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This thesis is focussed on the electronic properties of the new material class named topological insulators. Spin and angle resolved photoelectron spectroscopy have been applied to reveal several unique properties of the surface state of these materials. The first part of this thesis introduces the methodical background of these quite established experimental techniques.
In the following chapter, the theoretical concept of topological insulators is introduced. Starting from the prominent example of the quantum Hall effect, the application of topological invariants to classify material systems is illuminated. It is explained how, in presence of time reversal symmetry, which is broken in the quantum Hall phase, strong spin orbit coupling can drive a system into a topologically non trivial phase. The prediction of the spin quantum Hall effect in two dimensional insulators an the generalization to the three dimensional case of topological insulators is reviewed together with the first experimental realization of a three dimensional topological insulator in the Bi1-xSbx alloys given in the literature.
The experimental part starts with the introduction of the Bi2X3 (X=Se, Te) family of materials. Recent theoretical predictions and experimental findings on the bulk and surface electronic structure of these materials are introduced in close discussion to our own experimental results. Furthermore, it is revealed, that the topological surface state of Bi2Te3 shares its orbital symmetry with the bulk valence band and the observation of a temperature induced shift of the chemical potential is to a high probability unmasked as a doping effect due to residual gas adsorption.
The surface state of Bi2Te3 is found to be highly spin polarized with a polarization value of about 70% in a macroscopic area, while in Bi2Se3 the polarization appears reduced, not exceeding 50%. We, however, argue that the polarization is most likely only extrinsically limited in terms of the finite angular resolution and the lacking detectability of the out of plane component of the electron spin. A further argument is based on the reduced surface quality of the single crystals after cleavage and, for Bi2Se3 a sensitivity of the electronic structure to photon exposure.
We probe the robustness of the topological surface state in Bi2X3 against surface impurities in Chapter 5. This robustness is provided through the protection by the time reversal symmetry. Silver, deposited on the (111) surface of Bi2Se3 leads to a strong electron doping but the surface state is observed up to a deposited Ag mass equivalent to one atomic monolayer. The opposite sign of doping, i.e., hole-like, is observed by exposing oxygen to Bi2Te3. But while the n-type shift of Ag on Bi2Se3 appears to be more or less rigid, O2 is lifting the Dirac point of the topological surface state in Bi2Te3 out of the valence band minimum at $\Gamma$. After increasing the oxygen dose further, it is possible to shift the Dirac point to the Fermi level, while the valence band stays well beyond. The effect is found reversible, by warming up the samples which is interpreted in terms of physisorption of O2.
For magnetic impurities, i.e., Fe, we find a similar behavior as for the case of Ag in both Bi2Se3 and Bi2Te3. However, in that case the robustness is unexpected, since magnetic impurities are capable to break time reversal symmetry which should introduce a gap in the surface state at the Dirac point which in turn removes the protection. We argue, that the fact that the surface state shows no gap must be attributed to a missing magnetization of the Fe overlayer. In Bi2Te3 we are able to observe the surface state for deposited iron mass equivalents in the monolayer regime. Furthermore, we gain control over the sign of doping through the sample temperature during deposition.
Chapter6 is devoted to the lifetime broadening of the photoemission signal from the topological surface states of Bi2Se3 and Bi2Te3. It is revealed that the hexagonal warping of the surface state in Bi2Te3 introduces an anisotropy for electrons traveling along the two distinct high symmetry directions of the surface Brillouin zone, i.e., $\Gamma$K and $\Gamma$M. We show that the phonon coupling strength to the surface electrons in Bi2Te3 is in nice agreement with the theoretical prediction but, nevertheless, higher than one may expect. We argue that the electron-phonon coupling is one of the main contributions to the decay of photoholes but the relatively small size of the Fermi surface limits the number of phonon modes that may scatter off electrons. This effect is manifested in the energy dependence of the imaginary part of the electron self energy of the surface state which shows a decay to higher binding energies in contrast to the monotonic increase proportional to E$^2$ in the Fermi liquid theory due to electron-electron interaction.
Furthermore, the effect of the surface impurities of Chapter 5 on the quasiparticle life- times is investigated. We find that Fe impurities have a much stronger influence on the lifetimes as compared to Ag. Moreover, we find that the influence is stronger independently of the sign of the doping. We argue that this observation suggests a minor contribution of the warping on increased scattering rates in contrast to current belief. This is additionally confirmed by the observation that the scattering rates increase further with increasing silver amount while the doping stays constant and by the fact that clean Bi2Se3 and Bi2Te3 show very similar scattering rates regardless of the much stronger warping in Bi2Te3.
In the last chapter we report on a strong circular dichroism in the angle distribution of the photoemission signal of the surface state of Bi2Te3. We show that the color pattern obtained by calculating the difference between photoemission intensities measured with opposite photon helicity reflects the pattern expected for the spin polarization. However, we find a strong influence on strength and even sign of the effect when varying the photon energy. The sign change is qualitatively confirmed by means of one-step photoemission calculations conducted by our collaborators from the LMU München, while the calculated spin polarization is found to be independent of the excitation energy. Experiment and theory together unambiguously uncover the dichroism in these systems as a final state effect and the question in the title of the chapter has to be negated: Circular dichroism in the angle distribution is not a new spin sensitive technique.
This publications-based thesis summarizes my contribution to the scientific field of ultrafast structural dynamics. It consists of 16 publications, about the generation, detection and coupling of coherent gigahertz longitudinal acoustic phonons, also called hypersonic waves. To generate such high frequency phonons, femtosecond near infrared laser pulses were used to heat nanostructures composed of perovskite oxides on an ultrashort timescale. As a consequence the heated regions of such a nanostructure expand and a high frequency acoustic phonon pulse is generated. To detect such coherent acoustic sound pulses I use ultrafast variants of optical Brillouin and x-ray scattering. Here an incident optical or x-ray photon is scattered by the excited sound wave in the sample. The scattered light intensity measures the occupation of the phonon modes.
The central part of this work is the investigation of coherent high amplitude phonon wave packets which can behave nonlinearly, quite similar to shallow water waves which show a steepening of wave fronts or solitons well known as tsunamis. Due to the high amplitude of the acoustic wave packets in the solid, the acoustic properties can change significantly in the vicinity of the sound pulse. This may lead to a shape change of the pulse. I have observed by time-resolved Brillouin scattering, that a single cycle hypersound pulse shows a wavefront steepening. I excited hypersound pulses with strain amplitudes until 1% which I have calibrated by ultrafast x-ray diffraction (UXRD).
On the basis of this first experiment we developed the idea of the nonlinear mixing of narrowband phonon wave packets which we call "nonlinear phononics" in analogy with the nonlinear optics, which summarizes a kaleidoscope of surprising optical phenomena showing up at very high electric fields. Such phenomena are for instance Second Harmonic Generation, four-wave-mixing or solitons. But in case of excited coherent phonons the wave packets have usually very broad spectra which make it nearly impossible to look at elementary scattering processes between phonons with certain momentum and energy.
For that purpose I tested different techniques to excite narrowband phonon wave packets which mainly consist of phonons with a certain momentum and frequency. To this end epitaxially grown metal films on a dielectric substrate were excited with a train of laser pulses. These excitation pulses drive the metal film to oscillate with the frequency given by their inverse temporal displacement and send a hypersonic wave of this frequency into the substrate. The monochromaticity of these wave packets was proven by ultrafast optical Brillouin and x-ray scattering.
Using the excitation of such narrowband phonon wave packets I was able to observe the Second Harmonic Generation (SHG) of coherent phonons as a first example of nonlinear wave mixing of nanometric phonon wave packets.
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.
Changes in extratropical storm track activity and their implications for extreme weather events
(2016)
In this thesis, the two prototype catalysts Fe(CO)₅ and Cr(CO)₆ are investigated with time-resolved photoelectron spectroscopy at a high harmonic setup. In both of these metal carbonyls, a UV photon can induce the dissociation of one or more ligands of the complex. The mechanism of the dissociation has been debated over the last decades. The electronic dynamics of the first dissociation occur on the femtosecond timescale.
For the experiment, an existing high harmonic setup was moved to a new location, was extended, and characterized. The modified setup can induce dynamics in gas phase samples with photon energies of 1.55eV, 3.10eV, and 4.65eV. The valence electronic structure of the samples can be probed with photon energies between 20eV and 40eV. The temporal resolution is 111fs to 262fs, depending on the combination of the two photon energies.
The electronically excited intermediates of the two complexes, as well as of the reaction product Fe(CO)₄, could be observed with photoelectron spectroscopy in the gas phase for the first time. However, photoelectron spectroscopy gives access only to the final ionic states. Corresponding calculations to simulate these spectra are still in development. The peak energies and their evolution in time with respect to the initiation pump pulse have been determined, these peaks have been assigned based on literature data. The spectra of the two complexes show clear differences. The dynamics have been interpreted with the assumption that the motion of peaks in the spectra relates to the movement of the wave packet in the multidimensional energy landscape. The results largely confirm existing models for the reaction pathways. In both metal carbonyls, this pathway involves a direct excitation of the wave packet to a metal-to-ligand charge transfer state and the subsequent crossing to a dissociative ligand field state. The coupling of the electronic dynamics to the nuclear dynamics could explain the slower dissociation in Fe(CO)₅ as compared to Cr(CO)₆.
The cell interior is a highly packed environment in which biological macromolecules evolve and function. This crowded media has effects in many biological processes such as protein-protein binding, gene regulation, and protein folding. Thus, biochemical reactions that take place in such crowded conditions differ from diluted test tube conditions, and a considerable effort has been invested in order to understand such differences.
