Refine
Has Fulltext
- yes (60) (remove)
Year of publication
- 2020 (60) (remove)
Document Type
- Postprint (34)
- Doctoral Thesis (26)
Is part of the Bibliography
- yes (60)
Keywords
- diffusion (8)
- perovskite solar cells (3)
- random diffusivity (3)
- Datenassimilation (2)
- Galaktische Archäologie (2)
- Rashba effect (2)
- Rashba-Effekt (2)
- climate (2)
- impact (2)
- model (2)
- photoluminescence (2)
- physics education (2)
- 2D perovskites (1)
- 3D field calculations (1)
- 3D-Feldsimulationen (1)
- AG (1)
- ARPES (1)
- Amazon rainforest (1)
- Anisotropy (1)
- Antibiotics (1)
- Arctic-midlatitude linkages (1)
- Arktis (1)
- Astronomical instrumentation (1)
- Astrophysik (1)
- Astroteilchenphysik (1)
- Atlantic meridional overturning circulation (1)
- Atmosphäre (1)
- Atomic Force Microscope (1)
- Azobenzene containing surfactant (1)
- Azobenzol enthaltende Moleküle (1)
- Azobenzol enthaltendes Tensid (1)
- BESSY II. (1)
- BL Lacertae objects: individual (1)
- Bacterial biofilms (1)
- Bending energy (1)
- Beugungseffizienz (1)
- Biegeenergie (1)
- Biofilme (1)
- Biofilms (1)
- Biological defense mechanisms (1)
- Brownian motion (1)
- Cell-cell adhesion (1)
- Conic compartments (1)
- Cystic fibrosis (1)
- Data assimilation (1)
- Dielectric Relaxation Spectroscopy (DRS) (1)
- Dielektrophorese (1)
- Differential Scanning Calorimetry (DSC) (1)
- Diffusion (1)
- Donor-Acceptor (DA) interface (1)
- Dwarf galaxies (1)
- Einzel-Objekt-Nachweis (1)
- Einzelmolekül-Biosensor (1)
- El Niño-Southern Oscillation (ENSO) (1)
- El Niño-Südliche Oszillation (1)
- Elektret (1)
- Experimental Physics (1)
- Experimentalphysik (1)
- Experimente (1)
- FELS (1)
- Fibre-fed spectroscopy (1)
- Flow (1)
- Fluorescence fluctuation spectroscopy (1)
- Fluoreszenz-Mikroskopie (1)
- Fluoreszenzfluktuationsspektroskopie (1)
- Fluoreszenzproteine (1)
- Forecasting (1)
- Formgleichungen von Vesikeln (1)
- Forschend Entdeckendes Lernen (1)
- Fractals (1)
- Fraktale (1)
- Galactic Archaeology (1)
- Galactic archaeology (1)
- Galaxien (1)
- Galaxienbalken (1)
- Galaxienbulges (1)
- Galaxienentwicklung (1)
- Galaxienstruktur (1)
- Gammastrahlung (1)
- Gammastrahlungsastronomie (1)
- Geomagnetic activity (1)
- Geomagnetische Aktivität (1)
- Gitterdynamik (1)
- Graphen (1)
- Greenland (1)
- Grenzflächenrekombination (1)
- Halo der Milchstraße (1)
- Heterogenität (1)
- Hydrophobic and hydrophillic interactions (1)
- ICON (1)
- IMAGE EUV (1)
- ISOS-L-1I protocol (1)
- Inner magnetosphere (1)
- Innere Magnetosphäre (1)
- Integrale Feldspektroskopie (1)
- Isomerisierung Kinetik (1)
- Jahreszeitenvorhersage (1)
- Janus colloids (1)
- Janus-Kolloid (1)
- Kalman Filter (1)
- Kalman filter (1)
- Kegelförmige Geometrien (1)
- Kinetics of photoisomerization (1)
- Klima (1)
- Klimatologie (1)
- Kp index (1)
- Kp-Index (1)
- Kuramoto model (1)
- L-Asterisk (1)
- LDDO (1)
- Ladungsspeicherung und -transport (1)
- Lernumgebung (1)
- Licht-Materie-Wechselwirkung (1)
- Light-Matter Coupling (1)
- Machine learning (1)
- Magellanic Clouds (1)
- Magellansche Wolken (1)
- Maschinelles Lernen (1)
- Metal-poor stars (1)
- Metallarme Sterne (1)
- Meteorologie (1)
- Milchstraße (1)
- Milky Way (1)
- Milky Way Halo (1)
- Min-Proteine (1)
- Min-proteins (1)
- Modal expansion method (1)
- Modeling (1)
- Modellieren (1)
- Molecular motors (1)
- Molekulare Motoren (1)
- Multi-object spectroscopy (1)
- Multimode fibres (1)
- Nano-Elektroden (1)
- Nanoparticles (1)
- Nanopartikel (1)
- Neural networks (1)
- Neuronale Netze (1)
- Oberflächengitter (1)
- Ornstein-Uhlenbeck Process (1)
- Ornstein-Uhlenbeck Prozess (1)
- Ornstein–Uhlenbeck process (1)
- Perceived Relevance (1)
- Percolation (1)
- Perkolation (1)
