Universität Stuttgart
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Item Open Access Development and application of PICLas for combined optic-/plume-simulation of ion-propulsion systems(2019) Binder, Tilman; Fasoulas, Stefanos (Prof. Dr.-Ing.)Electric propulsion systems are an efficient option for altitude/attitude control and orbit transfers of spacecraft. One example is the gridded ion thruster which ionizes the propellant and accelerates the ions of the generated plasma by a high-voltage grid system. This work deals with the numerical simulation of the plasma flow starting near the grid system in the ionization chamber and leaving the thruster with high velocity. These simulations give direct insight into the modeled, physical interrelationships and can be used to investigate questions arising in the industrial development process of ion propulsion systems. The required simulation method is challenging due to the high degree of flow rarefaction and the plasma state itself, including freely moving ions and electrons. Applicable simulation methods belong to a particle-based, gas-kinetic approach, such as Particle-In-Cell (PIC) for the simulation of electromagnetic interaction and the Direct Simulation Monte Carlo (DSMC) for inter-particle collisions. The effects resulting from the finite size of a real system can only be investigated by simulating the complete, three-dimensional thruster geometry which requires a large and complex simulation domain. Acceptable simulation times are realized by expanding and using the framework of the coupled PIC-DSMC code PICLas in combination with high performance computing systems.Item Open Access Novel X-ray lenses for direct and coherent imaging(2019) Sanli, Umut Tunca; Schütz, Gisela (Prof. Dr.)Item Open Access Chirality effects in thermotropic and lyotropic nematic liquid crystals under confined geometries(2019) Dietrich, Clarissa; Giesselmann, Frank (Prof. Dr.)Chirality is a phenomenon in nature that appears across all disciplines of natural science, from biology to mathematics. The spontaneous formation of chiral structures in a system of achiral components is known as spontaneous mirror symmetry breaking and is by itself of fundamental interest leading also towards the question of the origin of homochirality in nature in general. In this work, we show that by means of the topology imposed by the confining geometry and by interfacial boundary conditions - in combination with the physical properties of a liquid crystal - spontaneous mirror symmetry broken structures can be obtained. They are analyzed, inter alia, with respect to the types of geometrical confinements used, e.g. how the confinement amplifies, induces, and influences the detection of chirality effects in order to facilitate the measurement of tiny amounts of chiral additives qualitatively and quantitatively.Item Open Access Functional renormalization group for strongly interacting Fermi systems(2019) Vilardi, Demetrio; Metzner, Walter (Prof. Dr.)The treatment of strongly interacting two-dimensional Fermi systems constitutes one of the most challenging problems in the field of condensed matter physics. Many theoretical works focused on the strongly coupled Hubbard model, since it is expected to capture the most important physics of cuprate superconductors. Due to our methodological improvements and understanding of the frequency dependence of the two-particle vertex function, the functional renormalization group combined with the dynamical mean field theory can now be used to study competing correlations in the strongly interacting regime. While limited to moderate interaction strengths, the functional renormalization group describes efficiently systems with a hierarchy of different energy scales and competing correlations. For instance, it provides definite evidence for d-wave superconductivity in the two-dimensional Hubbard model at moderate coupling. In a first project, we study the frequency dependence of the one-particle irreducible vertex function generated by the functional renormalization group flow. The frequency dependence, which becomes singular for strong interactions, appears to be important already for moderate couplings, and it cannot be represented by separate channels each depending on a single linear combination of frequencies. For strongly interacting systems, the dynamical mean field theory captures strong local correlation effects nonperturbatively. With this approximation, we study the impact of local correlations on the magnetic susceptibility. The local dynamics strongly affects the spin response function and its momentum dependence. In contrast to the widely used random-phase approximation with self-energy corrections that predicts Néel antiferromagnetic order, the local vertex corrections favor an incommensurate order similar to the ordering instability predicted by