
  year      = {2012},
  abstract  = {Which tensor fields G on a smooth manifold M can serve as a spacetime structure? In the first part of this thesis, it is found that only a severely restricted class of tensor fields can provide classical spacetime geometries, namely those that can carry predictive, interpretable and quantizable matter dynamics. The obvious dependence of this characterization of admissible tensorial spacetime geometries on specific matter is not a weakness, but rather presents an insight: it was Maxwell theory that justified Einstein to promote Lorentzian manifolds to the status of a spacetime geometry. Any matter that does not mimick the structure of Maxwell theory, will force us to choose another geometry on which the matter dynamics of interest are predictive, interpretable and quantizable. These three physical conditions on matter impose three corresponding algebraic conditions on the totally symmetric contravariant coefficient tensor field P that determines the principal symbol of the matter field equations in terms of the geometric tensor G: the tensor field P must be hyperbolic, time-orientable and energy-distinguishing. Remarkably, these physically necessary conditions on the geometry are mathematically already sufficient to realize all kinematical constructions familiar from Lorentzian geometry, for precisely the same structural reasons. This we were able to show employing a subtle interplay of convex analysis, the theory of partial differential equations and real algebraic geometry. In the second part of this thesis, we then explore general properties of any hyperbolic, time-orientable and energy-distinguishing tensorial geometry. Physically most important are the construction of freely falling non-rotating laboratories, the appearance of admissible modified dispersion relations to particular observers, and the identification of a mechanism that explains why massive particles that are faster than some massless particles can radiate off energy until they are slower than all massless particles in any hyperbolic, time-orientable and energy-distinguishing geometry. In the third part of the thesis, we explore how tensorial spacetime geometries fare when one wants to quantize particles and fields on them. This study is motivated, in part, in order to provide the tools to calculate the rate at which superluminal particles radiate off energy to become infraluminal, as explained above. Remarkably, it is again the three geometric conditions of hyperbolicity, time-orientability and energy-distinguishability that allow the quantization of general linear electrodynamics on an area metric spacetime and the quantization of massive point particles obeying any admissible dispersion relation. We explore the issue of field equations of all possible derivative order in rather systematic fashion, and prove a practically most useful theorem that determines Dirac algebras allowing the reduction of derivative orders. The final part of the thesis presents the sketch of a truly remarkable result that was obtained building on the work of the present thesis. Particularly based on the subtle duality maps between momenta and velocities in general tensorial spacetimes, it could be shown that gravitational dynamics for hyperbolic, time-orientable and energy distinguishable geometries need not be postulated, but the formidable physical problem of their construction can be reduced to a mere mathematical task: the solution of a system of homogeneous linear partial differential equations. This far-reaching physical result on modified gravity theories is a direct, but difficult to derive, outcome of the findings in the present thesis. Throughout the thesis, the abstract theory is illustrated through instructive examples.},
  language  = {en}
}
@phdthesis{Pick2020,
  author    = {Pick, Leonie Johanna Lisa},
  title     = {The centennial evolution of geomagnetic activity and its driving mechanisms},
  doi       = {10.25932/publishup-47275},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-472754},
  school      = {Universit{\"a}t Potsdam},
  pages     = {ix, 135},
  year      = {2020},
  abstract  = {This cumulative thesis is concerned with the evolution of geomagnetic activity since the beginning of the 20th century, that is, the time-dependent response of the geomagnetic field to solar forcing. The focus lies on the description of the magnetospheric response field at ground level, which is particularly sensitive to the ring current system, and an interpretation of its variability in terms of the solar wind driving. Thereby, this work contributes to a comprehensive understanding of long-term solar-terrestrial interactions. The common basis of the presented publications is formed by a reanalysis of vector magnetic field measurements from geomagnetic observatories located at low and middle geomagnetic latitudes. In the first two studies, new ring current targeting geomagnetic activity indices are derived, the Annual and Hourly Magnetospheric Currents indices (A/HMC). Compared to