- Perowskit (1)
- Perowskit Solarzellen (1)
- Photochemie (1)
- Photochemistry (1)
- Photopolymer (1)
- Photostrukturierung von Polymerfilmen (1)
- Photovoltaik (1)
- Physics Education (1)
- Physics Problems (1)
- Physikaufgaben (1)
- Physikdidaktik (1)
- Plasmasphere (1)
- Plasmasphäre (1)
- Plasmonen (1)
- Plasmons (1)
- Polypropylen (1)
- Polystyrol Nano-Sphären (1)
- Porous silica particles (1)
- Prognose (1)
- Pseudomonas aeruginosa (1)
- R-PE (1)
- REM (1)
- Random Environments (1)
- Random Walk (1)
- Rasterkraftmikroskopie (1)
- Repertory Grid (1)
- Resolved and unresolved sources as a function of wavelength (1)
- Ringstromelektronen (1)
- SEM (1)
- Seasonal prediction (1)
- Shape equations of vesicles (1)
- Smartphone (1)
- Solarzellen (1)
- Spin Textur (1)
- Sputum (1)
- Sterne (1)
- Sternenpopulationen (1)
- Strahlungsgürtel (1)
- Stratosphere-troposphere coupling (1)
- Stratospheric polar vortex (1)
- Stratosphären-Troposphären-Kopplung (1)
- Stratosphärischer Polarwirbel (1)
- Streuresonanzen (1)
- Supernovaüberrest (1)
- Surface Relief Grating (SRG) (1)
- Svalbard (1)
- Teilchenbeschleunigung (1)
- Topological Crystalline Insulator (1)
- Topological Insulator (1)
- Topologischer Isolator (1)
- Topologischer kristalliner Isolator (1)
- Validation (1)
- Validierung (1)
- Verbindungspfade zwischen der Arktis und den mittleren Breiten (1)
- Wellen-Teilchen Wechselwirkungen (1)
- Wismut (1)
- X-ray absorption (1)
- X-ray emission (1)
- X-ray photoelectron spectroscopy (1)
- X-ray probe (1)
- Zell-zell Adhäsion (1)
- Zufällige Stochastische Irrfahrt (1)
- Zufällige Umgebungen (1)
- Zwerg Galaxien (1)
- actin polymerization (1)
- activator–inhibitor models (1)
- anomalous diffusion (1)
- arctic (1)
- astroparticle physics (1)
- astrophysics (1)
- atmosphere (1)
- atomic force microscopy (AFM) (1)
- azobenzene (1)
- azobenzene containing molecules (1)
- bifurcation theory (1)
- biofilms (1)
- bismuth (1)
- blended learning (1)
- cesium lead halides (1)
- chains (1)
- channel (1)
- charge storage and transport (1)
- chemische Oberflächen-Modifikationen (1)
- climatology (1)
- complex networks (1)
- consequences (1)
- continuous time random walk (1)
- costs (1)
- coupled rotators (1)
- crystal orientation (1)
- data assimilation (1)
- dielectrophoresis (1)
- diffraction efficiency (1)
- diffusioosmotic flow (1)
- diffusioosmotischer Fluss (1)
- diselenide (1)
- donor (1)
- droughts (1)
- efficiency (1)
- electret (1)
- electron-transfer (1)
- electrons (1)
- energy (1)
- ensemble and time averaged mean squared displacement (1)
- entropy production (1)
- excitonic materials (1)
- expanding medium (1)
- experiment (1)
- exploit (1)
- extremal values (1)
- fastest first-passage time of N walkers (1)
- ferroelectric polymers (1)
- films (1)
- first passage (1)
- first-passage (1)
- flagellum (1)
- fluorescence microscopy (1)
- fluorescent proteins (1)
- fluorinated organic spacer (1)
- fractional Brownian motion (1)
- galaxies (1)
- galaxies: high-redshift (1)
- galaxy bars (1)
- galaxy bulges (1)
- galaxy evolution (1)
- galaxy structure (1)
- gamma rays (1)
- gamma-ray astronomy (1)
- gamma-rays: general (1)
- gas phase electron spectroscopy (1)
- global surface warming (1)
- grafted polymers (1)
- graphene (1)
- heterojunction silicon solar cells (1)
- hydrophoben und hydrophile Wechselwirkungen (1)
- hysteresis (1)
- inorganic perovskites (1)
- inquiry based learning (1)
- instability (1)
- integral field spectroscopy (1)
- interfaces (1)
- interfacial recombination (1)
- lattice dynamics (1)
- learning environment (1)
- magnetosphere (1)
- mass conservation (1)
- mathematical modeling (1)
- mathematische Modellierung (1)
- maximum and range (1)