In this work, we combine different computationally tools to disentangle the effects of molecular crowding on biochemical processes. First, we propose a lattice model to study the implications of molecular crowding on enzymatic reactions. We provide a detailed picture of how crowding affects binding and unbinding events and how the separate effects of crowding on binding equilibrium act together. Then, we implement a lattice model to study the effects of molecular crowding on facilitated diffusion. We find that obstacles on the DNA impair facilitated diffusion. However, the extent of this effect depends on how dynamic obstacles are on the DNA. For the scenario in which crowders are only present in the bulk solution, we find that at some conditions presence of crowding agents can enhance specific-DNA binding. Finally, we make use of structure-based techniques to look at the impact of the presence of crowders on the folding a protein. We find that polymeric crowders have stronger effects on protein stability than spherical crowders. The strength of this effect increases as the polymeric crowders become longer. The methods we propose here are general and can also be applied to more complicated systems.
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.
The high-latitudinal thermospheric processes driven by the solar wind and Interplanetary Magnetic Field (IMF) interaction with the Earth magnetosphere are highly variable parts of the complex dynamic plasma environment, which represent the coupled Magnetosphere – Ionosphere – Thermosphere (MIT) system. The solar wind and IMF interactions transfer energy to the MIT system via reconnection processes at the magnetopause. The Field Aligned Currents (FACs) constitute the energetic links between the magnetosphere and the Earth ionosphere. The MIT system depends on the highly variable solar wind conditions, in particular on changes of the strength and orientation of the IMF.
In my thesis, I perform an investigation on the physical background of the complex MIT system using the global physical - numerical, three-dimensional, time-dependent and self-consistent Upper Atmosphere Model (UAM). This model describes the thermosphere, ionosphere, plasmasphere and inner magnetosphere as well as the electrodynamics of the coupled MIT system for the altitudinal range from 80 (60) km up to the 15 Earth radii.
In the present study, I developed and investigated several variants of the high-latitudinal electrodynamic coupling by including the IMF dependence of FACs into the UAM model. For testing, the various variants were applied to simulations of the coupled MIT system for different seasons, geomagnetic activities, various solar wind and IMF conditions. Additionally, these variants of the theoretical model with the IMF dependence were compared with global empirical models. The modelling results for the most important thermospheric parameters like neutral wind and mass density were compared with satellite measurements. The variants of the UAM model with IMF dependence show a good agreement with the satellite observations. In comparison with the empirical models, the improved variants of the UAM model reproduce a more realistic meso-scale structures and dynamics of the coupled MIT system than the empirical models, in particular at high latitudes. The new configurations of the UAM model with IMF dependence contribute to the improvement of space weather prediction.
Organic bulk heterojunction (BHJ) solar cells based on polymer:fullerene blends are a promising alternative for a low-cost solar energy conversion. Despite significant improvements of the power conversion efficiency in recent years, the fundamental working principles of these devices are yet not fully understood. In general, the current output of organic solar cells is determined by the generation of free charge carriers upon light absorption and their transport to the electrodes in competition to the loss of charge carriers due to recombination.
The object of this thesis is to provide a comprehensive understanding of the dynamic processes and physical parameters determining the performance. A new approach for analyzing the characteristic current-voltage output was developed comprising the experimental determination of the efficiencies of charge carrier generation, recombination and transport, combined with numerical device simulations.
Central issues at the beginning of this work were the influence of an electric field on the free carrier generation process and the contribution of generation, recombination and transport to the current-voltage characteristics. An elegant way to directly measure the field dependence of the free carrier generation is the Time Delayed Collection Field (TDCF) method. In TDCF charge carriers are generated by a short laser pulse and subsequently extracted by a defined rectangular voltage pulse. A new setup was established with an improved time resolution compared to former reports in literature. It was found that charge generation is in general independent of the electric field, in contrast to the current view in literature and opposed to the expectations of the Braun-Onsager model that was commonly used to describe the charge generation process. Even in cases where the charge generation was found to be field-dependend, numerical modelling showed that this field-dependence is in general not capable to account for the voltage dependence of the photocurrent. This highlights the importance of efficient charge extraction in competition to non-geminate recombination, which is the second objective of the thesis.
Therefore, two different techniques were combined to characterize the dynamics and efficiency of non-geminate recombination under device-relevant conditions. One new approach is to perform TDCF measurements with increasing delay between generation and extraction of charges. Thus, TDCF was used for the first time to measure charge carrier generation, recombination and transport with the same experimental setup. This excludes experimental errors due to different measurement and preparation conditions and demonstrates the strength of this technique. An analytic model for the description of TDCF transients was developed and revealed the experimental conditions for which reliable results can be obtained. In particular, it turned out that the $RC$ time of the setup which is mainly given by the sample geometry has a significant influence on the shape of the transients which has to be considered for correct data analysis.
Secondly, a complementary method was applied to characterize charge carrier recombination under steady state bias and illumination, i.e. under realistic operating conditions. This approach relies on the precise determination of the steady state carrier densities established in the active layer. It turned out that current techniques were not sufficient to measure carrier densities with the necessary accuracy. Therefore, a new technique {Bias Assisted Charge Extraction} (BACE) was developed. Here, the charge carriers photogenerated under steady state illumination are extracted by applying a high reverse bias. The accelerated extraction compared to conventional charge extraction minimizes losses through non-geminate recombination and trapping during extraction. By performing numerical device simulations under steady state, conditions were established under which quantitative information on the dynamics can be retrieved from BACE measurements.
The applied experimental techniques allowed to sensitively analyse and quantify geminate and non-geminate recombination losses along with charge transport in organic solar cells. A full analysis was exemplarily demonstrated for two prominent polymer-fullerene blends.
The model system P3HT:PCBM spincast from chloroform (as prepared) exhibits poor power conversion efficiencies (PCE) on the order of 0.5%, mainly caused by low fill factors (FF) and currents. It could be shown that the performance of these devices is limited by the hole transport and large bimolecular recombination (BMR) losses, while geminate recombination losses are insignificant. The low polymer crystallinity and poor interconnection between the polymer and fullerene domains leads to a hole mobility of the order of 10^-7 cm^2/Vs which is several orders of magnitude lower than the electron mobility in these devices. The concomitant build up of space charge hinders extraction of both electrons and holes and promotes bimolecular recombination losses.
Thermal annealing of P3HT:PCBM blends directly after spin coating improves crystallinity and interconnection of the polymer and the fullerene phase and results in comparatively high electron and hole mobilities in the order of 10^-3 cm^2/Vs and 10^-4 cm^2/Vs, respectively. In addition, a coarsening of the domain sizes leads to a reduction of the BMR by one order of magnitude. High charge carrier mobilities and low recombination losses result in comparatively high FF (>65%) and short circuit current (J_SC ≈ 10 mA/cm^2). The overall device performance (PCE ≈ 4%) is only limited by a rather low spectral overlap of absorption and solar emission and a small V_OC, given by the energetics of the P3HT.
From this point of view the combination of the low bandgap polymer PTB7 with PCBM is a promising approach. In BHJ solar cells, this polymer leads to a higher V_OC due to optimized energetics with PCBM. However, the J_SC in these (unoptimized) devices is similar to the J_SC in the optimized blend with P3HT and the FF is rather low (≈ 50%). It turned out that the unoptimized PTB7:PCBM blends suffer from high BMR, a low electron mobility of the order of 10^-5 cm^2/Vs and geminate recombination losses due to field dependent charge carrier generation.
The use of the solvent additive DIO optimizes the blend morphology, mainly by suppressing the formation of very large fullerene domains and by forming a more uniform structure of well interconnected donor and acceptor domains of the order of a few nanometers. Our analysis shows that this results in an increase of the electron mobility by about one order of magnitude (3 x 10^-4 cm^2/Vs), while BMR and geminate recombination losses are significantly reduced. In total these effects improve the J_SC (≈ 17 mA/cm^2) and the FF (> 70%). In 2012 this polymer/fullerene combination resulted in a record PCE for a single junction OSC of 9.2%.
Remarkably, the numerical device simulations revealed that the specific shape of the J-V characteristics depends very sensitively to the variation of not only one, but all dynamic parameters. On the one hand this proves that the experimentally determined parameters, if leading to a good match between simulated and measured J-V curves, are realistic and reliable. On the other hand it also emphasizes the importance to consider all involved dynamic quantities, namely charge carrier generation, geminate and non-geminate recombination as well as electron and hole mobilities. The measurement or investigation of only a subset of these parameters as frequently found in literature will lead to an incomplete picture and possibly to misleading conclusions.
Importantly, the comparison of the numerical device simulation employing the measured parameters and the experimental $J-V$ characteristics allows to identify loss channels and limitations of OSC. For example, it turned out that inefficient extraction of charge carriers is a criticical limitation factor that is often disobeyed. However, efficient and fast transport of charges becomes more and more important with the development of new low bandgap materials with very high internal quantum efficiencies. Likewise, due to moderate charge carrier mobilities, the active layer thicknesses of current high-performance devices are usually limited to around 100 nm. However, larger layer thicknesses would be more favourable with respect to higher current output and robustness of production. Newly designed donor materials should therefore at best show a high tendency to form crystalline structures, as observed in P3HT, combined with the optimized energetics and quantum efficiency of, for example, PTB7.
The non-linear behaviour of the atmospheric dynamics is not well understood and makes the evaluation and usage of regional climate models (RCMs) difficult. Due to these non-linearities, chaos and internal variability (IV) within the RCMs are induced, leading to a sensitivity of RCMs to their initial conditions (IC). The IV is the ability of RCMs to realise different solutions of simulations that differ in their IC, but have the same lower and lateral boundary conditions (LBC), hence can be defined as the across-member spread between the ensemble members.
For the investigation of the IV and the dynamical and diabatic contributions generating the IV four ensembles of RCM simulations are performed with the atmospheric regional model HIRHAM5. The integration area is the Arctic and each ensemble consists of 20 members. The ensembles cover the time period from July to September for the years 2006, 2007, 2009 and 2012. The ensemble members have the same LBC and differ in their IC only. The different IC are arranged by an initialisation time that shifts successively by six hours. Within each ensemble the first simulation starts on 1st July at 00 UTC and the last simulation starts on 5th July at 18 UTC and each simulation runs until 30th September. The analysed time period ranges from 6th July to 30th September, the time period that is covered by all ensemble members. The model runs without any nudging to allow a free development of each simulation to get the full internal variability within the HIRHAM5.