existing indices (e.g., the Dst index), they do not only extend the covered period by at least three solar cycles but also constitute a qualitative improvement concerning the absolute index level and the ~11-year solar cycle variability. The analysis of A/HMC shows that (a) the annual geomagnetic activity experiences an interval-dependent trend with an overall linear decline during 1900-2010 of ~5 \% (b) the average trend-free activity level amounts to ~28 nT (c) the solar cycle related variability shows amplitudes of ~15-45 nT (d) the activity level for geomagnetically quiet conditions (Kp<2) lies slightly below 20 nT. The plausibility of the last three points is ensured by comparison to independent estimations either based on magnetic field measurements from LEO satellite missions (since the 1990s) or the modeling of geomagnetic activity from solar wind input (since the 1960s). An independent validation of the longterm trend is problematic mainly because the sensitivity of the locally measured geomagnetic activity depends on geomagnetic latitude. Consequently, A/HMC is neither directly comparable to global geomagnetic activity indices (e.g., the aa index) nor to the partly reconstructed open solar magnetic flux, which requires a homogeneous response of the ground-based measurements to the interplanetary magnetic field and the solar wind speed. The last study combines a consistent, HMC-based identification of geomagnetic storms from 1930-2015 with an analysis of the corresponding spatial (magnetic local time-dependent) disturbance patterns. Amongst others, the disturbances at dawn and dusk, particularly their evolution during the storm recovery phases, are shown to be indicative of the solar wind driving structure (Interplanetary Coronal Mass Ejections vs. Stream or Co-rotating Interaction Regions), which enables a backward-prediction of the storm driver classes. The results indicate that ICME-driven geomagnetic storms have decreased since 1930 which is consistent with the concurrent decrease of HMC. Out of the collection of compiled follow-up studies the inclusion of measurements from high-latitude geomagnetic observatories into the third study's framework seems most promising at this point.},
  language  = {en}
}
@phdthesis{Mergenthaler2009,
  author    = {Mergenthaler, Konstantin K.},
  title     = {The control of fixational eye movements},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-29397},
  school      = {Universit{\"a}t Potsdam},
  year      = {2009},
  abstract  = {In normal everyday viewing, we perform large eye movements (saccades) and miniature or fixational eye movements. Most of our visual perception occurs while we are fixating. However, our eyes are perpetually in motion. Properties of these fixational eye movements, which are partly controlled by the brainstem, change depending on the task and the visual conditions. Currently, fixational eye movements are poorly understood because they serve the two contradictory functions of gaze stabilization and counteraction of retinal fatigue. In this dissertation, we investigate the spatial and temporal properties of time series of eye position acquired from participants staring at a tiny fixation dot or at a completely dark screen (with the instruction to fixate a remembered stimulus); these time series were acquired with high spatial and temporal resolution. First, we suggest an advanced algorithm to separate the slow phases (named drift) and fast phases (named microsaccades) of these movements, which are considered to play different roles in perception. On the basis of this identification, we investigate and compare the temporal scaling properties of the complete time series and those time series where the microsaccades are removed. For the time series obtained during fixations on a stimulus, we were able to show that they deviate from Brownian motion. On short time scales, eye movements are governed by persistent behavior and on a longer time scales, by anti-persistent behavior. The crossover point between these two regimes remains unchanged by the removal of microsaccades but is different in the horizontal and the vertical components of the eyes. Other analyses target the properties of the microsaccades, e.g., the rate and amplitude distributions, and we investigate, whether microsaccades are triggered dynamically, as a result of earlier events in the drift, or completely randomly. The results obtained from using a simple box-count measure contradict the hypothesis of a purely random generation of microsaccades (Poisson process). Second, we set up a model for the slow part of the fixational eye movements. The model is based on a delayed random walk approach within the velocity related equation, which allows us to use the data to determine control loop durations; these durations appear to be different for the vertical and horizontal components of the eye movements. The model is also motivated by the known physiological representation of saccade generation; the difference between horizontal and vertical components concurs with the spatially separated representation of saccade generating regions. Furthermore, the control loop durations in the model suggest an external feedback loop for the horizontal but not for the vertical component, which is consistent with the fact that an internal feedback loop in the neurophysiology has only been identified for the vertical component. Finally, we confirmed the scaling properties of the model by semi-analytical calculations. In conclusion, we were able to identify several properties of the different parts of fixational eye movements and propose a model approach that is in accordance with the described neurophysiology and described limitations of fixational eye movement control.},