- mechanical and acoustical properties (1)
- mechanische und akustische Eigenschaften (1)
- metal halide perovskites (1)
- meteorology (1)
- mhd turbulence (1)
- mid-temperature transition (1)
- molecular bottle brushes (1)
- molecular dynamics (1)
- motivation (1)
- nano-electrodes (1)
- nanoscale heat transfer (1)
- nanoskaliger Wärmetransport (1)
- nichtstrahlende Verluste (1)
- noisy systems (1)
- non-Gaussianity (1)
- non-gaussianity (1)
- non-radiative interface recombination (1)
- nonfullerene acceptors (1)
- nonlinear waves (1)
- nonradiative losses (1)
- numerical simulations (1)
- ocean heat uptake (1)
- organic solar cell (1)
- organic solar cells (1)
- origins (1)
- parallel immobilization of biomolecules (1)
- parallele Immobilisierung von Biomolekülen (1)
- particle acceleration (1)
- pattern formation (1)
- perovskite (1)
- phase purity (1)
- phase transition (1)
- photo-isomerization kinetics (1)
- photo-sensitive surfactant (1)
- photo-structuring of polymer films (1)
- photon recycling (1)
- photosensitive Polymer (1)
- photostability (1)
- photovoltaics (1)
- piezoelectric polymers (1)
- poly (acrylic acid, sodium salt) (1)
- polypropylene (1)
- polystyrene nano-spheres (1)
- poröse Siliciumdioxidpartikel (1)
- power spectral analysis (1)
- power spectrum (1)
- pre-service teachers (1)
- prediction (1)
- projections (1)
- pyroelectric polymers (1)
- quantum mechanics (1)
- quasi-Fermi level (1)
- quasi-steady-state photoinduced absorptions (1)
- radiation belts (1)
- recombinations (1)
- repertory grid (1)
- resonant inelastic X-ray scattering; (1)
- ring current electrons (1)
- scattering resonances (1)
- seperation (1)
- sequence-controlled polymers (1)
- simulations (1)
- single chain folding (1)
- single trajectories (1)
- single-molecule biosensor (1)
- single-object detection (1)
- soft X-ray beamline (1)
- soft X-ray spectroscopy (1)
- solar cells (1)
- solar coronal mass ejections (1)
- solar storm (1)
- space-dependent diffusivity (1)
- spatial localization (1)
- spektrale Leistungsdichte (1)
- spin texture (1)
- splitting (1)
- star formation (1)
- stars (1)
- stationary stochastic process (1)
- stellar coronal mass ejections (1)
- stellar populations (1)
- sub-diffraction gratings (1)
- supernova remnant (1)
- surface (1)
- surface chemical treatment (1)
- synchronization transition (1)
- tandem solar cells (1)
- temperature dependence (1)
- thermodynamics (1)
- thin films (1)
- time (1)
- transcrystalline polypropylene (1)
- transfer dynamics (1)
- transkristallines Polypropylen (1)
- ultrafast magnetism (1)
- ultrafast x-ray diffraction (1)
- ultraschnelle Röntgendiffraktion (1)
- ultraschneller Magnetimus (1)
- vinylidenefluoride (VDF)-based polymers (1)
- voltage losses (1)
- wave-particle interactions (1)
- zufälligen Diffusivität (1)
- wahrgenommene Relevanz (1)
Institute
- Institut für Physik und Astronomie (60) (remove)
During the last decade, intracellular actin waves have attracted much attention due to their essential role in various cellular functions, ranging from motility to cytokinesis. Experimental methods have advanced significantly and can capture the dynamics of actin waves over a large range of spatio-temporal scales. However, the corresponding coarse-grained theory mostly avoids the full complexity of this multi-scale phenomenon. In this perspective, we focus on a minimal continuum model of activator–inhibitor type and highlight the qualitative role of mass conservation, which is typically overlooked. Specifically, our interest is to connect between the mathematical mechanisms of pattern formation in the presence of a large-scale mode, due to mass conservation, and distinct behaviors of actin waves.