As a measure of the model generated IV, the across-member standard deviation and the across-member variance is used and the dynamical and diabatic processes influencing the IV are estimated by applying a diagnostic budget study for the IV tendency of the potential temperature developed by Nikiema and Laprise [2010] and Nikiema and Laprise [2011]. The diagnostic budget study is based on the first law of thermodynamics for potential temperature and the mass-continuity equation. The resulting budget equation reveals seven contributions to the potential temperature IV tendency.
As a first study, this work analyses the IV within the HIRHAM5. Therefore, atmospheric circulation parameters and the potential temperature for all four ensemble years are investigated. Similar to previous studies, the IV fluctuates strongly in time. Further, due to the fact that all ensemble members are forced with the same LBC, the IV depends on the vertical level within the troposphere, with high values in the lower troposphere and at 500 hPa and low values in the upper troposphere and at the surface. By the same reason, the spatial distribution shows low values of IV at the boundaries of the model domain.
The diagnostic budget study for the IV tendency of potential temperature reveals that the seven contributions fluctuate in time like the IV. However, the individual terms reach different absolute magnitudes. The budget study identifies the horizontal and vertical ‘baroclinic’ terms as the main contributors to the IV tendency, with the horizontal ‘baroclinic’ term producing and the vertical ‘baroclinic’ term reducing the IV. The other terms fluctuate around zero, because they are small in general or are balanced due to the domain average.
The comparison of the results obtained for the four different ensembles (summers 2006, 2007, 2009 and 2012) reveals that on average the findings for each ensemble are quite similar concerning the magnitude and the general pattern of IV and its contributions. However, near the surface a weaker IV is produced with decreasing sea ice extent. This is caused by a smaller impact of the horizontal 'baroclinic' term over some regions and by the changing diabatic processes, particularly a more intense reducing tendency of the IV due to condensative heating. However, it has to be emphasised that the behaviour of the IV and its dynamical and diabatic contributions are influenced mainly by complex atmospheric feedbacks and large-scale processes and not by the sea ice distribution.
Additionally, a comparison with a second RCM covering the Arctic and using the same LBCs and IC is performed. For both models very similar results concerning the IV and its dynamical and diabatic contributions are found. Hence, this investigation leads to the conclusion that the IV is a natural phenomenon and is independent from the applied RCM.
The main goal of this cumulative thesis is the derivation of surface emissivity data in the infrared from radiance measurements of Venus. Since these data are diagnostic of the chemical composition and grain size of the surface material, they can help to improve knowledge of the planet’s geology. Spectrally resolved images of nightside emissions in the range 1.0-5.1 μm were recently acquired by the InfraRed Mapping channel of the Visible and InfraRed Thermal Imaging Spectrometer (VIRTIS-M-IR) aboard ESA’s Venus EXpress (VEX). Surface and deep atmospheric thermal emissions in this spectral range are strongly obscured by the extremely opaque atmosphere, but three narrow spectral windows at 1.02, 1.10, and 1.18 μm allow the sounding of the surface. Additional windows between 1.3 and 2.6 μm provide information on atmospheric parameters that is required to interpret the surface signals. Quantitative data on surface and atmosphere can be retrieved from the measured spectra by comparing them to simulated spectra. A numerical radiative transfer model is used in this work to simulate the observable radiation as a function of atmospheric, surface, and instrumental parameters. It is a line-by-line model taking into account thermal emissions by surface and atmosphere as well as absorption and multiple scattering by gases and clouds. The VIRTIS-M-IR measurements are first preprocessed to obtain an optimal data basis for the subsequent steps. In this process, a detailed detector responsivity analysis enables the optimization of the data consistency. The measurement data have a relatively low spectral information content, and different parameter vectors can describe the same measured spectrum equally well. A usual method to regularize the retrieval of the wanted parameters from a measured spectrum is to take into account a priori mean values and standard deviations of the parameters to be retrieved. This decreases the probability to obtain unreasonable parameter values. The multi-spectrum retrieval algorithm MSR is developed to additionally consider physically realistic spatial and temporal a priori correlations between retrieval parameters describing different measurements. Neglecting geologic activity, MSR also allows the retrieval of an emissivity map as a parameter vector that is common to several spectrally resolved images that cover the same surface target. Even applying MSR, it is difficult to obtain reliable emissivity maps in absolute values. A detailed retrieval error analysis based on synthetic spectra reveals that this is mainly due to interferences from parameters that cannot be derived from the spectra themselves, but that have to be set to assumed values to enable the radiative transfer simulations. The MSR retrieval of emissivity maps relative to a fixed emissivity is shown to effectively avoid most emissivity retrieval errors. Relative emissivity maps at 1.02, 1.10, and 1.18 μm are finally derived from many VIRTIS-M-IR measurements that cover a surface target at Themis Regio. They are interpreted as spatial variations relative to an assumed emissivity mean of the target. It is verified that the maps are largely independent of the choice of many interfering parameters as well as the utilized measurement data set. These are the first Venus IR emissivity data maps based on a consistent application of a full radiative transfer simulation and a retrieval algorithm that respects a priori information. The maps are sufficiently reliable for future geologic interpretations.
This thesis investigates the application of polyelectrolyte multilayers in plasmonics and picosecond acoustics. The observed samples were fabricated by the spin-assisted layer-by-layer deposition technique that allowed a precise tuning of layer thickness in the range of few nanometers.
The first field of interest deals with the interaction of light-induced localized surface plasmons (LSP) of rod-shaped gold nanoparticles with the particles' environment. The environment consists of an air phase and a phase of polyelectrolytes, whose ratio affects the spectral position of the LSP resonance.
Measured UV-VIS spectra showed the shift of the LSP absorption peak as a function of the cover layer thickness of the particles. The data are modeled using an average dielectric function instead of the dielectric functions of air and polyelectrolytes. In addition using a measured dielectric function of the gold nanoparticles, the position of the LSP absorption peak could be simulated with good agreement to the data.
The analytic model helps to understand the optical properties of metal nanoparticles in an inhomogeneous environment.
The second part of this work discusses the applicability of PAzo/PAH and dye-doped PSS/PAH polyelectrolyte multilayers as transducers to generate hypersound pulses. The generated strain pulses were detected by time-domain Brillouin scattering (TDBS) using a pump-probe laser setup. Transducer layers made of polyelectrolytes were compared qualitatively to common aluminum transducers in terms of measured TDBS signal amplitude, degradation due to laser excitation, and sample preparation.
The measurements proved that fast and easy prepared polyelectrolyte transducers provided stronger TDBS signals than the aluminum transducer. AFM topography measurements showed a degradation of the polyelectrolyte structures, especially for the PAzo/PAH sample.
To quantify the induced strain, optical barriers were introduced to separate the transducer material from the medium of the hypersound propagation. Difficulties in the sample preparation prohibited a reliable quantification. But the experiments showed that a coating with transparent polyelectrolytes increases the efficiency of aluminum transducers and modifies the excited phonon distribution.
The adoption of polyelectrolytes to the scientific field of picosecond acoustics enables a cheap and fast fabrication of transducer layers on most surfaces. In contrast to aluminum layers the polyelectrolytes are transparent over a wide spectral range. Thus, the strain modulation can be probed from surface and back.
Function by structure
(2015)
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.
Atmospheric interactions with land surface in the arctic based on regional climate model solutions
(2014)
Scientific inquiry requires that we formulate not only what we know, but also what we do not know and by how much. In climate data analysis, this involves an accurate specification of measured quantities and a consequent analysis that consciously propagates the measurement errors at each step. The dissertation presents a thorough analytical method to quantify errors of measurement inherent in paleoclimate data. An additional focus are the uncertainties in assessing the coupling between different factors that influence the global mean temperature (GMT).
Paleoclimate studies critically rely on `proxy variables' that record climatic signals in natural archives. However, such proxy records inherently involve uncertainties in determining the age of the signal. We present a generic Bayesian approach to analytically determine the proxy record along with its associated uncertainty, resulting in a time-ordered sequence of correlated probability distributions rather than a precise time series. We further develop a recurrence based method to detect dynamical events from the proxy probability distributions. The methods are validated with synthetic examples and
demonstrated with real-world proxy records. The proxy estimation step reveals the interrelations between proxy variability and uncertainty. The recurrence analysis of the East Asian Summer Monsoon during the last 9000 years confirms the well-known `dry' events at 8200 and 4400 BP, plus an additional significantly dry event at 6900 BP.
We also analyze the network of dependencies surrounding GMT. We find an intricate, directed network with multiple links between the different factors at multiple time delays. We further uncover a significant feedback from the GMT to the El Niño Southern Oscillation at quasi-biennial timescales. The analysis highlights the need of a more nuanced formulation of influences between different climatic factors, as well as the limitations in trying to estimate such dependencies.
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.
The atmosphere over the Arctic Ocean is strongly influenced by the distribution of sea ice and open water. Leads in the sea ice produce strong convective fluxes of sensible and latent heat and release aerosol particles into the atmosphere. They increase the occurrence of clouds and modify the structure and characteristics of the atmospheric boundary layer (ABL) and thereby influence the Arctic climate.
In the course of this study aircraft measurements were performed over the western Arctic Ocean as part of the campaign PAMARCMIP 2012 of the Alfred Wegener Institute for Polar and Marine Research (AWI). Backscatter from aerosols and clouds within the lower troposphere and the ABL were measured with the nadir pointing Airborne Mobile Aerosol Lidar (AMALi) and dropsondes were launched to obtain profiles of meteorological variables. Furthermore, in situ measurements of aerosol properties, meteorological variables and turbulence were part of the campaign. The measurements covered a broad range of atmospheric and sea ice conditions.