the Fermi surface geometry, as for weakly interacting systems. The dynamical mean field theory is also used as starting point for the functional renormalization group flow. We demonstrate that, due to our improvements for the parametrization of the vertex function, this approach is actually able to access the strong coupling physics. Moreover, we derive a flow scheme that conserves the local contributions and reduces the truncation error of the flow equations. In the strongly interacting regime, we capture strong d-wave pairing correlations driven by magnetic fluctuations with a mechanism similar to the one observed in the weakly interacting system.Item Open Access A scanning single-electron transistor array microscope probes the Hall potential profile in the fractional quantum Hall regime(2019) Gauß, Andreas W.; Weis, Jürgen (Prof. apl. Dr.)INTRO: In 1980 Klaus von Klitzing (Nobel prize in 1985) observed during low-temperature Hall measurements on two-dimensional electron systems hosted by MOSFETs, fixed values of the Hall resistances R_xy described with h/(ie^2) (i is integer) - nowadays denoted as integer quantum Hall effect (QHE). Since 1990 the QHE is used as a resistance standard and it played a key role in the redefinition of the Système Internationale d'unités (SI unit system), where from May 2019 the SI units are defined by fixing the values of fundamental physical constants as h, e, c and k_B. In 1982 Störmer, Tsui and Laughlin (Nobel prize in 1998) observed and discussed the fractional quantum Hall effect (FQHE) where further resistance plateaus are observable with R_xy=h/(ve^2) where v are special fractional numbers. The FQHE is currently understood on base of electron-electron interaction leading to quasi-particles with fractional effective charge. The main goal of this thesis was to use an one-dimensional single-electron transistor (SET) array as sensitive electrometer to locally probe Hall potential profiles in the fractional quantum Hall regime to determine where an externally biased current is distributed inside a two-dimensional electron system (2DES) hosted by an (Al,Ga)As heterostructure. MICROSCOPIC PICTURE: This thesis opens with an explanation of the microscopic picture of the integer quantum Hall effect where strong magnetic flux densities lead to the formation of Landau levels that are separated by an energy gap. This gap is responsible for the formation of electrically incompressible regions - with a well defined integer filling factor - within an otherwise compressible 2DES. A quantum Hall plateau shows two regimes: (1) the edge-dominated QH regime in the low magnetic field side and (2) the bulk-dominated QH regime in the high magnetic side of the plateau. SCANNING SET ARRAY MICROSCOPE: For experiments a scanning single-electron transistor (SET) array microscope with eight independent SETs on tips is used. The SET island sizes are about 155nm by 220nm, separated by 4µm. Single-electron charging energies up to 175µeV had been reached for these SETs. Measurements were performed at temperatures below 40mK in a 3He/4He dilution refrigerator with a 18T superconducting magnet, located in a highly vibrational reduced environment. MEASUREMENT PRINCIPLE: Electrostatic potential changes of the 2DES which result solely from an externally biased current are accessible via a two-step measurement technique probing calibrated Hall potential profiles. In this thesis a new method to extract and present local current density distributions from such Hall potential profiles is introduced. EXPERIMENTAL RESULTS: After systematic measurements of Hall potential profiles in the integer quantum Hall regime around filling factors v={3,2,1} the fractional quantum Hall regime with filling factors v=2/3 and v=3/5 is investigated for the first time with a scanning SET array microscope. Experimental results show a similar behavior for fractional and integer filling factors: (1) Hall potential profiles probed across the sample width evolve for varying magnetic flux densities in the same way, (2) the longitudinal resistance R_xx shows the same electrical breakdown behavior and (3) area scans in the fractional quantum Hall regime at fixed magnetic fields are spatially homogeneous. These similarities are seen for the first time and they contradict the widely used picture of a current transport along the edge. A final discussion at the end clarified, that the integer QH regime and the fractional QH regime have generally one thing in common: There is an evolution of the compressible/incompressible landscape within the 2DES which determines the current distribution in the 2DES. NEW SENSOR DEVELOPMENT: Additionally, a further milestone to the functionality of our scanning single-electron array microscope is developed: A free-standing Hall sensor tip which (i) allows another access to the current distribution inside