The Ornstein–Uhlenbeck process is a stationary and ergodic Gaussian process, that is fully determined by its covariance function and mean. We show here that the generic definitions of the ensemble- and time-averaged mean squared displacements fail to capture these properties consistently, leading to a spurious ergodicity breaking. We propose to remedy this failure by redefining the mean squared displacements such that they reflect unambiguously the statistical properties of any stochastic process. In particular we study the effect of the initial condition in the Ornstein–Uhlenbeck process and its fractional extension. For the fractional Ornstein–Uhlenbeck process representing typical experimental situations in crowded environments such as living biological cells, we show that the stationarity of the process delicately depends on the initial condition.
Over the last decades, the Arctic regions of the earth have warmed at a rate 2–3 times faster than the global average– a phenomenon called Arctic Amplification. A complex, non-linear interplay of physical processes and unique pecularities in the Arctic climate system is responsible for this, but the relative role of individual processes remains to be debated. This thesis focuses on the climate change and related processes on Svalbard, an archipelago in the North Atlantic sector of the Arctic, which is shown to be a "hotspot" for the amplified recent warming during winter. In this highly dynamical region, both oceanic and atmospheric large-scale transports of heat and moisture interfere with spatially inhomogenous surface conditions, and the corresponding energy exchange strongly shapes the atmospheric boundary layer. In the first part, Pan-Svalbard gradients in the surface air temperature (SAT) and sea ice extent (SIE) in the fjords are quantified and characterized. This analysis is based on observational data from meteorological stations, operational sea ice charts, and hydrographic observations from the adjacent ocean, which cover the 1980–2016 period. It is revealed that typical estimates of SIE during late winter range from 40–50% (80–90%) in the western (eastern) parts of Svalbard. However, strong SAT warming during winter of the order of 2–3K per decade dictates excessive ice loss, leaving fjords in the western parts essentially ice-free in recent winters. It is further demostrated that warm water currents on the west coast of Svalbard, as well as meridional winds contribute to regional differences in the SIE evolution. In particular, the proximity to warm water masses of the West Spitsbergen Current can explain 20–37% of SIE variability in fjords on west Svalbard, while meridional winds and associated ice drift may regionally explain 20–50% of SIE variability in the north and northeast. Strong SAT warming has overruled these impacts in recent years, though.
In the next part of the analysis, the contribution of large-scale atmospheric circulation changes to the Svalbard temperature development over the last 20 years is investigated. A study employing kinematic air-back trajectories for Ny-Ålesund reveals a shift in the source regions of lower-troposheric air over time for both the winter and the summer season. In winter, air in the recent decade is more often of lower-latitude Atlantic origin, and less frequent of Arctic origin. This affects heat- and moisture advection towards Svalbard, potentially manipulating clouds and longwave downward radiation in that region. A closer investigation indicates that this shift during winter is associated with a strengthened Ural blocking high and Icelandic low, and contributes about 25% to the observed winter warming on Svalbard over the last 20 years. Conversely, circulation changes during summer include a strengthened Greenland blocking high which leads to more frequent cold air advection from the central Arctic towards Svalbard, and less frequent air mass origins in the lower latitudes of the North Atlantic. Hence, circulation changes during winter are shown to have an amplifying effect on the recent warming on Svalbard, while summer circulation changes tend to mask warming.