In this thesis, properties of the ABL over Arctic sea ice with a focus on the influence of open leads are studied based on the data from the PAMARCMIP campaign. The height of the ABL is determined by different methods that are applied to dropsonde and AMALi backscatter profiles. ABL heights are compared for different flights representing different conditions of the atmosphere and of sea ice and open water influence. The different criteria for ABL height that are applied show large variation in terms of agreement among each other, depending on the characteristics of the ABL and its history. It is shown that ABL height determination from lidar backscatter by methods commonly used under mid-latitude conditions is applicable to the Arctic ABL only under certain conditions. Aerosol or clouds within the ABL are needed as a tracer for ABL height detection from backscatter. Hence an aerosol source close to the surface is necessary, that is typically found under the present influence of open water and therefore convective conditions. However it is not always possible to distinguish residual layers from the actual ABL. Stable boundary layers are generally difficult to detect.
To illustrate the complexity of the Arctic ABL and processes therein, four case studies are analyzed each of which represents a snapshot of the interplay between atmosphere and underlying sea ice or water surface. Influences of leads and open water on the aerosol and clouds within the ABL are identified and discussed. Leads are observed to cause the formation of fog and cloud layers within the ABL by humidity emission. Furthermore they decrease the stability and increase the height of the ABL and consequently facilitate entrainment of air and aerosol layers from the free troposphere.
During this work I built a four wave mixing setup for the time-resolved femtosecond spectroscopy of Raman-active lattice modes. This setup enables to study the selective excitation of phonon polaritons. These quasi-particles arise from the coupling of electro-magnetic waves and transverse optical lattice modes, the so-called phonons. The phonon polaritons were investigated in the optically non-linear, ferroelectric crystals LiNbO₃ and LiTaO₃.
The direct observation of the frequency shift of the scattered narrow bandwidth probe pulses proofs the role of the Raman interaction during the probe and excitation process of phonon polaritons. I compare this experimental method with the measurement where ultra-short laser pulses are used. The frequency shift remains obscured by the relative broad bandwidth of these laser pulses. In an experiment with narrow bandwidth probe pulses, the Stokes and anti-Stokes intensities are spectrally separated. They are assigned to the corresponding counter-propagating wavepackets of phonon polaritons. Thus, the dynamics of these wavepackets was separately studied. Based on these findings, I develop the mathematical description of the so-called homodyne detection of light for the case of light scattering from counter propagating phonon polaritons.
Further, I modified the broad bandwidth of the ultra-short pump pulses using bandpass filters to generate two pump pulses with non-overlapping spectra. This enables the frequency-selective excitation of polariton modes in the sample, which allows me to observe even very weak polariton modes in LiNbO₃ or LiTaO₃ that belong to the higher branches of the dispersion relation of phonon polaritons. The experimentally determined dispersion relation of the phonon polaritons could therefore be extended and compared to theoretical models. In addition, I determined the frequency-dependent damping of phonon polaritons.
The origin of cosmic rays was the subject of several studies for over a century. The investigations done within this dissertation are one small step to shed some more light on this mystery.
Locating the sources of cosmic rays is not trivial due to the interstellar magnetic field. However, the Hillas criterion allows us to arrive at the conclusion that supernova remnants are our main suspect for the origin of galactic cosmic rays. The mechanism by which they are accelerating particles is found within the field of shock physics as diffusive shock acceleration. To allow particles to enter this process also known as Fermi acceleration pre-acceleration processes like shock surfing acceleration and shock drift acceleration are necessary. Investigating the processes happening in the plasma shocks of supernova remnants is possible by utilising a simplified model which can be simulated on a computer using Particle-in-Cell simulations.
We developed a new and clean setup to simulate the formation of a double shock, i.e., consisting of a forward and a reverse shock and a contact discontinuity, by the collision of two counter-streaming plasmas, in which a magnetic field can be woven into. In a previous work, we investigated the processes at unmagnetised and at magnetised parallel shocks, whereas in the current work, we move our investigation on to magnetised perpendicular shocks.
Due to a much stronger confinement of the particles to the collision region the perpendicular shock develops much faster than the parallel shock. On the other hand, this leads to much weaker turbulence. We are able to find indications for shock surfing acceleration and shock drift acceleration happening at the two shocks leading to populations of pre-accelerated particles that are suitable as a seed population to be injected into further diffusive shock acceleration to be accelerated to even higher energies. We observe the development of filamentary structures in the shock ramp of the forward shock, but not at the reverse shock. This leads to the conclusion that the development of such structures in the shock ramp of quasi-perpendicular collisionless shocks might not necessarily be determined by the existence of a critical sonic Mach number but by a critical shock speed.
The results of the investigations done within this dissertation might be useful for further studies of oblique shocks and for studies using hybrid or magnetohydrodynamic simulations. Together with more sophisticated observational methods, these studies will help to bring us closer to an answer as to how particles can be accelerated in supernova remnants and eventually become cosmic rays that can be detected on Earth.
Synchronization is a fundamental phenomenon in nature. It can be considered as a general property of self-sustained oscillators to adjust their rhythm in the presence of an interaction.
In this work we investigate complex regimes of synchronization phenomena by means of theoretical analysis, numerical modeling, as well as practical analysis of experimental data.
As a subject of our investigation we consider chimera state, where due to spontaneous symmetry-breaking of an initially homogeneous oscillators lattice split the system into two parts with different dynamics. Chimera state as a new synchronization phenomenon was first found in non-locally coupled oscillators system, and has attracted a lot of attention in the last decade. However, the recent studies indicate that this state is also possible in globally coupled systems. In the first part of this work, we show under which conditions the chimera-like state appears in a system of globally coupled identical oscillators with intrinsic delayed feedback. The results of the research explain how initially monostable oscillators became effectivly bistable in the presence of the coupling and create a mean field that sustain the coexistence of synchronized and desynchronized states. Also we discuss other examples, where chimera-like state appears due to frequency dependence of the phase shift in the bistable system.
In the second part, we make further investigation of this topic by modeling influence of an external periodic force to an oscillator with intrinsic delayed feedback. We made stability analysis of the synchronized state and constructed Arnold tongues. The results explain formation of the chimera-like state and hysteric behavior of the synchronization area. Also, we consider two sets of parameters of the oscillator with symmetric and asymmetric Arnold tongues, that correspond to mono- and bi-stable regimes of the oscillator.
In the third part, we demonstrate the results of the work, which was done in collaboration with our colleagues from Psychology Department of University of Potsdam. The project aimed to study the effect of the cardiac rhythm on human perception of time using synchronization analysis. From our part, we made a statistical analysis of the data obtained from the conducted experiment on free time interval reproduction task. We examined how ones heartbeat influences the time perception and searched for possible phase synchronization between heartbeat cycles and time reproduction responses. The findings support the prediction that cardiac cycles can serve as input signals, and is used for reproduction of time intervals in the range of several seconds.
A dramatic efficiency improvement of bulk heterojunction solar cells based on electron-donating conjugated polymers in combination with soluble fullerene derivatives has been achieved over the past years. Certified and reported power conversion efficiencies now reach over 9% for single junctions and exceed the 10% benchmark for tandem solar cells. This trend brightens the vision of organic photovoltaics becoming competitive with inorganic solar cells including the realization of low-cost and large-area organic photovoltaics. For the best performing organic materials systems, the yield of charge generation can be very efficient. However, a detailed understanding of the free charge carrier generation mechanisms at the donor acceptor interface and the energy loss associated with it needs to be established. Moreover, organic solar cells are limited by the competition between charge extraction and free charge recombination, accounting for further efficiency losses. A conclusive picture and the development of precise methodologies for investigating the fundamental processes in organic solar cells are crucial for future material design, efficiency optimization, and the implementation of organic solar cells into commercial products.
In order to advance the development of organic photovoltaics, my thesis focuses on the comprehensive understanding of charge generation, recombination and extraction in organic bulk heterojunction solar cells summarized in 6 chapters on the cumulative basis of 7 individual publications.
The general motivation guiding this work was the realization of an efficient hybrid inorganic/organic tandem solar cell with sub-cells made from amorphous hydrogenated silicon and organic bulk heterojunctions. To realize this project aim, the focus was directed to the low band-gap copolymer PCPDTBT and its derivatives, resulting in the examination of the charge carrier dynamics in PCPDTBT:PC70BM blends in relation to by the blend morphology. The phase separation in this blend can be controlled by the processing additive diiodooctane, enhancing domain purity and size. The quantitative investigation of the free charge formation was realized by utilizing and improving the time delayed collection field technique. Interestingly, a pronounced field dependence of the free carrier generation for all blends is found, with the field dependence being stronger without the additive. Also, the bimolecular recombination coefficient for both blends is rather high and increases with decreasing internal field which we suggest to be caused by a negative field dependence of mobility. The additive speeds up charge extraction which is rationalized by the threefold increase in mobility.
By fluorine attachment within the electron deficient subunit of PCPDTBT, a new polymer F-PCPDTBT is designed. This new material is characterized by a stronger tendency to aggregate as compared to non-fluorinated PCPDTBT. Our measurements show that for F-PCPDTBT:PCBM blends the charge carrier generation becomes more efficient and the field-dependence of free charge carrier generation is weakened. The stronger tendency to aggregate induced by the fluorination also leads to increased polymer rich domains, accompanied in a threefold reduction in the non-geminate recombination coefficient at conditions of open circuit. The size of the polymer domains is nicely correlated to the field-dependence of charge generation and the Langevin reduction factor, which highlights the importance of the domain size and domain purity for efficient charge carrier generation. In total, fluorination of PCPDTBT causes the PCE to increase from 3.6 to 6.1% due to enhanced fill factor, short circuit current and open circuit voltage. Further optimization of the blend ratio, active layer thickness, and polymer molecular weight resulted in 6.6% efficiency for F-PCPDTBT:PC70BM solar cells.
Interestingly, the double fluorinated version 2F-PCPDTBT exhibited poorer FF despite a further reduction of geminate and non-geminate recombination losses. To further analyze this finding, a new technique is developed that measures the effective extraction mobility under charge carrier densities and electrical fields comparable to solar cell operation conditions. This method involves the bias enhanced charge extraction technique. With the knowledge of the carrier density under different electrical field and illumination conditions, a conclusive picture of the changes in charge carrier dynamics leading to differences in the fill factor upon fluorination of PCPDTBT is attained. The more efficient charge generation and reduced recombination with fluorination is counterbalanced by a decreased extraction mobility. Thus, the highest fill factor of 60% and efficiency of 6.6% is reached for F-PCPDTBT blends, while 2F-PCPDTBT blends have only moderate fill factors of 54% caused by the lower effective extraction mobility, limiting the efficiency to 6.5%.