  language  = {en}
}
@phdthesis{Khosravi2023,
  author    = {Khosravi, Sara},
  title     = {The effect of new turbulence parameterizations for the stable surface layer on simulations of the Arctic climate},
  doi       = {10.25932/publishup-64352},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-643520},
  school      = {Universit{\"a}t Potsdam},
  pages     = {XIV, 119},
  year      = {2023},
  abstract  = {Arctic climate change is marked by intensified warming compared to global trends and a significant reduction in Arctic sea ice which can intricately influence mid-latitude atmospheric circulation through tropo- and stratospheric pathways. Achieving accurate simulations of current and future climate demands a realistic representation of Arctic climate processes in numerical climate models, which remains challenging. Model deficiencies in replicating observed Arctic climate processes often arise due to inadequacies in representing turbulent boundary layer interactions that determine the interactions between the atmosphere, sea ice, and ocean. Many current climate models rely on parameterizations developed for mid-latitude conditions to handle Arctic turbulent boundary layer processes. This thesis focuses on modified representation of the Arctic atmospheric processes and understanding their resulting impact on large-scale mid-latitude atmospheric circulation within climate models. The improved turbulence parameterizations, recently developed based on Arctic measurements, were implemented in the global atmospheric circulation model ECHAM6. This involved modifying the stability functions over sea ice and ocean for stable stratification and changing the roughness length over sea ice for all stratification conditions. Comprehensive analyses are conducted to assess the impacts of these modifications on ECHAM6's simulations of the Arctic boundary layer, overall atmospheric circulation, and the dynamical pathways between the Arctic and mid-latitudes. Through a step-wise implementation of the mentioned parameterizations into ECHAM6, a series of sensitivity experiments revealed that the combined impacts of the reduced roughness length and the modified stability functions are non-linear. Nevertheless, it is evident that both modifications consistently lead to a general decrease in the heat transfer coefficient, being in close agreement with the observations. Additionally, compared to the reference observations, the ECHAM6 model falls short in accurately representing unstable and strongly stable conditions. The less frequent occurrence of strong stability restricts the influence of the modified stability functions by reducing the affected sample size. However, when focusing solely on the specific instances of a strongly stable atmosphere, the sensible heat flux approaches near-zero values, which is in line with the observations. Models employing commonly used surface turbulence parameterizations were shown to have difficulties replicating the near-zero sensible heat flux in strongly stable stratification. I also found that these limited changes in surface layer turbulence parameterizations have a statistically significant impact on the temperature and wind patterns across multiple pressure levels, including the stratosphere, in both the Arctic and mid-latitudes. These significant signals vary in strength, extent, and direction depending on the specific month or year, indicating a strong reliance on the background state. Furthermore, this research investigates how the modified surface turbulence parameterizations may influence the response of both stratospheric and tropospheric circulation to Arctic sea ice loss. The most suitable parameterizations for accurately representing Arctic boundary layer turbulence were identified from the sensitivity experiments. Subsequently, the model's response to sea ice loss is evaluated through extended ECHAM6 simulations with different prescribed sea ice conditions. The simulation with adjusted surface turbulence parameterizations better reproduced the observed Arctic tropospheric warming in vertical extent, demonstrating improved alignment with the reanalysis data. Additionally, unlike the control experiments, this simulation successfully reproduced specific circulation patterns linked to the stratospheric pathway for Arctic-mid-latitude linkages. Specifically, an increased occurrence of the Scandinavian-Ural blocking regime (negative phase of the North Atlantic Oscillation) in early (late) winter is observed. Overall, it can be inferred that improving turbulence parameterizations at the surface layer can improve the ECHAM6's response to sea ice loss.},