An observational case study using upper air soundings from the AWIPEV research station in Ny-Ålesund during May–June 2017 underlines that such circulation changes during summer are associated with tropospheric anomalies in temperature, humidity and boundary layer height.
In the last part of the analysis, the regional representativeness of the above described changes around Svalbard for the broader Arctic is investigated. Therefore, the terms in the diagnostic temperature equation in the Arctic-wide lower troposphere are examined for the Era-Interim atmospheric reanalysis product. Significant positive trends in diabatic heating rates, consistent with latent heat transfer to the atmosphere over regions of increasing ice melt, are found for all seasons over the Barents/Kara Seas, and in individual months in the vicinity of Svalbard. The above introduced warm (cold) advection trends during winter (summer) on Svalbard are successfully reproduced. Regarding winter, they are regionally confined to the Barents Sea and Fram Strait, between 70°–80°N, resembling a unique feature in the whole Arctic. Summer cold advection trends are confined to the area between eastern Greenland and Franz Josef Land, enclosing Svalbard.
The goal of this thesis was to thoroughly investigate the behavior of multimode fibres to aid the development of modern and forthcoming fibre-fed spectrograph systems. Based on the Eigenmode Expansion Method, a field propagation model was created that can emulate effects in fibres relevant for astronomical spectroscopy, such as modal noise, scrambling, and focal ratio degradation. These effects are of major concern for any fibre-coupled spectrograph used in astronomical research. Changes in the focal ratio, modal distribution of light or non-perfect scrambling limit the accuracy of measurements, e.g. the flux determination of the astronomical object, the sky-background subtraction and detection limit for faint galaxies, or the spectral line position accuracy used for the detection of extra-solar planets.
Usually, fibres used for astronomical instrumentation are characterized empirically through tests. The results of this work allow to predict the fibre behaviour under various conditions using sophisticated software tools to simulate the waveguide behaviour and mode transport of fibres.
The simulation environment works with two software interfaces. The first is the mode solver module FemSIM from Rsoft. It is used to calculate all the propagation modes and effective refractive indexes of a given system. The second interface consists of Python scripts which enable the simulation of the near- and far-field outputs of a given fibre. The characteristics of the input field can be manipulated to emulate real conditions. Focus variations, spatial translation, angular fluctuations, and disturbances through the mode coupling factor can also be simulated.
To date, complete coherent propagation or complete incoherent propagation can be simulated. Partial coherence was not addressed in this work. Another limitation of the simulations is that they work exclusively for the monochromatic case and that the loss coefficient of the fibres is not considered. Nevertheless, the simulations were able to match the results of realistic measurements.
To test the validity of the simulations, real fibre measurements were used for comparison. Two fibres with different cross-sections were characterized. The first fibre had a circular cross-section, and the second one had an octagonal cross-section. The utilized test-bench was originally developed for the prototype fibres of the 4MOST fibre feed characterization. It allowed for parallel laser beam measurements, light cone measurements, and scrambling measurements. Through the appropriate configuration, the acquisition of the near- and/or far-field was feasible.
By means of modal noise analysis, it was possible to compare the near-field speckle patterns of simulations and measurements as a function of the input angle. The spatial frequencies that originate from the modal interference could be analyzed by using the power spectral density analysis. Measurements and simulations yielded similar results. Measurements with induced modal scrambling were compared to simulations using incoherent propagation and once again similar results were achieved. Through both measurements and simulations, the enlargement of the near-field distribution could be observed and analyzed. The simulations made it possible to explain incoherent intensity fluctuations that appear in real measurements due to the field distribution of the active propagation modes.
By using the Voigt analysis in the far-field distribution, it was possible to separate the modal diffusion component in order to compare it with the simulations. Through an appropriate assessment, the modal diffusion component as a function of the input angle could be translated into angular divergence. The simulations gave the minimal angular divergence of the system. Through the mean of the difference between simulations and measurements, a figure of merit is given which can be used to characterize the angular divergence of real fibres using the simulations. Furthermore, it was possible to simulate light cone measurements. Due to the overall consistent results, it can be stated that the simulations represent a good tool to assist the fibre characterization process for fibre-fed spectrograph systems.
This work was possible through the BMBF Grant 05A14BA1 which was part of the phase A study of the fibre system for MOSAIC, a multi-object spectrograph for the Extremely Large Telescope (ELT-MOS).
