@misc{SchwarzlGodecMetzler2017,
  author    = {Schwarzl, Maria and Godec, Aljaž and Metzler, Ralf},
  title     = {Quantifying non-ergodicity of anomalous diffusion with higher order moments},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-402109},
  pages     = {18},
  year      = {2017},
  abstract  = {Anomalous diffusion is being discovered in a fast growing number of systems. The exact nature of this anomalous diffusion provides important information on the physical laws governing the studied system. One of the central properties analysed for finite particle motion time series is the intrinsic variability of the apparent diffusivity, typically quantified by the ergodicity breaking parameter EB. Here we demonstrate that frequently EB is insufficient to provide a meaningful measure for the observed variability of the data. Instead, important additional information is provided by the higher order moments entering by the skewness and kurtosis. We analyse these quantities for three popular anomalous diffusion models. In particular, we find that even for the Gaussian fractional Brownian motion a significant skewness in the results of physical measurements occurs and needs to be taken into account. Interestingly, the kurtosis and skewness may also provide sensitive estimates of the anomalous diffusion exponent underlying the data. We also derive a new result for the EB parameter of fractional Brownian motion valid for the whole range of the anomalous diffusion parameter. Our results are important for the analysis of anomalous diffusion but also provide new insights into the theory of anomalous stochastic processes.},
  language  = {en}
}
@misc{HempelKoseskaNikoloskietal.2017,
  author    = {Hempel, Sabrina and Koseska, Aneta and Nikoloski, Zoran and Kurths, J{\"u}rgen},
  title     = {Unraveling gene regulatory networks from time-resolved gene expression data},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-400924},
  pages     = {26},
  year      = {2017},
  abstract  = {Background: Inferring regulatory interactions between genes from transcriptomics time-resolved data, yielding reverse engineered gene regulatory networks, is of paramount importance to systems biology and bioinformatics studies. Accurate methods to address this problem can ultimately provide a deeper insight into the complexity, behavior, and functions of the underlying biological systems. However, the large number of interacting genes coupled with short and often noisy time-resolved read-outs of the system renders the reverse engineering a challenging task. Therefore, the development and assessment of methods which are computationally efficient, robust against noise, applicable to short time series data, and preferably capable of reconstructing the directionality of the regulatory interactions remains a pressing research problem with valuable applications. Results: Here we perform the largest systematic analysis of a set of similarity measures and scoring schemes within the scope of the relevance network approach which are commonly used for gene regulatory network reconstruction from time series data. In addition, we define and analyze several novel measures and schemes which are particularly suitable for short transcriptomics time series. We also compare the considered 21 measures and 6 scoring schemes according to their ability to correctly reconstruct such networks from short time series data by calculating summary statistics based on the corresponding specificity and sensitivity. Our results demonstrate that rank and symbol based measures have the highest performance in inferring regulatory interactions. In addition, the proposed scoring scheme by asymmetric weighting has shown to be valuable in reducing the number of false positive interactions. On the other hand, Granger causality as well as information-theoretic measures, frequently used in inference of regulatory networks, show low performance on the short time series analyzed in this study. Conclusions: Our study is intended to serve as a guide for choosing a particular combination of similarity measures and scoring schemes suitable for reconstruction of gene regulatory networks from short time series data. We show that further improvement of algorithms for reverse engineering can be obtained if one considers measures that are rooted in the study of symbolic dynamics or ranks, in contrast to the application of common similarity measures which do not consider the temporal character of the employed data. Moreover, we establish that the asymmetric weighting scoring scheme together with symbol based measures (for low noise level) and rank based measures (for high noise level) are the most suitable choices.},
  language  = {en}
}
@misc{LoebnerJelkenYadavallietal.2017,
  author    = {Loebner, Sarah and Jelken, Joachim and Yadavalli, Nataraja Sekhar and Sava, Elena and Hurduc, Nicolae and Santer, Svetlana},
  title     = {Motion of adsorbed nano-particles on azobenzene containing polymer films},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-400423},
  pages     = {11},
