Universität Stuttgart
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Item Open Access Investigating superconductivity by tunneling spectroscopy using oxide heterostructures(2017) Fillis-Tsirakis, Evangelos; Mannhart, Jochen (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 Structure and electronic properties of epitaxial monolayer WSe2(2019) Mohammed, Avaise; Takagi, Hidenori (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 Growth and characterization of transition metal oxide heterostructures with a tailored work function(2018) Rastegar Alam, Iman; Mannhart, Jochen (Prof. Dr.)Item Open Access Excitonic Fano resonances in Ta2NiSe5 and Ta2NiS5(2016) Larkin, Timofei I.; Keimer, Bernhard (Prof. Dr.)Item Open Access Neutron scattering studies on layered ruthenates(2018) Krautloher, Maximilian; Keimer, Bernhard (Prof. Dr.)Transition metal oxides (TMOs) exhibit a large variety of magnetic, electronic, and structural phases and have received much attention from the community. The tight competition between different interactions and ordering phenomena typical for such systems result in phase diagrams which are characterized by a multitude of transitions. These often depend on external variables, including temperature, magnetic or electric fields, pressure, and chemical doping. Early research focused on oxides of light transition metals exhibiting flat electronic bands and strongly correlated systems. Prominent examples include the families of copper oxides (cuprates) that exhibit high-temperature superconductivity, and manganites that show colossal magnetoresistance. For a long time, oxides of heavier transition metals were not expected to exhibit particular exciting phenomena: with increasing atomic mass and ionic radii, the Coulomb repulsion decreases while the extension of the d orbitals enlarges, consequently increasing the orbital overlap and the electronic bandwidth W. Such heavy-metal based systems were therefore expected to be metallic, without the intricate competition between different ordering phenomena seen in their lighter analogues. Recently, however, it was recognized that the spin-orbit coupling (SOC) can profoundly change the phase behavior of 4d- and 5d-electron materials. The strength of SOC scales with the atomic number Z as ∝Z^4 , which—in contrast to systems including 3d TMOs—renders SOC a driving force in oxides of heavy transition metals. As the interplay between SOC and electronic correlations brings about novel quantum ground states, these systems have received increasing interest during the last decade. One such systems is Sr2IrO4, where this interplay generates a Mott-insulating state with total angular momentum J_{eff} = 1/2 . 4d-electron compounds, which are characterized by moderate SOC, have until recently been modeled akin to oxides of 3d-electron systems, treating the SOC as a minor perturbation only. However, even moderate SOC proved to be enough to realize exotic phenomena that are not captured by such approaches, and can lead to a variety of competing structural and magnetic phases. Consequently, the role of SOC in 4d TMOs has been underestimated, calling for re-evaluation of the underlying physics. In this work we focus on the antiferromagnetic Mott insulator Ca2RuO4, in which the interaction is limited to the two-dimensional layers of RuO6 octahedra. The low-spin 4d^4 configuration of Ru^{4+} leads to a S = 1 spin, while the lattice symmetry results in an effective orbital momentum of L_{eff} = 1. Previous studies have shown that Ca2RuO4 undergoes an insulator-metal transition upon heating and exhibits a series of phase transitions upon isovalent substitution with Sr. The wide variety of phases makes Ca2RuO4 a prime material platform to investigate the role of moderate SOC in magnetism. We concentrate on the magnetic excitation spectrum, which reflects the combined influence of the exchange interactions between the Ru ions and the inter-ionic SOC. The first part of this PhD project is dedicated to the growth of high-quality crystals of Ca2RuO4 and related ruthenium oxides. To this end, we used the optical floating zone technique. The several hundred crystal shards were then co-aligned to be used in inelastic neutron scattering experiments. With a map of the magnetic scattering intensity in the full magnetic Brillouin zone, we observe and distinguish all transverse magnon (Goldstone) modes as well as a longitudinal amplitude (Higgs) mode. The results can be consistently interpreted in an excitonic magnetism model with a dominant influence of the SOC. We then used inelastic neutron scattering to investigate the magnetic excitations of the Sr-substituted Ca2RuO4 crystals and found a modified set of exchange interactions. We also investigated the closely related Ca3Ru2O7 system; here, a double-layers of RuO6 octahedra are interleaved by a CaO barrier layer. We find that this bilayer system exhibits a metallic phase where the impact of the SOC is less pronounced. Surprisingly, a chemical substitution of the 4d^4 Ru^{4+} ions with magnetically inactive 3d^0 Ti^{4+} ions renders the system insulating even for Ti concentrations less than 1 %. In this phase that the system’s magnetic excitations are similar to Ca2RuO4 suggesting the same excitonic magnetism. Our studies demonstrate the crucial role of SOC for the magnetic properties of ruthenium oxides, and call for a general reevaluation of the impact of SOC on the ground state and excitations of 4d-electron systems.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 Proximity effects and Josephson currents in ferromagnet : spin-triplet superconductors junctions(2015) Terrade, Damien; Metzner, Walter (Prof. Dr.)Spin-triplet superconductivity, first attached to the description of He3, is now generally considered to also occur in heavy-fermions compounds and in perovskite ruthenium oxide Sr2RuO4. The latter material is especially interesting since many experiments show strong evidences for a unitary chiral spin-triplet state. Moreover, the recent fabrication of thin heterostructures made of ferromagnetic SrRuO3 on the top of Sr2RuO4 strongly encourages new theoretical studies on the interplay between spin-triplet superconductor and ferromagnet in similar fashion to spin-singlet superconductors. Using an extended tight-binding Hamiltonian to model the superconductor, we discuss in this thesis the specific proximity effects of such interface by solving self-consistently the Bogoliubov-De Gennes equations on two- and three-dimensional lattices in the ballistic limit. We obtain the spatial profile of the superconducting order parameters at the interface as well as the spin-polarisation and the current across the Josephson junctions. In contrast to heterostructures made of spin-singlet superconductor, we show that the physical properties at the interface are not only controlled by the strength of the magnetization inside the ferromagnet but also by its orientation due to the existence of a finite pair spin projection of the spin-triplet Cooper pairs. We analyse in the first part the spin-polarisation and the Gibbs free energy at the three-dimensional ferromagnet-chiral spin-triplet superconductor interface. Then, the second part of the thesis is dedicated to the study of the Josephson junctions made of a chiral spin-triplet superconductor and a ferromagnetic barrier. More precisely, we analyse the existence of 0-Pi state transitions in two- and three-dimensional junctions with respect to the strength and the orientation of the magnetization. Finally, we study the proximity effects at the interface of helical spin-triplet superconductors. They differ from the chiral superconductor by the direction of the pair spin polarisation of the Cooper pairs and by the properties of the edge states, present at the boundaries, which can sustain dissipationless spin-current.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.