We consider the emerging dynamics of a separable continuous time random walk (CTRW) in the case when the random walker is biased by a velocity field in a uniformly growing domain. Concrete examples for such domains include growing biological cells or lipid vesicles, biofilms and tissues, but also macroscopic systems such as expanding aquifers during rainy periods, or the expanding Universe. The CTRW in this study can be subdiffusive, normal diffusive or superdiffusive, including the particular case of a Lévy flight. We first consider the case when the velocity field is absent. In the subdiffusive case, we reveal an interesting time dependence of the kurtosis of the particle probability density function. In particular, for a suitable parameter choice, we find that the propagator, which is fat tailed at short times, may cross over to a Gaussian-like propagator. We subsequently incorporate the effect of the velocity field and derive a bi-fractional diffusion-advection equation encoding the time evolution of the particle distribution. We apply this equation to study the mixing kinetics of two diffusing pulses, whose peaks move towards each other under the action of velocity fields acting in opposite directions. This deterministic motion of the peaks, together with the diffusive spreading of each pulse, tends to increase particle mixing, thereby counteracting the peak separation induced by the domain growth. As a result of this competition, different regimes of mixing arise. In the case of Lévy flights, apart from the non-mixing regime, one has two different mixing regimes in the long-time limit, depending on the exact parameter choice: in one of these regimes, mixing is mainly driven by diffusive spreading, while in the other mixing is controlled by the velocity fields acting on each pulse. Possible implications for encounter–controlled reactions in real systems are discussed.
We study the experimentally measured ciprofloxacin antibiotic diffusion through a gel-like artificial sputum medium (ASM) mimicking physiological conditions typical for a cystic fibrosis layer, in which regions occupied by Pseudomonas aeruginosa bacteria are present. To quantify the antibiotic diffusion dynamics we employ a phenomenological model using a subdiffusion-absorption equation with a fractional time derivative. This effective equation describes molecular diffusion in a medium structured akin Thompson’s plumpudding model; here the ‘pudding’ background represents the ASM and the ‘plums’ represent the bacterial biofilm. The pudding is a subdiffusion barrier for antibiotic molecules that can affect bacteria found in plums. For the experimental study we use an interferometric method to determine the time evolution of the amount of antibiotic that has diffused through the biofilm. The theoretical model shows that this function is qualitatively different depending on whether or not absorption of the antibiotic in the biofilm occurs. We show that the process can be divided into three successive stages: (1) only antibiotic subdiffusion with constant biofilm parameters, (2) subdiffusion and absorption of antibiotic molecules with variable biofilm transport parameters, (3) subdiffusion and absorption in the medium but the biofilm parameters are constant again. Stage 2 is interpreted as the appearance of an intensive defence build–up of bacteria against the action of the antibiotic, and in the stage 3 it is likely that the bacteria have been inactivated. Times at which stages change are determined from the experimentally obtained temporal evolution of the amount of antibiotic that has diffused through the ASM with bacteria. Our analysis shows good agreement between experimental and theoretical results and is consistent with the biologically expected biofilm response. We show that an experimental method to study the temporal evolution of the amount of a substance that has diffused through a biofilm is useful in studying the processes occurring in a biofilm. We also show that the complicated biological process of antibiotic diffusion in a biofilm can be described by a fractional subdiffusion-absorption equation with subdiffusion and absorption parameters that change over time.
The current thesis is focused on the properties of graphene supported by metallic substrates and specifically on the behaviour of electrons in such systems. Methods of scanning tunneling microscopy, electron diffraction and photoemission spectroscopy were applied to study the structural and electronic properties of graphene. The purpose of the first part of this work is to introduce the most relevant aspects of graphene physics and the methodical background of experimental techniques used in the current thesis.
The scientific part of this work starts with the extensive study by means of scanning tunneling microscopy of the nanostructures that appear in Au intercalated graphene on Ni(111). This study was aimed to explore the possible structural explanations of the Rashba-type spin splitting of ~100 meV experimentally observed in this system — much larger than predicted by theory. It was demonstrated that gold can be intercalated under graphene not only as a dense monolayer, but also in the form of well-periodic arrays of nanoclusters, a structure previously not reported. Such nanocluster arrays are able to decouple graphene from the strongly interacting Ni substrate and render it quasi-free-standing, as demonstrated by our DFT study. At the same time calculations confirm strong enhancement of the proximity-induced SOI in graphene supported by such nanoclusters in comparison to monolayer gold. This effect, attributed to the reduced graphene-Au distance in the case of clusters, provides a large Rashba-type spin splitting of ~60 meV.