  year      = {2017},
  abstract  = {We demonstrate in situ recorded motion of nano-objects adsorbed on a photosensitive polymer film. The motion is induced by a mass transport of the underlying photoresponsive polymer material occurring during irradiation with interference pattern. The polymer film contains azobenzene molecules that undergo reversible photoisomerization reaction from trans- to cis-conformation. Through a multi-scale chain of physico-chemical processes, this finally results in the macro-deformations of the film due to the changing elastic properties of polymer. The topographical deformation of the polymer surface is sensitive to a local distribution of the electrical field vector that allows for the generation of dynamic changes in the surface topography during irradiation with different light interference patterns. Polymer film deformation together with the motion of the adsorbed nano-particles are recorded using a homemade set-up combining an optical part for the generation of interference patterns and an atomic force microscope for acquiring the surface deformation. The particles undergo either translational or rotational motion. The direction of particle motion is towards the topography minima and opposite to the mass transport within the polymer film. The ability to relocate particles by photo-induced dynamic topography fluctuation offers a way for a non-contact simultaneous manipulation of a large number of adsorbed particles just in air at ambient conditions.},
  language  = {en}
}
@phdthesis{Guber2017,
  author    = {Guber, Christoph Rudolf},
  title     = {Dust depletion of Ca and Ti in quasar absorption-line systems},
  school      = {Universit{\"a}t Potsdam},
  pages     = {178},
  year      = {2017},
  language  = {en}
}
@phdthesis{KianiAlibagheri2017,
  author    = {Kiani Alibagheri, Bahareh},
  title     = {On structural properties of magnetosome chains},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-398849},
  school      = {Universit{\"a}t Potsdam},
  pages     = {XIV, 117},
  year      = {2017},
  abstract  = {Magnetotaktische Bakterien besitzen eine intrazellul{\"a}re Struktur, die Magnetosomenkette genannt wird. Magnetosomenketten enthalten Nanopartikel von Eisenkristallen, die von einer Membran umschlossen und entlang eines Zytoskelettfilaments ausgerichtet sind. Dank der Magnetosomenkette ist es magnetotaktischen Bakterien m{\"o}glich sich in Magnetfeldern auszurichten und entlang magnetischer Feldlinien zu schwimmen. Die ausf{\"u}hrliche Untersuchung der strukturellen Eigenschaften der Magnetosomenkette in magnetotaktischen Bakterien sind von grundlegendem wissenschaftlichen Interesse, weil sie Einblicke in die Anordnung des Zytoskeletts von Bakterien erlauben. In dieser Studie haben wir ein neues theoretisches Modell entwickelt, dass sich dazu eignet, die strukturellen Eigenschaften der Magnetosomenketten in magnetotaktischen Bakterien zu erforschen. Zuerst wenden wir uns der Biegesteifigkeit von Magnetosomenketten zu, die von zwei Faktoren beeinflusst wird: Die magnetische Wechselwirkung der Magnetosomenpartikel und der Biegesteifigkeit des Zytoskelettfilaments auf welchem die Magnetosome verankert sind. Unsere Analyse zeigt, dass sich die lineare Konfiguration von Magnetosomenpartikeln ohne die Stabilisierung durch das Zytoskelett zu einer ring{\"o}rmigen Struktur biegen w{\"u}rde, die kein magnetisches Moment aufweist und daher nicht die Funktion eines Kompass in der zellul{\"a}ren Navigation einnehmen k{\"o}nnte. Wir schlussfolgern, dass das Zytoskelettfilament eine stabilisierende Wirkung auf die lineare Konfiguration hat und eine ringf{\"o}rmige Anordnung verhindert. Wir untersuchen weiter die Gleichgewichtskonfiguration der Magnetosomenpartikel in einer linearen Kette und in einer geschlossenen ringf{\"o}rmigen Struktur. Dabei beobachteten wir ebenfalls, dass f{\"u}r eine stabile lineare Anordnung eine Bindung an ein Zytoskelettfilament notwendig ist. In einem externen magnetischen Feld wird die Stabilit{\"a}t der Magnetosomenketten durch die Dipol-Dipol-Wechselwirkung, {\"u}ber die Steifheit und die Bindungsenergie der Proteinstruktur, die die Partikel des Magnetosomen mit dem Filament verbinden, erreicht. Durch Beobachtungen w{\"a}hrend und nach der Behandlung einer Magnetosomenkette mit einem externen magnetischen Feld, l{\"a}sst sich begr{\"u}nden, dass die Stabilisierung von Magnetosomenketten durch Zytoskelettfilamente {\"u}ber proteinhaltige Bindeglieder und die dynamischen Eigenschaften dieser Strukturen realisiert wird. Abschließend wenden wir unser Modell bei der Untersuchung von ferromagnetischen Resonanz-Spektren von Magnetosomenketten in einzelnen Zellen von magnetotaktischen Bakterien an. Wir erforschen den Effekt der magnetokristallinen Anistropie in ihrer dreifach-Symmetrie, die in ferromagnetischen Ressonanz Spektren beobachtet wurden und die Besonderheit von verschiedenen Spektren, die bei Mutanten dieser Bakterien auftreten.},
  language  = {en}
}
@article{CherstvyVinodAghionetal.2017,