To understand the details of the charge generation mechanism and the related losses, we evaluated the yield and field-dependence of free charge generation using time delayed collection field in combination with sensitive measurements of the external quantum efficiency and absorption coefficients for a variety of blends. Importantly, both the yield and field-dependence of free charge generation is found to be unaffected by excitation energy, including direct charge transfer excitation below the optical band gap. To access the non-detectable absorption at energies of the relaxed charge transfer emission, the absorption was reconstructed from the CT emission, induced via the recombination of thermalized charges in electroluminescence. For a variety of blends, the quantum yield at energies of charge transfer emission was identical to excitations with energies well above the optical band-gap. Thus, the generation proceeds via the split-up of the thermalized charge transfer states in working solar cells. Further measurements were conducted on blends with fine-tuned energy levels and similar blend morphologies by using different fullerene derivatives. A direct correlation between the efficiency of free carrier generation and the energy difference of the relaxed charge transfer state relative to the energy of the charge separated state is found. These findings open up new guidelines for future material design as new high efficiency materials require a minimum energetic offset between charge transfer and the charge separated state while keeping the HOMO level (and LUMO level) difference between donor and acceptor as small as possible.
Bacteria respond to changing environmental conditions by switching the global pattern of expressed genes. In response to specific environmental stresses the cell activates several stress-specific molecules such as sigma factors. They reversibly bind the RNA polymerase to form the so-called holoenzyme and direct it towards the appropriate stress response genes. In exponentially growing E. coli cells, the majority of the transcriptional activity is carried out by the housekeeping sigma factor, while stress responses are often under the control of alternative sigma factors. Different sigma factors compete for binding to a limited pool of RNA polymerase (RNAP) core enzymes, providing a mechanism for cross talk between genes or gene classes via the sharing of expression machinery. To quantitatively analyze the contribution of sigma factor competition to global changes in gene expression, we develop a thermodynamic model that describes binding between sigma factors and core RNAP at equilibrium, transcription, non-specific binding to DNA and the modulation of the availability of the molecular components.
Association of housekeeping sigma factor to RNAP is generally favored by its abundance and higher binding affinity to the core. In order to promote transcription by alternative sigma subunits, the bacterial cell modulates the transcriptional efficiency in a reversible manner through several strategies such as anti-sigma factors, 6S RNA and generally any kind of transcriptional regulators (e.g. activators or inhibitors). By shifting the outcome of sigma factor competition for the core, these modulators bias the transcriptional program of the cell. The model is validated by comparison with in vitro competition experiments, with which excellent agreement is found. We observe that transcription is affected via the modulation of the concentrations of the different types of holoenzymes, so saturated promoters are only weakly affected by sigma factor competition. However, in case of overlapping promoters or promoters recognized by two types of sigma factors, we find that even saturated promoters are strongly affected.
Active transcription effectively lowers the affinity between the sigma factor driving it and the core RNAP, resulting in complex cross talk effects and raising the question of how their in vitro measure is relevant in the cell. We also estimate that sigma factor competition is not strongly affected by non-specific binding of core RNAPs, sigma factors, and holoenzymes to DNA. Finally, we analyze the role of increased core RNAP availability upon the shut-down of ribosomal RNA transcription during stringent response. We find that passive up-regulation of alternative sigma-dependent transcription is not only possible, but also displays hypersensitivity based on the sigma factor competition. Our theoretical analysis thus provides support for a significant role of passive control during that global switch of the gene expression program and gives new insights into RNAP partitioning in the cell.
Despite remarkable progress made in the past century, which has revolutionized our understanding of the universe, there are numerous open questions left in theoretical physics. Particularly important is the fact that the theories describing the fundamental interactions of nature are incompatible. Einstein's theory of general relative describes gravity as a dynamical spacetime, which is curved by matter and whose curvature determines the motion of matter. On the other hand we have quantum field theory, in form of the standard model of particle physics, where particles interact via the remaining interactions - electromagnetic, weak and strong interaction - on a flat, static spacetime without gravity. A theory of quantum gravity is hoped to cure this incompatibility by heuristically replacing classical spacetime by quantum spacetime'. Several approaches exist attempting to define such a theory with differing underlying premises and ideas, where it is not clear which is to be preferred. Yet a minimal requirement is the compatibility with the classical theory, they attempt to generalize. Interestingly many of these models rely on discrete structures in their definition or postulate discreteness of spacetime to be fundamental. Besides the direct advantages discretisations provide, e.g. permitting numerical simulations, they come with serious caveats requiring thorough investigation: In general discretisations break fundamental diffeomorphism symmetry of gravity and are generically not unique. Both complicates establishing the connection to the classical continuum theory. The main focus of this thesis lies in the investigation of this relation for spin foam models. This is done on different levels of the discretisation / triangulation, ranging from few simplices up to the continuum limit. In the regime of very few simplices we confirm and deepen the connection of spin foam models to discrete gravity. Moreover, we discuss dynamical, e.g. diffeomorphism invariance in the discrete, to fix the ambiguities of the models. In order to satisfy these conditions, the discrete models have to be improved in a renormalisation procedure, which also allows us to study their continuum dynamics. Applied to simplified spin foam models, we uncover a rich, non--trivial fixed point structure, which we summarize in a phase diagram. Inspired by these methods, we propose a method to consistently construct the continuum theory, which comes with a unique vacuum state.
The H.E.S.S. array is a third generation Imaging Atmospheric Cherenkov Telescope (IACT) array. It is located in the Khomas Highland in Namibia, and measures very high energy (VHE) gamma-rays. In Phase I, the array started data taking in 2004 with its four identical 13 m telescopes. Since then, H.E.S.S. has emerged as the most successful IACT experiment to date. Among the almost 150 sources of VHE gamma-ray radiation found so far, even the oldest detection, the Crab Nebula, keeps surprising the scientific community with unexplained phenomena such as the recently discovered very energetic flares of high energy gamma-ray radiation. During its most recent flare, which was detected by the Fermi satellite in March 2013, the Crab Nebula was simultaneously observed with the H.E.S.S. array for six nights. The results of the observations will be discussed in detail during the course of this work. During the nights of the flare, the new 24 m × 32 m H.E.S.S. II telescope was still being commissioned, but participated in the data taking for one night. To be able to reconstruct and analyze the data of the H.E.S.S. Phase II array, the algorithms and software used by the H.E.S.S. Phase I array had to be adapted. The most prominent advanced shower reconstruction technique developed by de Naurois and Rolland, the template-based model analysis, compares real shower images taken by the Cherenkov telescope cameras with shower templates obtained using a semi-analytical model. To find the best fitting image, and, therefore, the relevant parameters that describe the air shower best, a pixel-wise log-likelihood fit is done. The adaptation of this advanced shower reconstruction technique to the heterogeneous H.E.S.S. Phase II array for stereo events (i.e. air showers seen by at least two telescopes of any kind), its performance using MonteCarlo simulations as well as its application to real data will be described.
In the presented thesis, the most advanced photon reconstruction technique of ground-based γ-ray astronomy is adapted to the H.E.S.S. 28 m telescope. The method is based on a semi-analytical model of electromagnetic particle showers in the atmosphere. The properties of cosmic γ-rays are reconstructed by comparing the camera image of the telescope with the Cherenkov emission that is expected from the shower model. To suppress the dominant background from charged cosmic rays, events are selected based on several criteria. The performance of the analysis is evaluated with simulated events. The method is then applied to two sources that are known to emit γ-rays. The first of these is the Crab Nebula, the standard candle of ground-based γ-ray astronomy. The results of this source confirm the expected performance of the reconstruction method, where the much lower energy threshold compared to H.E.S.S. I is of particular importance. A second analysis is performed on the region around the Galactic Centre. The analysis results emphasise the capabilities of the new telescope to measure γ-rays in an energy range that is interesting for both theoretical and experimental astrophysics. The presented analysis features the lowest energy threshold that has ever been reached in ground-based γ-ray astronomy, opening a new window to the precise measurement of the physical properties of time-variable sources at energies of several tens of GeV.
One of the most significant current discussions in Astrophysics relates to the origin of high-energy cosmic rays. According to our current knowledge, the abundance distribution of the elements in cosmic rays at their point of origin indicates, within plausible error limits, that they were initially formed by nuclear processes in the interiors of stars. It is also believed that their energy distribution up to 1018 eV has Galactic origins. But even though the knowledge about potential sources of cosmic rays is quite poor above „ 1015 eV, that is the “knee” of the cosmic-ray spectrum, up to the knee there seems to be a wide consensus that supernova remnants are the most likely candidates. Evidence of this comes from observations of non-thermal X-ray radiation, requiring synchrotron electrons with energies up to 1014 eV, exactly in the remnant of supernovae. To date, however, there is not conclusive evidence that they produce nuclei, the dominant component of cosmic rays, in addition to electrons. In light of this dearth of evidence, γ-ray observations from supernova remnants can offer the most promising direct way to confirm whether or not these astrophysical objects are indeed the main source of cosmic-ray nuclei below the knee. Recent observations with space- and ground-based observatories have established shell-type supernova remnants as GeV-to- TeV γ-ray sources. The interpretation of these observations is however complicated by the different radiation processes, leptonic and hadronic, that can produce similar fluxes in this energy band rendering ambiguous the nature of the emission itself. The aim of this work is to develop a deeper understanding of these radiation processes from a particular shell-type supernova remnant, namely RX J1713.7–3946, using observations of the LAT instrument onboard the Fermi Gamma-Ray Space Telescope. Furthermore, to obtain accurate spectra and morphology maps of the emission associated with this supernova remnant, an improved model of the diffuse Galactic γ-ray emission background is developed. The analyses of RX J1713.7–3946 carried out with this improved background show that the hard Fermi-LAT spectrum cannot be ascribed to the hadronic emission, leading thus to the conclusion that the leptonic scenario is instead the most natural picture for the high-energy γ-ray emission of RX J1713.7–3946. The leptonic scenario however does not rule out the possibility that cosmic-ray nuclei are accelerated in this supernova remnant, but it suggests that the ambient density may not be high enough to produce a significant hadronic γ-ray emission. Further investigations involving other supernova remnants using the improved back- ground developed in this work could allow compelling population studies, and hence prove or disprove the origin of Galactic cosmic-ray nuclei in these astrophysical objects. A break- through regarding the identification of the radiation mechanisms could be lastly achieved with a new generation of instruments such as CTA.
