Universität Stuttgart
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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 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 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 Surface-governed dynamics of atomic-scale magnetic moments(2025) Arnhold, Lukas; Loth, Sebastian (Prof. Dr.)This thesis explores how atomic-scale environments influence the behavior of individual magnetic moments adsorbed on surfaces, focusing on both their static and dynamic properties. Such magnetic systems, from individual atoms to molecules and nanostructures, exhibit properties that are shaped by interactions with their immediate surroundings. Their energy level diagrams change depending on the symmetry of their binding site, but also their stochastic dynamics and the respective scattering mechanisms are extremely sensitive to variations in the environment. Here, we use time-resolved scanning tunneling microscopy (STM) to address these individual atomic-scale magnets with picometer spatial resolution and uncover how their behavior is governed by the properties of the surface they are adsorbed on. We employ multiplet calculations and rate equation modeling to gain insights into the inner workings of an atom's magnetism. Combining experimental and theoretical approaches, we reveal how surfaces impose magnetic anisotropy, change electronic properties, and define the spin and orbital properties of these magnetic systems. We study the influence of four different rotational symmetries of the magnetic moments environment: 4-fold, 3-fold, 2-fold, and asymmetric. Further, we compare how the symmetries affect magnetic systems with different numbers of electrons. Fe atoms on bilayer MgO have 6 electrons in the 3$d$ shell. The orbital momentum of the atoms mediate magnetic anisotropy, induced by the 4-fold symmetric surface geometry, which stabilizes the magnetic states. The energy separation of the eigenstates enables distinct regimes of scattering with the local environment. Fe atoms are efficiently excited by spin-flip interactions with the tunneling current, while the relaxation of magnetization preferably occurs by tunneling of magnetization through the anisotropy barrier. The scattering regimes are determined by the coupling strengths of the magnetic moment to the surface and the STM tip. In contrast, Ti atoms on MgO have only one electron in the 3$d$ shell but adsorb on two different binding sites. Ti atoms adsorbed on 4-fold symmetric binding sites experience out-of-plane anisotropy, and we can observe and assign transitions that rotate the atoms electron spin momentum and orbital momentum relative to each other. Distinctively, atoms adsorbed on 2-fold symmetric binding sites exhibit quenched orbital momentum, resulting in an in-plane anisotropy and transitions of the magnetic moment that are highly sensitive to environmental scattering contributions. These findings highlight how the surface geometry can lead to contrasting behaviors. Molecular systems, such as CpTicot on Pb nanoislands, demonstrate the potential of protecting a magnetic moment by ligand encapsulation. In this case, the local geometry leads to an asymmetric environment for the Ti spin center. The ligands stabilize the Ti atom's single valence electron in a non-bonding orbital that interacts with the superconducting substrate through a rare, spatially-dependent hybridization process, providing valuable insights into local spin-superconductor coupling. Lastly, we implement 3-fold symmetric ZnO films as a promising substrate for stabilizing magnetic moments in single atoms. Self-assembled Co-Co and Co-H nanostructures on the surface reveal how the symmetry of orbital momentum and chemical bonding modifies the magnetic anisotropy. For, a Co atom adsorbed on the hollow site of ZnO experiences a single in-plane anisotropy component by the crystal field. Adsorbing a single hydrogen on this Co atom contributes a dominant charge contributions above the atom, changing the anisotropy axis to the out-of-plane orientation. This investigation adds valuable understanding for quantum state engineering on the atomic scale. Through a systematic exploration of surface symmetries and atomic electron configurations in both experiment and theory, this thesis deepens the understanding of how atomic-scale environments condition magnetic moments. These findings advance the description of atomic-scale magnetic dynamics and explicitly introduce the orbital momentum as a valuable design resource for such quantum systems.Item Open Access Effective Kugel-Khomskii type models for d4 and d5 materials(2023) Strobel, Pascal; Daghofer, Maria (Prof. Dr.)Item Open Access Excitonic antiferromagnetism in two-dimensional t4 2g systems(2020) Feldmaier, Teresa; Daghofer, Maria (Prof. Dr.)Item Open Access Robustness of topological features against environment(2025) Sattler, Alexander; 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 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 Charge and spin order in an infinite-layer nickelate NdNiO2(2025) Plienbumrung, Tharathep; Daghofer, Maria (Prof. Dr.)