  author    = {Cherstvy, Andrey G. and Vinod, Deepak and Aghion, Erez and Chechkin, Aleksei V. and Metzler, Ralf},
  title     = {Time averaging, ageing and delay analysis of financial time series},
  series = {New journal of physics},
  volume    = {19},
  journal   = {New journal of physics},
  publisher = {IOP},
  address   = {London},
  issn      = {1367-2630},
  doi       = {10.1088/1367-2630/aa7199},
  pages     = {1 -- 11},
  year      = {2017},
  abstract  = {We introduce three strategies for the analysis of financial time series based on time averaged observables. These comprise the time averaged mean squared displacement (MSD) as well as the ageing and delay time methods for varying fractions of the financial time series. We explore these concepts via statistical analysis of historic time series for several Dow Jones Industrial indices for the period from the 1960s to 2015. Remarkably, we discover a simple universal law for the delay time averaged MSD. The observed features of the financial time series dynamics agree well with our analytical results for the time averaged measurables for geometric Brownian motion, underlying the famed Black-Scholes-Merton model. The concepts we promote here are shown to be useful for financial data analysis and enable one to unveil new universal features of stock market dynamics.},
  language  = {en}
}
@misc{CherstvyVinodAghionetal.2017,
  author    = {Cherstvy, Andrey G. and Vinod, Deepak and Aghion, Erez and Chechkin, Aleksei V. and Metzler, Ralf},
  title     = {Time averaging, ageing and delay analysis of financial time series},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-400541},
  pages     = {11},
  year      = {2017},
  abstract  = {We introduce three strategies for the analysis of financial time series based on time averaged observables. These comprise the time averaged mean squared displacement (MSD) as well as the ageing and delay time methods for varying fractions of the financial time series. We explore these concepts via statistical analysis of historic time series for several Dow Jones Industrial indices for the period from the 1960s to 2015. Remarkably, we discover a simple universal law for the delay time averaged MSD. The observed features of the financial time series dynamics agree well with our analytical results for the time averaged measurables for geometric Brownian motion, underlying the famed Black-Scholes-Merton model. The concepts we promote here are shown to be useful for financial data analysis and enable one to unveil new universal features of stock market dynamics.},
  language  = {en}
}
@phdthesis{Qiu2017,
  author    = {Qiu, Xunlin},
  title     = {Ferroelectrets: heterogenous polymer electrets with high electromechanical response},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-398425},
  school      = {Universit{\"a}t Potsdam},
  pages     = {viii, 172},
  year      = {2017},
  abstract  = {Ferroelectrets are internally charged polymer foams or cavity-containing polymer-_lm systems that combine large piezoelectricity with mechanical flexibility and elastic compliance. The term "ferroelectret" was coined based on the fact that it is a space-charge electret that also shows ferroic behavior. In this thesis, comprehensive work on ferroelectrets, and in particular on their preparation, their charging, their piezoelectricity and their applications is reported. For industrial applications, ferroelectrets with well-controlled distributions or even uniform values of cavity size and cavity shape and with good thermal stability of the piezoelectricity are very desirable. Several types of such ferroelectrets are developed using techniques such as straightforward thermal lamination, sandwiching sticky templates with electret films, and screen printing. In particular, uoroethylenepropylene (FEP) _lm systems with tubular-channel openings, prepared by means of the thermal lamination technique, show piezoelectric d33 coefficients of up to 160 pC/N after charging through dielectric barrier discharges (DBDs) . For samples charged at suitable elevated temperatures, the piezoelectricity is stable at temperatures of at least 130°C. These preparation methods are easy to implement at laboratory or industrial scales, and are quite flexible in terms of material selection and cavity geometry design. Due to the uniform and well-controlled cavity structures, samples are also very suitable for fundamental studies on ferroelectrets. Charging of ferroelectrets is achieved via a series of dielectric barrier discharges (DBDs) inside the cavities. In the present work, the DBD charging process is comprehensively studied by means of optical, electrical and electro-acoustic methods. The spectrum of the transient light from the DBDs in cellular polypropylene (PP) ferroelectrets directly confirms the ionization of molecular nitrogen, and allows the determination of the electric field in the discharge. Detection of the light emission reveals not only DBDs under high applied voltage but also back discharges when the applied voltage is reduced to sufficiently low values. Back discharges