The obtained results not only provide a possible mechanism of SOI enhancement in this particular system, but they can be also generalized for graphene on other strongly interacting substrates intercalated by nanostructures of heavy noble d metals.
Even more intriguing is the proximity of graphene to heavy sp-metals that were predicted to induce an intrinsic SOI and realize a spin Hall effect in graphene. Bismuth is the heaviest stable sp-metal and its compounds demonstrate a plethora of exciting physical phenomena. This was the motivation behind the next part of the current thesis, where structural and electronic properties of a previously unreported phase of Bi-intercalated graphene on Ir(111) were studied by means of scanning tunneling microscopy, spin- and angle-resolved photoemission spectroscopy and electron diffraction. Photoemission experiments revealed a remarkable, nearly ideal graphene band structure with strongly suppressed signatures of interaction between graphene and the Ir(111) substrate, moreover, the characteristic moiré pattern observed in graphene on Ir(111) by electron diffraction and scanning tunneling microscopy was strongly suppressed after intercalation. The whole set of experimental data evidences that Bi forms a dense intercalated layer that efficiently decouples graphene from the substrate. The interaction manifests itself only in the n-type charge doping (~0.4 eV) and a relatively small band gap at the Dirac point (~190 meV). The origin of this minor band gap is quite intriguing and in this work it was possible to exclude a wide range of mechanisms that could be responsible for it, such as induced intrinsic spin-orbit interaction, hybridization with the substrate states and corrugation of the graphene lattice. The main origin of the band gap was attributed to the A-B symmetry breaking and this conclusion found support in the careful analysis of the interference effects in photoemission that provided the band gap estimate of ~140 meV.
While the previous chapters were focused on adjusting the properties of graphene by proximity to heavy metals, graphene on its own is a great object to study various physical effects at crystal surfaces. The final part of this work is devoted to a study of surface scattering resonances by means of photoemission spectroscopy, where this effect manifests itself as a distinct modulation of photoemission intensity. Though scattering resonances were widely studied in the past by means of electron diffraction, studies about their observation in photoemission experiments started to appear only recently and they are very scarce.
For a comprehensive study of scattering resonances graphene was selected as a versatile model system with adjustable properties. After the theoretical and historical introduction to the topic of scattering resonances follows a detailed description of the unusual features observed in the photoemission spectra obtained in this work and finally the equivalence between these features and scattering resonances is proven. The obtained photoemission results are in a good qualitative agreement with the existing theory, as verified by our calculations in the framework of the interference model. This simple model gives a suitable explanation for the general experimental observations.
The possibilities of engineering the scattering resonances were also explored. A systematic study of graphene on a wide range of substrates revealed that the energy position of the resonances is in a direct relation to the magnitude of charge transfer between graphene and the substrate. Moreover, it was demonstrated that the scattering resonances in graphene on Ir(111) can be suppressed by nanopatterning either by a superlattice of Ir nanoclusters or by atomic hydrogen. These effects were attributed to strong local variations of tork function and/or destruction of long-range order of thephene lattice. The tunability of scattering resonances can be applied for optoelectronic devices based on graphene. Moreover, the results of this study expand the general understanding of the phenomenon of scattering resonances and are applicable to many other materials besides graphene.
In the present study, we employ the angle-resolved photoemission spectroscopy (ARPES) technique to study the electronic structure of topological states of matter. In particular, the so-called topological crystalline insulators (TCIs) Pb1-xSnxSe and Pb1-xSnxTe, and the Mn-doped Z2 topological insulators (TIs) Bi2Te3 and Bi2Se3. The Z2 class of strong topological insulators is protected by time-reversal symmetry and is characterized by an odd number of metallic Dirac type surface states in the surface Brillouin zone. The topological crystalline insulators on the other hand are protected by the individual crystal symmetries and exhibit an even number of Dirac cones.