the 2DES and (ii) makes also diamagnetic currents, which are already present in equilibrium, accessible. A calculation shows that this sensor can detect electrostatics and magnetic fields separately when a feedback loop is used.Item Open Access High-density hydrogen monolayer formation and isotope diffusion in porous media(2019) Balderas-Xicohténcatl, Rafael; Schütz, Gisela (Prof. Dr.)This thesis is focused on two fundamental aspects of hydrogen physisorption. First, the density of a single layer of adsorbed hydrogen molecules is studied at temperatures close the boiling point (20 K). These systematic experiments give an explanation to high H2 monolayer capacity as a high-density phase of adsorbed hydrogen. Secondly, the diffusion of the hydrogen molecules through a porous material (ZIF-8) is studied using a commercially available adsorption apparatus. Establishing that gas adsorption experiments can be used to study hydrogen diffusion and isotope separation at temperatures close the boiling point.Item Open Access Modeling and simulation of closed low-pressure adsorbers for thermal energy storage(2019) Schäfer, Micha; Thess, André (Prof. Dr. rer. nat.)Closed low-pressure adsorption systems can be applied for thermal energy storage. Their performance is determined by the mass and heat transport processes in the adsorber. Therefore, thorough knowledge of these transport processes is required for further storage development. The present thesis contributes to this by providing detailed models of closed low-pressure adsorbers and by conducting simulations over a broad range of parameters and configurations. The focus is on adsorbers of larger scale (length L = 0.1 . . . 1 m) and on the discharging process. As the adsorption pair, binderless zeolite 13X with water is examined. The models are developed in a stepwise manner from pore to storage scale. The Finite-Difference-Method is implemented to numerically solve the models. Simulations are conducted for defined reference cases as well as over a broad range of geometric and process parameters. The reference cases are analyzed in detail to gain a better understanding of the transport processes. Furthermore, the results are analyzed with respect to two particular modeling aspects: equilibrium assumptions and rarefaction effects (e. g. slip effect). With respect to the application, the discharging performance is analyzed in terms of thermal power and a defined discharging degree. Both the adsorber and the adsorbent configurations are varied. In addition, the effect of the discharging conditions is evaluated. Finally, one exemplary charging process is examined. The detailed analysis of the reference cases reveals that the mass and heat transport and the adsorption processes are strongly coupled and can only be understood in their interaction. For onedimensional adsorber configurations, that is the mass and heat transport are in the same direction, the discharging process is generally limited by the heat transport. This leads to insufficient thermal power and unsuitable discharging durations of up to one year. In contrast, for two-dimensional adsorber configurations, that is the mass and heat transport are in perpendicular directions, the discharging process can be limited either by the mass or heat transport or by the adsorption. The limitation depends on the configuration of the adsorber and adsorbent. Moreover, the twodimensional adsorber configurations can provide sufficient thermal power. With respect to the modeling, it is found that the assumption of a uniform pressure distribution is applicable for one-dimensional adsorber configurations. In contrast, for two-dimensional configurations, no equilibrium assumptions can be applied in general. However, for powder adsorbent it is always valid to assume local adsorption equilibrium. Regarding the rarefaction effects in twodimensional adsorber configurations with honeycombs and granules, the slip effect is relevant for small channel and particle diameters (d = 1 mm). For adsorbers with powder adsorbent, the reduction of the effective heat conductivity due to the rarefaction effect becomes relevant. With respect to the application, the variation of the adsorber configuration shows that the volumetric thermal power generally decreases with increasing adsorber length. Furthermore, the power decreases with increasing width between the parallel heat exchanger plates in the adsorber. Regarding the adsorbent configuration in two-dimensional adsorber configurations, it is found that the volumetric thermal power can be optimized by variation of the channel or particle diameter. Interestingly, the optima for peak and mean power do not coincide. In addition, the discharging degree is found to strongly depend on the discharging conditions in terms of discharging temperature and volume flow of the heat transfer fluid extracting