are triggered by the internally deposited charges, as the breakdown inside the cavities is controlled by the sum of the applied electric field and the electric field of the deposited charges. The remanent effective polarization is determined by the breakdown strength of the gas-filled cavities. These findings form the basis of more efficient charging techniques for ferroelectrets such as charging with high-pressure air, thermal poling and charging assisted by gas exchange. With the proposed charging strategies, the charging efficiency of ferroelectrets can be enhanced significantly. After charging, the cavities can be considered as man-made macroscopic dipoles whose direction can be reversed by switching the polarity of the applied voltage. Polarization-versus-electric-field (P(E)) hysteresis loops in ferroelectrets are observed by means of an electro-acoustic method combined with dielectric resonance spectroscopy. P(E) hysteresis loops in ferrroelectrets are also obtained by more direct measurements using a modified Sawyer-Tower circuit. Hysteresis loops prove the ferroic behavior of ferroelectrets. However, repeated switching of the macroscopic dipoles involves complex physico-chemical processes. The DBD charging process generates a cold plasma with numerous active species and thus modifies the inner polymer surfaces of the cavities. Such treatments strongly affect the chargeability of the cavities. At least for cellular PP ferroelectrets, repeated DBDs in atmospheric conditions lead to considerable fatigue of the effective polarization and of the resulting piezoelectricity. The macroscopic dipoles in ferroelectrets are highly compressible, and hence the piezoelectricity is essentially the primary effect. It is found that the piezoelectric d33 coefficient is proportional to the polarization and the elastic compliance of the sample, providing hints for developing materials with higher piezoelectric sensitivity in the future. Due to their outstanding electromechanical properties, there has been constant interest in the application of ferroelectrets. The antiresonance frequencies (fp) of ferroelectrets are sensitive to the boundary conditions during measurement. A tubular-channel FEP ferroelectret is conformably attached to a self-organized minimum-energy dielectric elastomer actuator (DEA). It turns out that the antiresonance frequency (fp) of the ferroelectret film changes noticeably with the bending angle of the DEA. Therefore, the actuation of DEAs can be used to modulate the fp value of ferroelectrets, but fp can also be exploited for in-situ diagnosis and for precise control of the actuation of the DEA. Combination of DEAs and ferroelectrets opens up various new possibilities for application.},
  language  = {en}
}
@phdthesis{Roland2017,
  author    = {Roland, Steffen},
  title     = {Charge carrier recombination and open circuit voltage in organic solar cells},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-397721},
  school      = {Universit{\"a}t Potsdam},
  pages     = {VI, 145},
  year      = {2017},
  abstract  = {Tremendous progress in the development of thin film solar cell techniques has been made over the last decade. The field of organic solar cells is constantly developing, new material classes like Perowskite solar cells are emerging and different types of hybrid organic/inorganic material combinations are being investigated for their physical properties and their applicability in thin film electronics. Besides typical single-junction architectures for solar cells, multi-junction concepts are also being investigated as they enable the overcoming of theoretical limitations of a single-junction. In multi-junction devices each sub-cell operates in different wavelength regimes and should exhibit optimized band-gap energies. It is exactly this tunability of the band-gap energy that renders organic solar cell materials interesting candidates for multi-junction applications. Nevertheless, only few attempts have been made to combine inorganic and organic solar cells in series connected multi-junction architectures. Even though a great diversity of organic solar cells exists nowadays, their open circuit voltage is usually low compared to the band-gap of the active layer. Hence, organic low band-gap solar cells in particular show low open circuit voltages and the key factors that determine the voltage losses are not yet fully understood. Besides open circuit voltage losses the recombination of charges in organic solar cells is also a prevailing research topic, especially with respect to the influence of trap states. The exploratory focus of this work is therefore set, on the one hand, on the development of hybrid organic/inorganic multi-junctions and, on the other hand, on gaining a deeper understanding of the open circuit voltage and the recombination processes of organic solar cells. In the first part of this thesis, the development of a hybrid organic/inorganic triple-junction will be discussed which showed at that time (Jan. 2015) a record power conversion efficiency of 11.7\%. The