Universität Stuttgart
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Item Open Access Structure and electronic properties of epitaxial monolayer WSe2(2019) Mohammed, Avaise; Takagi, Hidenori (Prof. Dr.)Item Open Access From Hermitian to non-Hermitian topological phases of matter(2019) Rui, Wenbin; Metzner, Walter (Prof. Dr.)The focus of this thesis lies on extending the theory of topological phases of matter from the Hermitian to the non-Hermitian regime. This includes not only the extension of conventional concepts such as topological invariants and topological boundary states in the theory of Hermitian topological phases, but also the exploration and characterization of entirely new topological phases unique to non-Hermitian systems.Item Open Access Realistic calculations for correlated materials(2019) Toloui-Mantadakis, Daniil; Hansmann, Philipp (Dr.)Strongly correlated fermionic systems nowadays stand in the forefront of condensed matter physics. A plethora of phenomena, ranging from unconventional superconductivity, gigantic and colossal magneto-resistance and metal-to-insulator transitions, are attributed to the effects of electron correlation. Given the spectacular progress on the experimental side, today - more than ever - the understanding of the underlying microscopic mechanisms, and the explanation or even prediction of experimental observations becomes a necessity. The advancements of theoretical and computational methodologies together with a concurrent increase of computational power, allows for both the ab initio study of realistic materials and the investigation of low-energy effective Hamiltonians inspired and designed to resemble whole classes of compounds. This work is conceptually divided into two major parts. In Chapter 3 and Chapter 4, we present our results, obtained by the state-of-the-art merger of density functional and dynamical mean-field theory, for two realistic systems: the layered LaNiO2/LaGaO3 superstructure, where we focus on the orbital resolved single-particle spectral functions and study the effect of electron and hole doping; and the ruthenate system Ca2RuO4, for which we provide a clear understanding and theoretical support of the experimentally observed semi-metallic state under the application of DC current. The second conceptual part of this work deals with the study of low-energy effective Hamiltonians. In Chapter 4, we investigate a generic t2g model Hamiltonian in the presence of non-spherical crystal-field potentials and/or spin-orbit coupling in order to shed more light on the distinct features that arise on the single-particle level and, most importantly, on the two-particle observables, such as the uniform and static magnetic susceptibilities. In Chapter 5, we investigate the multi-orbital extension of the periodic Anderson model, as inspired by the family of cerium-based heavy-fermion compounds, with a clear focus on the evolution of the dynamic behavior of the systems' moments.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 Dynamics of finite-dimensional mechanical systems(2019) Winandy, Tom; Leine, Remco I. (Prof. Dr. ir. habil.)This monograph deals with the description of mechanical systems having finitely many degrees of freedom using the language of global differential geometry. The mechanical systems may be explicitly time-dependent and involve nonpotential forces. The focus is on the mathematically rigorous formulation of the physical theory dealing with the aforementioned mechanical systems with the objective to introduce the involved physical quantities as well-defined mathematical objects. The geometric presentation of the physical theory is erected upon a generalized space-time known as Galilean manifold. The state space of a mechanical system is defined as an affine subbundle of the tangent bundle of its associated Galilean manifold. The system's motion is considered to be an integral curve of a second-order vector field on the state space. With the coordinate-free characterization of the motion in terms of second-order vector fields, differential forms appear on stage. A one-to-one correspondence between second-order vector fields and action forms is established. Action forms are differential two-forms with additional properties. The definition of action forms and the derivation of this bijective relation relies on the geometry of double tangent bundles, in which vector bundle homomorphisms and their differential concomitants play an important role. A coordinate-free definition of forces is given and different geometric interpretations are discussed. With the definition of kinetic energy and of potential forces, the equations of motion are postulated in a coordinate-free way using the action form of the mechanical system. Lagrange's, Hamel's, and Hamilton's equations become local representations of this postulate in terms of a respective chart of the state space. Moreover, the connection between action forms and the concept of virtual work is established. This allows to obtain Lagrange's and Hamel's central equation. This variational perspective is pursued by showing that motions characterized by an exact action form satisfy Hamilton's principle. For this purpose, a coordinate-free definition of the action integral is given. Finally, constraints are defined as distributions compatible with the time structure of the Galilean manifold on which they are defined. Consequently, the distinction between holonomic and nonholonomic constraints is made using the Frobenius theorem.Item Open Access Interfaces in fluids of ionic liquid crystals(2019) Bartsch, Hendrik; Dietrich, Siegfried (Prof. Dr.)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.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 Novel X-ray lenses for direct and coherent imaging(2019) Sanli, Umut Tunca; Schütz, Gisela (Prof. Dr.)