  language  = {en}
}
@phdthesis{Heinig2003,
  author    = {Heinig, Peter},
  title     = {The geometry of interacting liquid domains in Langmuir monolayers},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-0000814},
  school      = {Universit{\"a}t Potsdam},
  year      = {2003},
  abstract  = {Es werden die Strukturbildung und Benetzung zweidimensionaler (2D) Phasen von Langmuir-Monolagen im lokalen thermodynamischen Gleichgewicht untersucht. Eine Langmuir-Monolage ist ein isoliertes 2D System von Surfaktanten an der Wasser/Luft-Grenzfl{\"a}che, in dem kristalline, fl{\"u}ssigkristalline, fl{\"u}ssige oder gasf{\"o}rmige Phasen auftreten, die sich in Positionsordnung und/oder Orientierungsordnung unterscheiden. Permanente elektrische Dipolmomente der Surfaktanten f{\"u}hren zu einer langreichweitigen repulsiven Selbstwechselwirkung der Monolage und zur Bildung mesoskopischer Strukturen. Es wird ein Wechselwirkungsmodell verwendet, das die Strukturbildung als Wechselspiel kurzreichweitiger Anziehung (nackte Linienspannung) und langreichweitiger Abstoßung (Oberfl{\"a}chenpotential) auf einer Skala Delta beschreibt. Physikalisch trennt Delta die beiden L{\"a}ngenskalen der lang- und kurzreichweitigen Wechselwirkung. In dieser Arbeit werden die thermodynamischen Stabilit{\"a}tsbedingungen f{\"u}r die Form einer Phasengrenzlinie (Young-Laplace-Gleichung) und Dreiphasenkontaktpunkt (Young-Bedingung) hergeleitet und zur Beschreibung experimenteller Daten genutzt: Die Linienspannung benetzender 2D Tropfen wird mit Hilfe h{\"a}ngender-Tropfen-Tensiometrie gemessen. Die Blasenform und -gr{\"o}ße von 2D Sch{\"a}umen wird theoretisch modelliert und mit experimentellen 2D Sch{\"a}umen verglichen. Kontaktwinkel werden durch die Anpassung von experimentellen Tropfen mit numerischen L{\"o}sungen der Young-Laplace-Gleichung auf Mikrometerskalen gemessen. Das Skalenverhalten des Kontaktwinkels erm{\"o}glicht die Bestimmung einer unteren Schranke von Delta. Weiterhin wird diskutiert, inwieweit das Schalten von 2D Benetzungsmodi in biologischen Membranen zur Steuerung der Reaktionskinetik ein Rolle spielen k{\"o}nnte. Hierzu werden Experimente aus unserer Gruppe, die in einer Langmuir-Monolage durchgef{\"u}hrt wurden, herangezogen. Abschließend wird die scheinbare Verletzung der Gibbs\&prime;schen Phasenregel in Langmuir-Monolagen (nicht-horizontales Plateau der Oberfl{\"a}chendruck-Fl{\"a}che Isotherme, ausgedehntes Dreiphasengebiet in Einkomponentensystemen) quantitativ untersucht. Eine Verschmutzung der verwendeten Substanzen ist demnach die wahscheinlichste Erkl{\"a}rung, w{\"a}hrend Finite-Size-Effekte oder der Einfluss der langreichweitigen Elektrostatik die Gr{\"o}ßenordnung des Effektes nicht beschreiben k{\"o}nnen.},
  language  = {en}
}
@phdthesis{Yin2009,
  author    = {Yin, Chunhong},
  title     = {The interplay of nanostructure and efficiency of polymer solar cells},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus-29054},
  school      = {Universit{\"a}t Potsdam},
  year      = {2009},
  abstract  = {The aim of this thesis is to achieve a deep understanding of the working mechanism of polymer based solar cells and to improve the device performance. Two types of the polymer based solar cells are studied here: all-polymer solar cells comprising macromolecular donors and acceptors based on poly(p-phenylene vinylene) and hybrid cells comprising a PPV copolymer in combination with a novel small molecule electron acceptor. To understand the interplay between morphology and photovoltaic properties in all-polymer devices, I compared the photocurrent characteristics and excited state properties of bilayer and blend devices with different nano-morphology, which was fine tuned by using solvents with different boiling points. The main conclusion from these complementary measurements was that the performance-limiting step is the field-dependent generation of free charge carriers, while bimolecular recombination and charge extraction do not compromise device performance. These findings imply that the proper design of the donor-acceptor heterojunction is of major importance towards the goal of high photovoltaic efficiencies. Regarding polymer-small molecular hybrid solar cells I combined the hole-transporting polymer M3EH-PPV with a novel Vinazene-based electron acceptor. This molecule can be either deposited from solution or by thermal evaporation, allowing for a large variety of layer architectures to be realized. I then demonstrated that the layer architecture has a large influence on the photovoltaic properties. Solar cells with very high fill factors of up to 57 \% and an open circuit voltage of 1V could be achieved by realizing a sharp and well-defined donor-acceptor heterojunction. In the past, fill factors exceeding 50 \% have only been observed for polymers in combination with soluble fullerene-derivatives or nanocrystalline inorganic semiconductors as the electron-accepting component. The finding that proper processing of polymer-vinazene devices leads to similar high values is a major step towards the design of efficient polymer-based solar cells.},