The topological properties of the lead tin chalcogenides topological crystalline insulators can be tuned by temperature and composition. Here, we demonstrate that Bi-doping of the Pb1-xSnxSe(111) epilayers induces a quantum phase transition from a topological crystalline insulator to a Z2 topological insulator. This occurs because Bi-doping lifts the fourfold valley degeneracy in the bulk. As a consequence a gap appears at ⌈¯, while the three Dirac cones at the M̅ points of the surface Brillouin zone remain intact. We interpret this new phase transition is caused by lattice distortion. Our findings extend the topological phase diagram enormously and make strong topological insulators switchable by distortions or electric field. In contrast, the bulk Bi doping of epitaxial Pb1-xSnxTe(111) films induces a giant Rashba splitting at the surface that can be tuned by the doping level. Tight binding calculations identify their origin as Fermi level pinning by trap states at the surface.
Magnetically doped topological insulators enable the quantum anomalous Hall effect (QAHE) which provide quantized edge states for lossless charge transport applications. The edge states are hosted by a magnetic energy gap at the Dirac point which has not been experimentally observed to date. Our low temperature ARPES studies unambiguously reveal the magnetic gap of Mn-doped Bi2Te3. Our analysis shows a five times larger gap size below the Tc than theoretically predicted. We assign this enhancement to a remarkable structure modification induced by Mn doping. Instead of a disordered impurity system, a self-organized alternating sequence of MnBi2Te4 septuple and Bi2Te3quintuple layers is formed. This enhances the wave-function overlap and gives rise to a large magnetic gap. Mn-doped Bi2Se3 forms similar heterostructure, but only a nonmagnetic gap is observed in this system. This correlates with the difference in magnetic anisotropy due to the much larger spin-orbit interaction in Bi2Te3 compared to Bi2Se3. These findings provide crucial insights for pushing lossless transport in topological insulators towards room-temperature applications.
Giant unilamellar vesicles are an important tool in todays experimental efforts to understand the structure and behaviour of biological cells. Their simple structure allows the isolation of the physical elastic properties of the lipid membrane. A central physical
property is the bending energy of the membrane, since the many different shapes of giant vesicles can be obtained by finding the minimum of the bending energy. In the spontaneous curvature model the bending energy is a function of the bending rigidity as well as the mean curvature and an additional parameter called the spontaneous curvature, which describes an internal preference of the lipid-bilayer to bend towards one side or the other. The spontaneous and mean curvature are local properties of the membrane.
Additional constraints arise from the conservation of the membrane surface area and the enclosed volume, which are global properties.
In this thesis the spontaneous curvature model is used to explain the experimental observation of a periodic shape oscillation of a giant unilamellar vesicle that was filled with a protein complex that periodically binds to and unbinds from the membrane.
By assuming that the binding of the proteins to the membrane induces a change in the spontaneous curvature the experimentally observed shapes could successfully be explained. This involves the numerical solution of the differential equations as obtained from the minimization of the bending energy respecting the area and volume constraints, the so called shape equations. Vice versa this approach can be used to estimate the spontaneous curvature from experimentally measurable quantities.
The second topic of this thesis is the analysis of concentration gradients in rigid conic membrane compartments. Gradients of an ideal gas due to gravity and gradients generated by the directed stochastic movement of molecular motors along a microtubulus were considered. It was possible to calculate the free energy and the bending energy analytically for the ideal gas. In the case of the non-equilibrium system with molecular motors, the characteristic length of the density profile, the jam-length, and its dependency on the opening angle of the conic compartment have been calculated in the mean-field limit.
The mean field results agree qualitatively with stochastic particle simulations.
We report on the detection of very high energy (VHE; E > 100 GeV) gamma-ray emission from the BL Lac objects KUV 00311-1938 and PKS 1440-389 with the High Energy Stereoscopic System (H.E.S.S.). H.E.S.S. observations were accompanied or preceded by multiwavelength observations with Fermi/LAT, XRT and UVOT onboard the Swift satellite, and ATOM. Based on an extrapolation of the Fermi/LAT spectrum towards the VHE gamma-ray regime, we deduce a 95 per cent confidence level upper limit on the unknown redshift of KUV 00311-1938 of z < 0.98 and of PKS 1440-389 of z < 0.53. When combined with previous spectroscopy results, the redshift of KUV 00311-1938 is constrained to 0.51 <= z < 0.98 and of PKS 1440-389 to 0.14 (sic) z < 0.53.