In processing and data storage mainly ferromagnetic (FM) materials are being used. Approaching physical limits, new concepts have to be found for faster, smaller switches, for higher data densities and more energy efficiency. Some of the discussed new concepts involve the material classes of correlated oxides and materials with antiferromagnetic coupling. Their applicability depends critically on their switching behavior, i.e., how fast and how energy efficient material properties can be manipulated. This thesis presents investigations of ultrafast non-equilibrium phase transitions on such new materials. In transition metal oxides (TMOs) the coupling of different degrees of freedom and resulting low energy excitation spectrum often result in spectacular changes of macroscopic properties (colossal magneto resistance, superconductivity, metal-to-insulator transitions) often accompanied by nanoscale order of spins, charges, orbital occupation and by lattice distortions, which make these material attractive. Magnetite served as a prototype for functional TMOs showing a metal-to-insulator-transition (MIT) at T = 123 K. By probing the charge and orbital order as well as the structure after an optical excitation we found that the electronic order and the structural distortion, characteristics of the insulating phase in thermal equilibrium, are destroyed within the experimental resolution of 300 fs. The MIT itself occurs on a 1.5 ps timescale. It shows that MITs in functional materials are several thousand times faster than switching processes in semiconductors. Recently ferrimagnetic and antiferromagnetic (AFM) materials have become interesting. It was shown in ferrimagnetic GdFeCo, that the transfer of angular momentum between two opposed FM subsystems with different time constants leads to a switching of the magnetization after laser pulse excitation. In addition it was theoretically predicted that demagnetization dynamics in AFM should occur faster than in FM materials as no net angular momentum has to be transferred out of the spin system. We investigated two different AFM materials in order to learn more about their ultrafast dynamics. In Ho, a metallic AFM below T ≈ 130 K, we found that the AFM Ho can not only be faster but also ten times more energy efficiently destroyed as order in FM comparable metals. In EuTe, an AFM semiconductor below T ≈ 10 K, we compared the loss of magnetization and laser-induced structural distortion in one and the same experiment. Our experiment shows that they are effectively disentangled. An exception is an ultrafast release of lattice dynamics, which we assign to the release of magnetostriction. The results presented here were obtained with time-resolved resonant soft x-ray diffraction at the Femtoslicing source of the Helmholtz-Zentrum Berlin and at the free-electron laser in Stanford (LCLS). In addition the development and setup of a new UHV-diffractometer for these experiments will be reported.
When azobenzene-modified photosensitive polymer films are irradiated with light interference patterns, topographic variations in the film develop that follow the electric field vector distribution resulting in the formation of surface relief grating (SRG). The exact correspondence of the electric field vector orientation in interference pattern in relation to the presence of local topographic minima or maxima of SRG is in general difficult to determine. In my thesis, we have established a systematic procedure to accomplish the correlation between different interference patterns and the topography of SRG. For this, we devise a new setup combining an atomic force microscope and a two-beam interferometer (IIAFM). With this set-up, it is possible to track the topography change in-situ, while at the same time changing polarization and phase of the impinging interference pattern. To validate our results, we have compared two photosensitive materials named in short as PAZO and trimer. This is the first time that an absolute correspondence between the local distribution of electric field vectors of interference pattern and the local topography of the relief grating could be established exhaustively. In addition, using our IIAFM we found that for a certain polarization combination of two orthogonally polarized interfering beams namely SP (↕, ↔) interference pattern, the topography forms SRG with only half the period of the interference patterns. Exploiting this phenomenon we are able to fabricate surface relief structures below diffraction limit with characteristic features measuring only 140 nm, by using far field optics with a wavelength of 491 nm. We have also probed for the stresses induced during the polymer mass transport by placing an ultra-thin gold film on top (5–30 nm). During irradiation, the metal film not only deforms along with the SRG formation, but ruptures in regular and complex manner. The morphology of the cracks differs strongly depending on the electric field distribution in the interference pattern even when the magnitude and the kinetic of the strain are kept constant. This implies a complex local distribution of the opto-mechanical stress along the topography grating. The neutron reflectivity measurements of the metal/polymer interface indicate the penetration of metal layer within the polymer resulting in the formation of bonding layer that confirms the transduction of light induced stresses in the polymer layer to a metal film.
LCST-type synthetic thermoresponsive polymers can reversibly respond to certain stimuli in aqueous media with a massive change of their physical state. When fluorophores, that are sensitive to such changes, are incorporated into the polymeric structure, the response can be translated into a fluorescence signal. Based on this idea, this thesis presents sensing schemes which transduce the stimuli-induced variations in the solubility of polymer chains with covalently-bound fluorophores into a well-detectable fluorescence output. Benefiting from the principles of different photophysical phenomena, i.e. of fluorescence resonance energy transfer and solvatochromism, such fluorescent copolymers enabled monitoring of stimuli such as the solution temperature and ionic strength, but also of association/disassociation mechanisms with other macromolecules or of biochemical binding events through remarkable changes in their fluorescence properties. For instance, an aqueous ratiometric dual sensor for temperature and salts was developed, relying on the delicate supramolecular assembly of a thermoresponsive copolymer with a thiophene-based conjugated polyelectrolyte. Alternatively, by taking advantage of the sensitivity of solvatochromic fluorophores, an increase in solution temperature or the presence of analytes was signaled as an enhancement of the fluorescence intensity. A simultaneous use of the sensitivity of chains towards the temperature and a specific antibody allowed monitoring of more complex phenomena such as competitive binding of analytes. The use of different thermoresponsive polymers, namely poly(N-isopropylacrylamide) and poly(meth)acrylates bearing oligo(ethylene glycol) side chains, revealed that the responsive polymers differed widely in their ability to perform a particular sensing function. In order to address questions regarding the impact of the chemical structure of the host polymer on the sensing performance, the macromolecular assembly behavior below and above the phase transition temperature was evaluated by a combination of fluorescence and light scattering methods. It was found that although the temperature-triggered changes in the macroscopic absorption characteristics were similar for these polymers, properties such as the degree of hydration or the extent of interchain aggregations differed substantially. Therefore, in addition to the demonstration of strategies for fluorescence-based sensing with thermoresponsive polymers, this work highlights the role of the chemical structure of the two popular thermoresponsive polymers on the fluorescence response. The results are fundamentally important for the rational choice of polymeric materials for a specific sensing strategy.
Donor-acceptor (D-A) copolymers have revolutionized the field of organic electronics over the last decade. Comprised of a electron rich and an electron deficient molecular unit, these copolymers facilitate the systematic modification of the material's optoelectronic properties. The ability to tune the optical band gap and to optimize the molecular frontier orbitals as well as the manifold of structural sites that enable chemical modifications has created a tremendous variety of copolymer structures. Today, these materials reach or even exceed the performance of amorphous inorganic semiconductors. Most impressively, the charge carrier mobility of D-A copolymers has been pushed to the technologically important value of 10 cm^{2}V^{-1}s^{-1}. Furthermore, owed to their enormous variability they are the material of choice for the donor component in organic solar cells, which have recently surpassed the efficiency threshold of 10%. Because of the great number of available D-A copolymers and due to their fast chemical evolution, there is a significant lack of understanding of the fundamental physical properties of these materials. Furthermore, the complex chemical and electronic structure of D-A copolymers in combination with their semi-crystalline morphology impede a straightforward identification of the microscopic origin of their superior performance. In this thesis, two aspects of prototype D-A copolymers were analysed. These are the investigation of electron transport in several copolymers and the application of low band gap copolymers as acceptor component in organic solar cells. In the first part, the investigation of a series of chemically modified fluorene-based copolymers is presented. The charge carrier mobility varies strongly between the different derivatives, although only moderate structural changes on the copolymers structure were made. Furthermore, rather unusual photocurrent transients were observed for one of the copolymers. Numerical simulations of the experimental results reveal that this behavior arises from a severe trapping of electrons in an exponential distribution of trap states. Based on the comparison of simulation and experiment, the general impact of charge carrier trapping on the shape of photo-CELIV and time-of-flight transients is discussed. In addition, the high performance naphthalenediimide (NDI)-based copolymer P(NDI2OD-T2) was characterized. It is shown that the copolymer posses one of the highest electron mobilities reported so far, which makes it attractive to be used as the electron accepting component in organic photovoltaic cells.\par Solar cells were prepared from two NDI-containing copolymers, blended with the hole transporting polymer P3HT. I demonstrate that the use of appropriate, high boiling point solvents can significantly increase the power conversion efficiency of these devices. Spectroscopic studies reveal that the pre-aggregation of the copolymers is suppressed in these solvents, which has a strong impact on the blend morphology. Finally, a systematic study of P3HT:P(NDI2OD-T2) blends is presented, which quantifies the processes that limit the efficiency of devices. The major loss channel for excited states was determined by transient and steady state spectroscopic investigations: the majority of initially generated electron-hole pairs is annihilated by an ultrafast geminate recombination process. Furthermore, exciton self-trapping in P(NDI2OD-T2) domains account for an additional reduction of the efficiency. The correlation of the photocurrent to microscopic morphology parameters was used to disclose the factors that limit the charge generation efficiency. Our results suggest that the orientation of the donor and acceptor crystallites relative to each other represents the main factor that determines the free charge carrier yield in this material system. This provides an explanation for the overall low efficiencies that are generally observed in all-polymer solar cells.