  language  = {en}
}
@phdthesis{Ruppert2016,
  author    = {Ruppert, Jan},
  title     = {The Low-Mass Young Stellar Content in the Extended Environment of the Galactic Starburst Region NGC3603},
  school      = {Universit{\"a}t Potsdam},
  pages     = {148},
  year      = {2016},
  language  = {en}
}
@phdthesis{Schlemm2022,
  author    = {Schlemm, Tanja},
  title     = {The marine ice cliff instability of the Antarctic ice sheet},
  doi       = {10.25932/publishup-58633},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-586333},
  school      = {Universit{\"a}t Potsdam},
  pages     = {107},
  year      = {2022},
  abstract  = {The Antarctic ice sheet is the largest freshwater reservoir worldwide. If it were to melt completely, global sea levels would rise by about 58 m. Calculation of projections of the Antarctic contribution to sea level rise under global warming conditions is an ongoing effort which yields large ranges in predictions. Among the reasons for this are uncertainties related to the physics of ice sheet modeling. These uncertainties include two processes that could lead to runaway ice retreat: the Marine Ice Sheet Instability (MISI), which causes rapid grounding line retreat on retrograde bedrock, and the Marine Ice Cliff Instability (MICI), in which tall ice cliffs become unstable and calve off, exposing even taller ice cliffs. In my thesis, I investigated both marine instabilities (MISI and MICI) using the Parallel Ice Sheet Model (PISM), with a focus on MICI.},
  language  = {en}
}
@phdthesis{DeAndradeQueiroz2023,
  author    = {De Andrade Queiroz, Anna Barbara},
  title     = {The Milky Way disks, bulge, and bar sub-populations},
  doi       = {10.25932/publishup-59061},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-590615},
  school      = {Universit{\"a}t Potsdam},
  pages     = {xii, 187},
  year      = {2023},
  abstract  = {In recent decades, astronomy has seen a boom in large-scale stellar surveys of the Galaxy. The detailed information obtained about millions of individual stars in the Milky Way is bringing us a step closer to answering one of the most outstanding questions in astrophysics: how do galaxies form and evolve? The Milky Way is the only galaxy where we can dissect many stars into their high-dimensional chemical composition and complete phase space, which analogously as fossil records can unveil the past history of the genesis of the Galaxy. The processes that lead to large structure formation, such as the Milky Way, are critical for constraining cosmological models; we call this line of study Galactic archaeology or near-field cosmology. At the core of this work, we present a collection of efforts to chemically and dynamically characterise the disks and bulge of our Galaxy. The results we present in this thesis have only been possible thanks to the advent of the Gaia astrometric satellite, which has revolutionised the field of Galactic archaeology by precisely measuring the positions, parallax distances and motions of more than a billion stars. Another, though not less important, breakthrough is the APOGEE survey, which has observed spectra in the near-infrared peering into the dusty regions of the Galaxy, allowing us to determine detailed chemical abundance patterns in hundreds of thousands of stars. To accurately depict the Milky Way structure, we use and develop the Bayesian isochrone fitting tool/code called StarHorse; this software can predict stellar distances, extinctions and ages by combining astrometry, photometry and spectroscopy based on stellar evolutionary models. The StarHorse code is pivotal to calculating distances where Gaia parallaxes alone cannot allow accurate estimates. We show that by combining Gaia, APOGEE, photometric surveys and using StarHorse, we can produce a chemical cartography of the Milky way disks from their outermost to innermost parts. Such a map is unprecedented in the inner Galaxy. It reveals a continuity of the bimodal chemical pattern previously detected in the solar neighbourhood, indicating two populations with distinct formation histories. Furthermore, the data reveals a chemical gradient within the thin disk where the content of 𝛼-process elements and metals is higher towards the centre. Focusing on a sample in the inner MW we confirm the extension of the chemical duality to the innermost regions of the Galaxy. We find stars with bar shape orbits to show both high- and low-𝛼 abundances, suggesting the bar formed by secular evolution trapping stars that already existed. By analysing the chemical orbital space of the inner Galactic regions, we disentangle the multiple populations that inhabit this complex