the heat from the adsorber. In general, the discharging degree decreases with increasing discharging temperature. Similarly, the discharging degree decreases with increasing volume flow of the heat transfer fluid. Finally, the analysis of an exemplary charging process revealed that the pressure in the adsorber can increase significantly (> 50%) due to the desorption.Item Open Access Mid-infrared resonant nanostructures for in-vitro monitoring of polypeptides(2019) Semenyshyn, Rostyslav; Giessen, Harald (Prof. Dr.)Infrared vibrational spectroscopy is a technique based on the molecular vibrations, that is, the oscillation of individual atoms with respect to each other. Each of these vibrations has a characteristic resonance frequency which leads to the distinct vibrational fingerprint of a molecule and thus enables a label-free, non-destructive, and chemically specific detection of molecular species. The infrared absorption cross-sections, which characterize the optical interaction strength, are relatively small. This is of minor importance for conventional spectroscopy, where large ensembles of molecules can be measured and thus contribute to the overall signal. However, the small infrared absorption hampers detection of molecules at low concentrations, which is of large importance for medical diagnostics, for instance, where the determination of the secondary structures of proteins is crucial due to their role in many incurable diseases. A key to overcome this limitation is to utilize plasmonic nanostructures, which confine the electromagnetic radiation on the nanometer scale and allow higher overall absorption. In this dissertation, it is demonstrated that even a monolayer of proteins can be detected using mid-infrared resonant gold nanostructures. We use polypeptides as a model system and were able to investigate the secondary structure of molecular monolayers in-vitro. Applying different external stimuli, we are able to induce structural changes of polypeptides in aqueous environments. In addition to a mid-infrared resonant nanoantenna, nanoslits (or inverse antennas) can also enhance the optical response of polypeptides, which allowed us to detect the secondary structure of minicollagen monolayers. Both nanostructure designs provided the possibility even to monitor reversible conformational transitions of molecular monolayers. Scaling this approach down to a single nanostructure allows to detect only a few thousands of polypeptides in liquid environments. The demonstrated concept could lead to integrated chip-level technology for biological and even medical applications, where biosamples with minute concentrations are investigated. With further advances, it could be possible to scale the process to a few or single proteins and observe the structural changes of individual entities.Item Open Access Pressure-dependent dielectric spectroscopy measurements on organic spin liquid compounds(2019) Rösslhuber, Roland; Dressel, Martin (Prof. Dr.)The investigations carried out in this thesis unveil a percolating phase coexistence at the bandwidth-tuned Mott insulator-metal transition (IMT) of the organic spin liquid compound 𝜅-(BEDT-TTF)2Cu2(CN)3 and unequivocally proof the first oder nature of the genuine Mott transition. Our findings are in remarkable agreement with DMFT and finally rebut the controversy of spinons interfering with the Mott transition. In addition, we shed light onto the anomalous dielectric response (ADR) observed in the insulating phase of many organic charge transfer salts subject to electronic correlations, which puzzled the community for a decade. More specifically, we performed comprehensive dielectric spectroscopy measurements on the organic dimer Mott insulators 𝜅k-(BEDT-TTF)2Cu2(CN)3 (k-CuCN), 𝜅k-[(BEDT-STF) 𝑥-(BEDT-TTF)1-x]2Cu2(CN)3 (𝜅k-STF𝑥-CuCN) and 𝜅k-(BEDT-TTF)2Ag2(CN)3 (k-AgCN). In addition to varying temperature and frequency as experimental parameters, we applied hydrostatic pressure to tune 𝜅k-CuCN across the Mott IMT and to scrutinize the dielectric response in the entire phase diagram. These investigations are complemented by measuring a set of 𝜅k-STF𝑥-CuCN crystals, wherein varying the substitution level x is used to tune the compound across the Mott IMT. Additional pressure-dependent measurements of 𝜅k-AgCN extend our investigations further into the insulating state of the organic spin liquid compounds allowing us to thoroughly inspect the ADR.Item Open Access Probing the electronic structure of new 3D Dirac semimetals(2019) Topp, Andreas; Ast, Christian R. (Dr. habil.)In this thesis, ARPES was used to measure the band structure of novel 3D Dirac semimetals, many of which were previously unknown concerning their electronic structure. The main results were obtained characterizing materials of space group (SG) no. 129 and more specifically ZrSiS and related compounds.