In the presence of a solid-liquid or liquid-air interface, bacteria can choose between a planktonic and a sessile lifestyle. Depending on environmental conditions, cells swimming in close proximity to the interface can irreversibly attach to the surface and grow into three-dimensional aggregates where the majority of cells is sessile and embedded in an extracellular polymer matrix (biofilm). We used microfluidic tools and time lapse microscopy to perform experiments with the polarly flagellated soil bacterium Pseudomonas putida (P. putida), a bacterial species that is able to form biofilms. We analyzed individual trajectories of swimming cells, both in the bulk fluid and in close proximity to a glass-liquid interface. Additionally, surface related growth during the early phase of biofilm formation was investigated. In the bulk fluid, P.putida shows a typical bacterial swimming pattern of alternating periods of persistent displacement along a line (runs) and fast reorientation events (turns) and cells swim with an average speed around 24 micrometer per second. We found that the distribution of turning angles is bimodal with a dominating peak around 180 degrees. In approximately six out of ten turning events, the cell reverses its swimming direction. In addition, our analysis revealed that upon a reversal, the cell systematically changes its swimming speed by a factor of two on average. Based on the experimentally observed values of mean runtime and rotational diffusion, we presented a model to describe the spreading of a population of cells by a run-reverse random walker with alternating speeds. We successfully recover the mean square displacement and, by an extended version of the model, also the negative dip in the directional autocorrelation function as observed in the experiments. The analytical solution of the model demonstrates that alternating speeds enhance a cells ability to explore its environment as compared to a bacterium moving at a constant intermediate speed. As compared to the bulk fluid, for cells swimming near a solid boundary we observed an increase in swimming speed at distances below d= 5 micrometer and an increase in average angular velocity at distances below d= 4 micrometer. While the average speed was maximal with an increase around 15% at a distance of d= 3 micrometer, the angular velocity was highest in closest proximity to the boundary at d=1 micrometer with an increase around 90% as compared to the bulk fluid. To investigate the swimming behavior in a confinement between two solid boundaries, we developed an experimental setup to acquire three-dimensional trajectories using a piezo driven objective mount coupled to a high speed camera. Results on speed and angular velocity were consistent with motility statistics in the presence of a single boundary. Additionally, an analysis of the probability density revealed that a majority of cells accumulated near the upper and lower boundaries of the microchannel. The increase in angular velocity is consistent with previous studies, where bacteria near a solid boundary were shown to swim on circular trajectories, an effect which can be attributed to a wall induced torque. The increase in speed at a distance of several times the size of the cell body, however, cannot be explained by existing theories which either consider the drag increase on cell body and flagellum near a boundary (resistive force theory) or model the swimming microorganism by a multipole expansion to account for the flow field interaction between cell and boundary. An accumulation of swimming bacteria near solid boundaries has been observed in similar experiments. Our results confirm that collisions with the surface play an important role and hydrodynamic interactions alone cannot explain the steady-state accumulation of cells near the channel walls. Furthermore, we monitored the number growth of cells in the microchannel under medium rich conditions. We observed that, after a lag time, initially isolated cells at the surface started to grow by division into colonies of increasing size, while coexisting with a comparable smaller number of swimming cells. After 5:50 hours, we observed a sudden jump in the number of swimming cells, which was accompanied by a breakup of bigger clusters on the surface. After approximately 30 minutes where planktonic cells dominated in the microchannel, individual swimming cells reattached to the surface. We interpret this process as an emigration and recolonization event. A number of complementary experiments were performed to investigate the influence of collective effects or a depletion of the growth medium on the transition. Similar to earlier observations on another bacterium from the same family we found that the release of cells to the swimming phase is most likely the result of an individual adaption process, where syntheses of proteins for flagellar motility are upregulated after a number of division cycles at the surface.
Organic semiconductors combine the benefits of organic materials, i.e., low-cost production, mechanical flexibility, lightweight, and robustness, with the fundamental semiconductor properties light absorption, emission, and electrical conductivity. This class of material has several advantages over conventional inorganic semiconductors that have led, for instance, to the commercialization of organic light-emitting diodes which can nowadays be found in the displays of TVs and smartphones. Moreover, organic semiconductors will possibly lead to new electronic applications which rely on the unique mechanical and electrical properties of these materials. In order to push the development and the success of organic semiconductors forward, it is essential to understand the fundamental processes in these materials. This thesis concentrates on understanding how the charge transport in thiophene-based semiconductor layers depends on the layer morphology and how the charge transport properties can be intentionally modified by doping these layers with a strong electron acceptor. By means of optical spectroscopy, the layer morphologies of poly(3-hexylthiophene), P3HT, P3HT-fullerene bulk heterojunction blends, and oligomeric polyquaterthiophene, oligo-PQT-12, are studied as a function of temperature, molecular weight, and processing conditions. The analyses rely on the decomposition of the absorption contributions from the ordered and the disordered parts of the layers. The ordered-phase spectra are analyzed using Spano’s model. It is figured out that the fraction of aggregated chains and the interconnectivity of these domains is fundamental to a high charge carrier mobility. In P3HT layers, such structures can be grown with high-molecular weight, long P3HT chains. Low and medium molecular weight P3HT layers do also contain a significant amount of chain aggregates with high intragrain mobility; however, intergranular connectivity and, therefore, efficient macroscopic charge transport are absent. In P3HT-fullerene blend layers, a highly crystalline morphology that favors the hole transport and the solar cell efficiency can be induced by annealing procedures and the choice of a high-boiling point processing solvent. Based on scanning near-field and polarization optical microscopy, the morphology of oligo-PQT-12 layers is found to be highly crystalline which explains the rather high field-effect mobility in this material as compared to low molecular weight polythiophene fractions. On the other hand, crystalline dislocations and grain boundaries are identified which clearly limit the charge carrier mobility in oligo-PQT-12 layers. The charge transport properties of organic semiconductors can be widely tuned by molecular doping. Indeed, molecular doping is a key to highly efficient organic light-emitting diodes and solar cells. Despite this vital role, it is still not understood how mobile charge carriers are induced into the bulk semiconductor upon the doping process. This thesis contains a detailed study of the doping mechanism and the electrical properties of P3HT layers which have been p-doped by the strong molecular acceptor tetrafluorotetracyanoquinodimethane, F4TCNQ. The density of doping-induced mobile holes, their mobility, and the electrical conductivity are characterized in a broad range of acceptor concentrations. A long-standing debate on the nature of the charge transfer between P3HT and F4TCNQ is resolved by showing that almost every F4TCNQ acceptor undergoes a full-electron charge transfer with a P3HT site. However, only 5% of these charge transfer pairs can dissociate and induce a mobile hole into P3HT which contributes electrical conduction. Moreover, it is shown that the left-behind F4TCNQ ions broaden the density-of-states distribution for the doping-induced mobile holes, which is due to the longrange Coulomb attraction in the low-permittivity organic semiconductors.
The problem under consideration in the thesis is a two level atom in a photonic crystal and a pumping laser. The photonic crystal provides an environment for the atom, that modifies the decay of the exited state, especially if the atom frequency is close to the band gap. The population inversion is investigated als well as the emission spectrum. The dynamics is analysed in the context of open quantum systems. Due to the multiple reflections in the photonic crystal, the system has a finite memory that inhibits the Markovian approximation. In the Heisenberg picture the equations of motion for the system variables form a infinite hierarchy of integro-differential equations. To get a closed system, approximations like a weak coupling approximation are needed. The thesis starts with a simple photonic crystal that is amenable to analytic calculations: a one-dimensional photonic crystal, that consists of alternating layers. The Bloch modes inside and the vacuum modes outside a finite crystal are linked with a transformation matrix that is interpreted as a transfer matrix. Formulas for the band structure, the reflection from a semi-infinite crystal, and the local density of states in absorbing crystals are found; defect modes and negative refraction are discussed. The quantum optics section of the work starts with the discussion of three problems, that are related to the full resonance fluorescence problem: a pure dephasing model, the driven atom and resonance fluorescence in free space. In the lowest order of the system-environment coupling, the one-time expectation values for the full problem are calculated analytically and the stationary states are discussed for certain cases. For the calculation of the two time correlation functions and spectra, the additional problem of correlations between the two times appears. In the Markovian case, the quantum regression theorem is valid. In the general case, the fluctuation dissipation theorem can be used instead. The two-time correlation functions are calculated by the two different methods. Within the chosen approximations, both methods deliver the same result. Several plots show the dependence of the spectrum on the parameters. Some examples for squeezing spectra are shown with different approximations. A projection operator method is used to establish two kinds of Markovian expansion with and without time convolution. The lowest order is identical with the lowest order of system environment coupling, but higher orders give different results.
The life of microorganisms is characterized by two main tasks, rapid growth under conditions permitting growth and survival under stressful conditions. The environments, in which microorganisms dwell, vary in space and time. The microorganisms innovate diverse strategies to readily adapt to the regularly fluctuating environments. Phenotypic heterogeneity is one such strategy, where an isogenic population splits into subpopulations that respond differently under identical environments. Bacterial persistence is a prime example of such phenotypic heterogeneity, whereby a population survives under an antibiotic attack, by keeping a fraction of population in a drug tolerant state, the persister state. Specifically, persister cells grow more slowly than normal cells under growth conditions, but survive longer under stress conditions such as the antibiotic administrations. Bacterial persistence is identified experimentally by examining the population survival upon an antibiotic treatment and the population resuscitation in a growth medium. The underlying population dynamics is explained with a two state model for reversible phenotype switching in a cell within the population. We study this existing model with a new theoretical approach and present analytical expressions for the time scale observed in population growth and resuscitation, that can be easily used to extract underlying model parameters of bacterial persistence. In addition, we recapitulate previously known results on the evolution of such structured population under periodically fluctuating environment using our simple approximation method. Using our analysis, we determine model parameters for Staphylococcus aureus population under several antibiotics and interpret the outcome of cross-drug treatment. Next, we consider the expansion of a population exhibiting phenotype switching in a spatially structured environment consisting of two growth permitting patches separated by an antibiotic patch. The dynamic interplay of growth, death and migration of cells in different patches leads to distinct regimes in population propagation speed as a function of migration rate. We map out the region in parameter space of phenotype switching and migration rate to observe the condition under which persistence is beneficial. Furthermore, we present an extended model that allows mutation from the two phenotypic states to a resistant state. We find that the presence of persister cells may enhance the probability of resistant mutation in a population. Using this model, we explain the experimental results showing the emergence of antibiotic resistance in a Staphylococcus aureus population upon tobramycin treatment. In summary, we identify several roles of bacterial persistence, such as help in spatial expansion, development of multidrug tolerance and emergence of antibiotic resistance. Our study provides a theoretical perspective on the dynamics of bacterial persistence in different environmental conditions. These results can be utilized to design further experiments, and to develop novel strategies to eradicate persistent infections.