region. We reveal the presence of the thin disk, thick disk, bar, and a counter-rotating population, which resembles the outcome of a perturbed proto-Galactic disk. Our study also finds that the inner Galaxy holds a high quantity of super metal-rich stars up to three times solar suggesting it is a possible repository of old super-metal-rich stars found in the solar neighbourhood. We also enter into the complicated task of deriving individual stellar ages. With StarHorse, we calculate the ages of main-sequence turn-off and sub-giant stars for several public spectroscopic surveys. We validate our results by investigating linear relations between chemical abundances and time since the 𝛼 and neutron capture elements are sensitive to age as a reflection of the different enrichment timescales of these elements. For further study of the disks in the solar neighbourhood, we use an unsupervised machine learning algorithm to delineate a multidimensional separation of chrono-chemical stellar groups revealing the chemical thick disk, the thin disk, and young 𝛼-rich stars. The thick disk is shown to have a small age dispersion indicating its fast formation contrary to the thin disk that spans a wide range of ages. With groundbreaking data, this thesis encloses a detailed chemo-dynamical view of the disk and bulge of our Galaxy. Our findings on the Milky Way can be linked to the evolution of high redshift disk galaxies, helping to solve the conundrum of galaxy formation.},
  language  = {en}
}
@phdthesis{Sauter2016,
  author    = {Sauter, J{\"o}rg},
  title     = {The molecular origin of plant cell wall swelling},
  school      = {Universit{\"a}t Potsdam},
  pages     = {iii, 127 S.},
  year      = {2016},
  abstract  = {In dieser Arbeit werden die Eigenschaften von hydratisierten Hemicellulose Polysacchariden mittels Computersimulation untersucht. Die hohe Quellfähigkeit von Materialien die aus diesen Molek{\"u}len bestehen, erlaubt die Erzeugung von zielgerichteter Bewegung in Planzenmaterialien, ausschließlich gesteuert durch Wasseraufnahme. Um den molekularen Ursprung dieses Quellvermögens zu untersuchen wird, im Vergleich mit Experimenten, ein atomistisches Modell f{\"u}r Hemicellulose Polysaccharide entwickelt und getestet. Unter Verwendung dieses Modells werden Simulationen von kleinen Polysacchariden benutzt um die Wechselwirkungen mit Wasser, den Einfluss von Wasser auf die Konformationsfreiheit der Molek{\"u}le, und die Quellfähigkeit, quantifiziert durch den osmotischen Druck, zu verstehen. Es wird gezeigt, dass verzweigte und lineare Polysaccharide unterschiedliche Hydratisierungseingenschaften im Vergleich zu lineare Polysacchariden aufweisen. Um das Quellverhalten auf Längen- und Zeitskalen untersuchen zu können die {\"u}ber die Begrenzungen atomistischer Simulationen hinausgehen, wurde eine Prozedur entwickelt um {\"u}bertragbare vergröberte Modelle herzuleiten. Die Übertragbarkeit der vegröberten Modelle wird gezeigt, sowohl {\"u}ber unterschiedliche Polysaccharidkonzentrationen als auch {\"u}ber unterschiedliche Polymerlängen. Daher erlaubt die Prozedur die Konstruktion von großen vergröberter Systemen ausgehend von kleinen atomistischen Referenzsystemen. Abschließend wird das vergröberte Modell verwendet um zu zeigen, dass lineare und verzweigte Polysaccharide ein unterschiedliches Quellverhalten aufweisen, wenn sie mit einem Wasserbad gekoppelt werden.},
  language  = {en}
}
@phdthesis{Haase2019,
  author    = {Haase, Nadin},
  title     = {The nascent peptide chain in the ribosomal exit tunnel},
  school      = {Universit{\"a}t Potsdam},
  pages     = {105},
  year      = {2019},
  language  = {en}
}
@phdthesis{Breuer2016,
  author    = {Breuer, David},
  title     = {The plant cytoskeleton as a transportation network},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-93583},
  school      = {Universit{\"a}t Potsdam},
  pages     = {164},
  year      = {2016},
  abstract  = {The cytoskeleton is an essential component of living cells. It is composed of different types of protein filaments that form complex, dynamically rearranging, and interconnected networks. The cytoskeleton serves a multitude of cellular functions which further depend on the cell context. In animal cells, the cytoskeleton prominently shapes the cell's mechanical properties and movement. In plant cells, in contrast, the presence of a rigid cell wall as well as their larger sizes highlight the role of the cytoskeleton in long-distance intracellular transport. As it provides the basis for cell growth and biomass production, cytoskeletal transport in plant cells is of direct environmental and economical relevance. However, while knowledge about the molecular details of the cytoskeletal transport is growing rapidly, the organizational principles that shape these processes on a whole-cell level remain elusive. This thesis is devoted to the following question: How does the complex architecture of the plant cytoskeleton relate to its transport functionality? The answer requires a systems level perspective