Universität Stuttgart
Permanent URI for this communityhttps://elib.uni-stuttgart.de/handle/11682/1
Browse
12 results
Search Results
Item Open Access From ground state properties to high energy spectroscopy : extending the application of DMFT for correlated quantum materials(2020) Schmid, Michael; Daghofer, Maria (Prof. Dr.)Strongly correlated electron systems exhibit rich physical phenomena reaching from superconductivity, Kondo- and, Mott physics to novel magnetic phases, which lie beyond most single-particle approaches such as density functional theory (DFT) or static mean-field theory. For many transition metal oxides (TMOs) such as Ca2RuO4 or LiV2O4 this is often a result of the partially filled d shells, leading to many-body wave functions, which cannot expressed as a single-slater determinant. Moreover, within this compounds there is often no clear hierarchy of energy scales, e.g. strong spin-orbit coupling, Hund’s coupling, and crystal-field splitting, making the description with minimal models difficult. The breakdown of the single-particle picture triggered the development of numerous numerical methods (DMFT, DMRG, VCA, . . . ) within the last decades, all aimed at tackling the aforementioned phenomena with complementary approximations. One of the most prominent methods for describing real compounds has become dynamical mean-field theory (DMFT), which in many cases has proven to describe local electronic phenomena in good agreement with experimental results. In this thesis we perform state of the art DFT+DMFT calculations in its single shot approach to complement theoretical k-resolved one-particle spectral functions to neutron and x-ray diffraction experiments on Ca2RuO4 . In the experiment small DC currents were applied to a Ca2RuO4 single-crystal resulting in the stabilization of new nonequilibrium phases. Based on experimentally refined structures, DFT calculations are performed to extract a tight binding model by projecting the correlated t2g -subspace onto maximally localized Wannier orbitals. Within our DMFT calculations spin-orbit coupling (SOC) and the spherical invariant Coulomb interaction are added to calculate spectral functions. The results indicate a semimetalic state with partially gapped Fermi surface in the nonequilibrium phases with elongated RuO6 octahedra. Additionally, we extend the DFT+DMFT scheme by a discretization scheme to obtain core-level spectroscopy data, such as XAS or RIXS spectra. This concept is based on the discretization of the DMFT hybridization function to construct an Anderson impurity model of finite bath sites. The discretized model is then extended by the core levels and core-valence interaction. To include sufficiently large amounts of bath sites, despite using an exact diagonalization (ED) solver, we choose the natural orbital basis as the single particle basis of choice to compute RIXS and XAS spectra.Item Open Access Variational cluster approximation at finite temperatures(2022) Lotze, Jan; Daghofer, Maria (Prof. Dr.)Being able to describe thermodynamics and dynamics of ordered systems at finite temperature allows capturing the signatures of different phases as well as thermal transitions between them. Systems of strongly correlated electrons residing in multiple orbitals where spin-orbit coupling is of significance can exhibit a multitude of exotic phases. Modelling these systems and capturing their properties for the entire temperature range is a non-trivial task. In this thesis, the implementation details of several cluster solvers used for the variational cluster approximation (VCA) at finite temperature are described, since this method is capable of modelling the systems mentioned before while incorporating local quantum fluctuations. The most reliable, sufficiently benchmarked and best performing solver among them is then used to investigate the magnetic and orbital properties of Sr2IrO4 and Ca2RuO4 described by three-band Hubbard models, as well as the Kondo lattice model at half-filling.Item Open Access Spin-orbital entanglement and molecular orbital formation in 4d and 5d transition metal oxides(2020) Krajewska, Aleksandra; Takagi, Hidenori (Prof. Dr.)Item Open Access Triplons in the excitonic Kitaev-Heisenberg model on the honeycomb lattice: condensation, interactions and topology(2019) Anisimov, Pavel S.; Daghofer, Maria (Prof. Dr.)Item Open Access Effective Kugel-Khomskii type models for d4 and d5 materials(2023) Strobel, Pascal; Daghofer, Maria (Prof. Dr.)Item Open Access From classical to quantum stochastic resonance(2022) McMurtrie, Gregory; Loth, Sebastian (Prof. Dr.)The open quantum system presented by atomic spins on surfaces is a unique platform to investigate the interplay between stochastic and deterministic behavior. This work investigates this interplay at the transition from classical to quantum behavior in tailored magnetic nanostructures. The structures are assembled with Fe atoms on a Cu2N surface grown on Cu(100) by using atom manipulation with a cryogenic-temperature scanning tunneling microscope. The spin state of the structures can be resolved with a spin-polarized tip, allowing their dynamic response to be measured. The stochastic evolution of the spin states is brought into a resonant regime by means of either a modulated exchange field or a modulated voltage applied with the tip. Undergoing this stochastic resonance yields insight into how these structures interact with their environment, with clear signatures of