inorganic sub-cells of these devices consist of hydrogenated amorphous silicon and were delivered by the Competence Center Thin-Film and Nanotechnology for Photovoltaics in Berlin. Different recombination contacts and organic sub-cells were tested in conjunction with these inorganic sub-cells on the basis of optical modeling predictions for the optimal layer thicknesses to finally reach record efficiencies for this type of solar cells. In the second part, organic model systems will be investigated to gain a better understanding of the fundamental loss mechanisms that limit the open circuit voltage of organic solar cells. First, bilayer systems with different orientation of the donor and acceptor molecules were investigated to study the influence of the donor/acceptor orientation on non-radiative voltage loss. Secondly, three different bulk heterojunction solar cells all comprising the same amount of fluorination and the same polymer backbone in the donor component were examined to study the influence of long range electrostatics on the open circuit voltage. Thirdly, the device performance of two bulk heterojunction solar cells was compared which consisted of the same donor polymer but used different fullerene acceptor molecules. By this means, the influence of changing the energetics of the acceptor component on the open circuit voltage was investigated and a full analysis of the charge carrier dynamics was presented to unravel the reasons for the worse performance of the solar cell with the higher open circuit voltage. In the third part, a new recombination model for organic solar cells will be introduced and its applicability shown for a typical low band-gap cell. This model sheds new light on the recombination process in organic solar cells in a broader context as it re-evaluates the recombination pathway of charge carriers in devices which show the presence of trap states. Thereby it addresses a current research topic and helps to resolve alleged discrepancies which can arise from the interpretation of data derived by different measurement techniques.},
  language  = {en}
}
@phdthesis{Hakansson2017,
  author    = {H{\aa}kansson, Nils},
  title     = {A Dark Matter line search using 3D-modeling of Cherenkov showers below 10 TeV with VERITAS},
  url       = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-397670},
  school      = {Universit{\"a}t Potsdam},
  pages     = {107, xxxvi},
  year      = {2017},
  abstract  = {Dark matter, DM, has not yet been directly observed, but it has a very solid theoretical basis. There are observations that provide indirect evidence, like galactic rotation curves that show that the galaxies are rotating too fast to keep their constituent parts, and galaxy clusters that bends the light coming from behind-lying galaxies more than expected with respect to the mass that can be calculated from what can be visibly seen. These observations, among many others, can be explained with theories that include DM. The missing piece is to detect something that can exclusively be explained by DM. Direct observation in a particle accelerator is one way and indirect detection using telescopes is another. This thesis is focused on the latter method. The Very Energetic Radiation Imaging Telescope Array System, V ERITAS, is a telescope array that detects Cherenkov radiation. Theory predicts that DM particles annihilate into, e.g., a γγ pair and create a distinctive energy spectrum when detected by such telescopes, e.i., a monoenergetic line at the same energy as the particle mass. This so called "smoking-gun" signature is sought with a sliding window line search within the sub-range ∼ 0.3 - 10 TeV of the VERITAS energy range, ∼ 0.01 - 30 TeV. Standard analysis within the VERITAS collaboration uses Hillas analysis and look-up tables, acquired by analysing particle simulations, to calculate the energy of the particle causing the Cherenkov shower. In this thesis, an improved analysis method has been used. Modelling each shower as a 3Dgaussian should increase the energy recreation quality. Five dwarf spheroidal galaxies were chosen as targets with a total of ∼ 224 hours of data. The targets were analysed individually and stacked. Particle simulations were based on two simulation packages, CARE and GrISU. Improvements have been made to the energy resolution and bias correction, up to a few percent each, in comparison to standard analysis. Nevertheless, no line with a relevant significance has been detected. The most promising line is at an energy of ∼ 422 GeV with an upper limit cross section of 8.10 · 10^-24 cm^3 s^-1 and a significance of ∼ 2.73 σ, before trials correction and ∼ 1.56 σ after. Upper limit cross sections have also been calculated for the γγ annihilation process and four other outcomes. The limits are in line with current limits using other methods, from ∼ 8.56 · 10^-26 - 6.61 · 10^-23 cm^3s^-1. Future larger telescope arrays, like the upcoming Cherenkov Telescope Array, CTA, will provide better results with the help of this analysis method.},
  language  = {en}
}