of plant cytoskeletal structure and transport. To this end, I combined state-of-the-art confocal microscopy, quantitative digital image analysis, and mathematically powerful, intuitively accessible graph-theoretical approaches. This thesis summarizes five of my publications that shed light on the plant cytoskeleton as a transportation network: (1) I developed network-based frameworks for accurate, automated quantification of cytoskeletal structures, applicable in, e.g., genetic or chemical screens; (2) I showed that the actin cytoskeleton displays properties of efficient transport networks, hinting at its biological design principles; (3) Using multi-objective optimization, I demonstrated that different plant cell types sustain cytoskeletal networks with cell-type specific and near-optimal organization; (4) By investigating actual transport of organelles through the cell, I showed that properties of the actin cytoskeleton are predictive of organelle flow and provided quantitative evidence for a coordination of transport at a cellular level; (5) I devised a robust, optimization-based method to identify individual cytoskeletal filaments from a given network representation, allowing the investigation of single filament properties in the network context. The developed methods were made publicly available as open-source software tools. Altogether, my findings and proposed frameworks provide quantitative, system-level insights into intracellular transport in living cells. Despite my focus on the plant cytoskeleton, the established combination of experimental and theoretical approaches is readily applicable to different organisms. Despite the necessity of detailed molecular studies, only a complementary, systemic perspective, as presented here, enables both understanding of cytoskeletal function in its evolutionary context as well as its future technological control and utilization.},
  language  = {en}
}
@phdthesis{Sposini2020,
  author    = {Sposini, Vittoria},
  title     = {The random diffusivity approach for diffusion in heterogeneous systems},
  doi       = {10.25932/publishup-48780},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-487808},
  school      = {Universit{\"a}t Potsdam},
  year      = {2020},
  abstract  = {The two hallmark features of Brownian motion are the linear growth < x2(t)> = 2Ddt of the mean squared displacement (MSD) with diffusion coefficient D in d spatial dimensions, and the Gaussian distribution of displacements. With the increasing complexity of the studied systems deviations from these two central properties have been unveiled over the years. Recently, a large variety of systems have been reported in which the MSD exhibits the linear growth in time of Brownian (Fickian) transport, however, the distribution of displacements is pronouncedly non-Gaussian (Brownian yet non-Gaussian, BNG). A similar behaviour is also observed for viscoelastic-type motion where an anomalous trend of the MSD, i.e., <x2(t)> ~ ta, is combined with a priori unexpected non-Gaussian distributions (anomalous yet non-Gaussian, ANG). This kind of behaviour observed in BNG and ANG diffusions has been related to the presence of heterogeneities in the systems and a common approach has been established to address it, that is, the random diffusivity approach. This dissertation explores extensively the field of random diffusivity models. Starting from a chronological description of all the main approaches used as an attempt of describing BNG and ANG diffusion, different mathematical methodologies are defined for the resolution and study of these models. The processes that are reported in this work can be classified in three subcategories, i) randomly-scaled Gaussian processes, ii) superstatistical models and iii) diffusing diffusivity models, all belonging to the more general class of random diffusivity models. Eventually, the study focuses more on BNG diffusion, which is by now well-established and relatively well-understood. Nevertheless, many examples are discussed for the description of ANG diffusion, in order to highlight the possible scenarios which are known so far for the study of this class of processes. The second part of the dissertation deals with the statistical analysis of random diffusivity processes. A general description based on the concept of moment-generating function is initially provided to obtain standard statistical properties of the models. Then, the discussion moves to the study of the power spectral analysis and the first passage statistics for some particular random diffusivity models. A comparison between the results coming from the random diffusivity approach and the ones for standard Brownian motion is discussed. In this way, a deeper physical understanding of the systems described by random diffusivity models is also outlined. To conclude, a discussion based on the possible origins of the heterogeneity is sketched, with the main goal of inferring which kind of systems can actually be described by the random diffusivity approach.},
  language  = {en}
}