Within the course of this thesis, I have investigated the complex interplay between electron and lattice dynamics in nanostructures of perovskite oxides. Femtosecond hard X-ray pulses were utilized to probe the evolution of atomic rearrangement directly, which is driven by ultrafast optical excitation of electrons. The physics of complex materials with a large number of degrees of freedom can be interpreted once the exact fingerprint of ultrafast lattice dynamics in time-resolved X-ray diffraction experiments for a simple model system is well known. The motion of atoms in a crystal can be probed directly and in real-time by femtosecond pulses of hard X-ray radiation in a pump-probe scheme. In order to provide such ultrashort X-ray pulses, I have built up a laser-driven plasma X-ray source. The setup was extended by a stable goniometer, a two-dimensional X-ray detector and a cryogen-free cryostat. The data acquisition routines of the diffractometer for these ultrafast X-ray diffraction experiments were further improved in terms of signal-to-noise ratio and angular resolution. The implementation of a high-speed reciprocal-space mapping technique allowed for a two-dimensional structural analysis with femtosecond temporal resolution. I have studied the ultrafast lattice dynamics, namely the excitation and propagation of coherent phonons, in photoexcited thin films and superlattice structures of the metallic perovskite SrRuO3. Due to the quasi-instantaneous coupling of the lattice to the optically excited electrons in this material a spatially and temporally well-defined thermal stress profile is generated in SrRuO3. This enables understanding the effect of the resulting coherent lattice dynamics in time-resolved X-ray diffraction data in great detail, e.g. the appearance of a transient Bragg peak splitting in both thin films and superlattice structures of SrRuO3. In addition, a comprehensive simulation toolbox to calculate the ultrafast lattice dynamics and the resulting X-ray diffraction response in photoexcited one-dimensional crystalline structures was developed in this thesis work. With the powerful experimental and theoretical framework at hand, I have studied the excitation and propagation of coherent phonons in more complex material systems. In particular, I have revealed strongly localized charge carriers after above-bandgap femtosecond photoexcitation of the prototypical multiferroic BiFeO3, which are the origin of a quasi-instantaneous and spatially inhomogeneous stress that drives coherent phonons in a thin film of the multiferroic. In a structurally imperfect thin film of the ferroelectric Pb(Zr0.2Ti0.8)O3, the ultrafast reciprocal-space mapping technique was applied to follow a purely strain-induced change of mosaicity on a picosecond time scale. These results point to a strong coupling of in- and out-of-plane atomic motion exclusively mediated by structural defects.
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.
Nucleation and growth of unsubstituted metal phthalocyanine films from solution on planar substrates
(2012)
In den vergangenen Jahren wurden kosteneffiziente nasschemische Beschichtungsverfahren für die Herstellung organischer Dünnfilme für verschiedene opto-elektronische Anwendungen entdeckt und weiterentwickelt. Unter anderem wurden Phthalocyanin-Moleküle in photoaktiven Schichten für die Herstellung von Solarzellen intensiv erforscht. Aufgrund der kleinen bzw. unbekannten Löslichkeit wurden Phthalocyanin-Schichten durch Aufdampfverfahren im Vakuum hergestellt. Des Weiteren wurde die Löslichkeit durch chemische Synthese erhöht, was aber die Eigenschaften von Pc beeinträchtigte. In dieser Arbeit wurde die Löslichkeit, optische Absorption und Stabilität von 8 verschiedenen unsubstituierten Metall-Phthalocyaninen in 28 verschiedenen Lösungsmitteln quantitativ gemessen. Wegen ausreichender Löslichkeit, Stabilität und Anwendbarkeit in organischen Solarzellen wurde Kupferphthalocyanin (CuPc) in Trifluoressigsäure (TFA) für weitere Untersuchungen ausgewählt. Durch die Rotationsbeschichtung von CuPc aus TFA Lösung wurde ein dünner Film aus der verdampfenden Lösung auf dem Substrat platziert. Nach dem Verdampfen des Lösungsmittels, die Nanobändern aus CuPc bedecken das Substrat. Die Nanobänder haben eine Dicke von etwa ~ 1 nm (typische Dimension eines CuPc-Molekül) und variierender Breite und Länge, je nach Menge des Materials. Solche Nanobändern können durch Rotationsbeschichtung oder auch durch andere Nassbeschichtungsverfahren, wie Tauchbeschichtung, erzeugt werden. Ähnliche Fibrillen-Strukturen entstehen durch Nassbeschichtung von anderen Metall-Phthalocyaninen, wie Eisen- und Magnesium-Phthalocyanin, aus TFA-Lösung sowie auf anderen Substraten, wie Glas oder Indium Zinnoxid. Materialeigenschaften von aufgebrachten CuPc aus TFA Lösung und CuPc in der Lösung wurden ausführlich mit Röntgenbeugung, Spektroskopie- und Mikroskopie Methoden untersucht. Es wird gezeigt, dass die Nanobänder nicht in der Lösung, sondern durch Verdampfen des Lösungsmittels und der Übersättigung der Lösung entstehen. Die Rasterkraftmikroskopie wurde dazu verwendet, um die Morphologie des getrockneten Films bei unterschiedlicher Konzentration zu studieren. Der Mechanismus der Entstehung der Nanobändern wurde im Detail studiert. Gemäß der Keimbildung und Wachstumstheorie wurde die Entstehung der CuPc Nanobänder aus einer übersättigt Lösung diskutiert. Die Form der Nanobändern wurde unter Berücksichtigung der Wechselwirkung zwischen den Molekülen und dem Substrat diskutiert. Die nassverarbeitete CuPc-Dünnschicht wurde als Donorschicht in organischen Doppelschicht Solarzellen mit C60-Molekül, als Akzeptor eingesetzt. Die Effizienz der Energieumwandlung einer solchen Zelle wurde entsprechend den Schichtdicken der CuPc Schicht untersucht.
Estimation of the self-similarity exponent has attracted growing interest in recent decades and became a research subject in various fields and disciplines. Real-world data exhibiting self-similar behavior and/or parametrized by self-similarity exponent (in particular Hurst exponent) have been collected in different fields ranging from finance and human sciencies to hydrologic and traffic networks. Such rich classes of possible applications obligates researchers to investigate qualitatively new methods for estimation of the self-similarity exponent as well as identification of long-range dependencies (or long memory). In this thesis I present the Bayesian estimation of the Hurst exponent. In contrast to previous methods, the Bayesian approach allows the possibility to calculate the point estimator and confidence intervals at the same time, bringing significant advantages in data-analysis as discussed in this thesis. Moreover, it is also applicable to short data and unevenly sampled data, thus broadening the range of systems where the estimation of the Hurst exponent is possible. Taking into account that one of the substantial classes of great interest in modeling is the class of Gaussian self-similar processes, this thesis considers the realizations of the processes of fractional Brownian motion and fractional Gaussian noise. Additionally, applications to real-world data, such as the data of water level of the Nile River and fixational eye movements are also discussed.
Actin is one of the most abundant and highly conserved proteins in eukaryotic cells. The globular protein assembles into long filaments, which form a variety of different networks within the cytoskeleton. The dynamic reorganization of these networks - which is pivotal for cell motility, cell adhesion, and cell division - is based on cycles of polymerization (assembly) and depolymerization (disassembly) of actin filaments. Actin binds ATP and within the filament, actin-bound ATP is hydrolyzed into ADP on a time scale of a few minutes. As ADP-actin dissociates faster from the filament ends than ATP-actin, the filament becomes less stable as it grows older. Recent single filament experiments, where abrupt dynamical changes during filament depolymerization have been observed, suggest the opposite behavior, however, namely that the actin filaments become increasingly stable with time. Several mechanisms for this stabilization have been proposed, ranging from structural transitions of the whole filament to surface attachment of the filament ends. The key issue of this thesis is to elucidate the unexpected interruptions of depolymerization by a combination of experimental and theoretical studies. In new depolymerization experiments on single filaments, we confirm that filaments cease to shrink in an abrupt manner and determine the time from the initiation of depolymerization until the occurrence of the first interruption. This duration differs from filament to filament and represents a stochastic variable. We consider various hypothetical mechanisms that may cause the observed interruptions. These mechanisms cannot be distinguished directly, but they give rise to distinct distributions of the time until the first interruption, which we compute by modeling the underlying stochastic processes. A comparison with the measured distribution reveals that the sudden truncation of the shrinkage process neither arises from blocking of the ends nor from a collective transition of the whole filament. Instead, we predict a local transition process occurring at random sites within the filament. The combination of additional experimental findings and our theoretical approach confirms the notion of a local transition mechanism and identifies the transition as the photo-induced formation of an actin dimer within the filaments. Unlabeled actin filaments do not exhibit pauses, which implies that, in vivo, older filaments become destabilized by ATP hydrolysis. This destabilization can be identified with an acceleration of the depolymerization prior to the interruption. In the final part of this thesis, we theoretically analyze this acceleration to infer the mechanism of ATP hydrolysis. We show that the rate of ATP hydrolysis is constant within the filament, corresponding to a random as opposed to a vectorial hydrolysis mechanism.