classical, semi-classical and quantum behavior being observed. This work sets the stage for a new way of interacting with incoherently evolving spin systems, by synchronizing their dynamics, and tailoring their interaction with their environment.Item Open Access Excitonic antiferromagnetism in two-dimensional t4 2g systems(2020) Feldmaier, Teresa; Daghofer, Maria (Prof. Dr.)Item Open Access Molekulardynamische Simulationen der Laserablation an Aluminium unter Einbeziehung von Plasmaeffekten(2020) Eisfeld, Eugen; Roth, Johannes (Prof. Dr.)Die vorliegende Arbeit widmet sich der Modellierung und Simulation der ultrakurz gepulsten Laserablation am Modellmaterial Aluminium. Das Zweitemperaturmodell, welches das anfängliche thermische Nichtgleichgewicht zwischen den angeregten Elektronen und dem kalten Metallgitter beschreibt wird mit der klassischen Molekulardynamik in einem Hybridansatz gekoppelt. Auf diese Weise wird eine plausible Beschreibung der hierbei stattfindenden, mehrere Zeit- und Längenskalen überspannenden physikalischen Prozesse ermöglicht, ohne auf a priori Annahmen hinsichtlich der metastabilen Phasenübergänge und sonstiger Ablationsmechanismen angewiesen zu sein. Ergänzt wird dieser Ansatz mit unterschiedlichen Modellen für die thermophysikalischen, optischen und Transporteigenschaften des Elektronensystems um einen weiten Temperatur- und Dichtebereich, ausgehend vom kalten Festkörper- bis hin in den heißen Plasmazustand zu berücksichtigen. Das neue Modell wird in das, am ehemaligen Institut für theoretische und angewandte Physik entwickelte Programmpaket IMD implementiert und erweitert den Anwendungsbereich auf Ablations-Simulationen bei hohen Laserintensitäten und Mehrfachpulse. In dieser Arbeit wird es unter anderem dafür eingesetzt, die experimentell beobachtete Sättigung der Ablationseffizienzbei hohen Intensitäten sowie die Abnahme der Abtragstiefe bei Doppelpulsen mit zunehmendem Pulsabstand zu untersuchen. Im Vordergrund steht dabei immer der Vergleich mit dem Experiment. Im Falle von ultrakurzen Pulsen kann eine sehr gute Übereinstimmung mit experimentellen Messungen erzielt werden. Als essentiell erweist sich hierbei das Einbeziehen der verminderten Elektronen-Ionen Stoßfrequenz beim allmählichen Übergang in den Plasmazustand sowie eine vollständig wellenoptische Behandlung der Licht-Materie-Wechselwirkung. Für Pulsdauern oberhalb von 1 ps führt eine Erweiterung des Modells zur Berücksichtigung des ballistischen Transports angeregter Elektronen, sowie deren verzögerte Thermalisierung zu zuverlässigen Vorhersagen der Abtragstiefen. Auf Grundlage der Simulationen sowie simpler thermodynamischer Überlegungen werden Kriterien, in Form von Schwellenwerten für die isochore Temperaturzunahme formuliert, die eine systematische Kategorisierung der unterschiedlichen Ablationsmechanismen erlauben. In diesem Zusammenhang kann ferner festgestellt werden, dass die Phasenexplosion entgegen der Behauptung einiger Autoren, eine Besonderheit der ultrakurzen Laserablation ist. Bei Pulsdauern im Bereich einiger Pikosekunden hingegen wird festgestellt, dass die Ablation hauptsächlich auf einem Mechanismus beruht, der in der Literatur als Fragmentierung bekannt ist.Item Open Access Molecular dynamics simulations of laser ablation in covalent materials(2017) Kiselev, Alexander; Roth, Johannes (Prof. Dr.)Item Open Access Laser ablation of covalent materials(2023) Klein, Dominic; Roth, Johannes (Apl. Prof. Dr.)Ultra-fast laser ablation is the process of material removal from solid surfaces by pulsed sub-picosecond laser irradiation. In contrast to longer pulse durations, ultra-fast laser ablation shows the distinguishing feature of the timescale of excitation being below the timescale of consequent material heating. Excited charge carriers distribute the thermal energy over a larger volume than the optical penetration depth suggests, while the lattice remains in a cold state. Spatial energy distribution is followed by a fast carrier-lattice energy relaxation, which induces overheated and meta-stable states of matter. These meta-stable states are induced simultaneously in the laser-affected zone, forcing the material to relax in a variety of mechanisms, ranging from ultra-fast melting over hydrodynamic expansion to material ejection in a complex mixture of chunks, droplets or vapor. While a multitude of publications successfully study the laser irradiation induced material dynamics of metals, we investigate laser ablation of covalent materials. In contrast to metals, covalent materials show a band gap, excitation-dependent carrier heat conduction and strong excitation-dependent interatomic bonding strengths, rendering the theoretical description of such materials a difficult task. However, it also gives rise to a number of unique dynamics like non-thermal melting, Coulomb explosions and altered carrier heat conduction due to charge carrier confinement. In this work we choose silicon as our prototypical covalent material and perform molecular dynamics simulations of laser irradiated silicon, while applying an excitation-dependent interatomic potential. We present new parametrizations of the optical properties, as well as the extension of established charge carrier transport models for silicon, which are both tailored for the application on large scale massive multi-parallel high-performance computers. Finally we observe and characterize the novel and non-thermal ablation mechanics of laser irradiated silicon.