Universität Stuttgart
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Item Open Access Electronic and structural instabilities in the putative excitonic insulator Ta2NiSe5(2025) Zhang, Yuanshan; Takagi, Hidenori (Prof. Dr.)Item Open Access Novel theoretical methods for transition metal chemistry(2025) Safari, Arta A.; Alavi, Ali (Prof. Dr.)Item Open Access Automatic generation of coupled cluster algorithms with application to transcorrelated Hamiltonians(2025) Schraivogel, Thomas; Kats, Daniel (PD Dr.)We have used automatic code generation techniques to derive and implement full transcorrelated coupled and distinguishable cluster methods for accelerated basis set convergence and wave function compactification, leading to increased accuracy with manageable cost. The methods were benchmarked on thermochemical properties and two integral approximations based on normal ordering have been explored. The integral approximations have been shown to reduce the cost of the calculations without significant sacrifices in accuracy. They have paved the way for the very versatile transcorrelated method via exclusion of explicit three-body components (xTC). It has been demonstrated that the distinguishable cluster approximation improves the accuracy of transcorrelated coupled cluster methods. The transcorrelated coupled and distinguishable cluster methods with single and double excitations have been shown to outperform their F12 counterparts and approach the accuracy of CCSD(T)-F12. A two determinant distinguishable cluster method, alongside the traditional version, has been implemented in a new Julia package for electronic structure methods, called ElemCo.jl. The methods have been benchmarked on singlet and triplet excited states of closed-shell molecules and singlet-triplet gaps of diradicals using state-specific orbitals in the delta coupled cluster framework. The distinguishable cluster approximation improved the accuracy of the two determinant coupled cluster method considerably and the two determinant distinguishable cluster method has been shown to provide comparable and sometimes better accuracy than the equation-of-motion coupled cluster methods.Item Open Access Exploration and control of optical, magnetic, and structural properties of ruthenium-oxide Mott insulators(2025) Rabinovich, Ksenia S.; Keimer, Bernhard (Prof. Dr.)Item Open Access The geometric memory of quantum wave functions(2025) Heinsdorf, Niclas; Metzner, Walter (Prof. Dr.)Quantum geometry - including both the quantum metric and Berry curvature - arises from the non-zero overlap of well-defined eigenstates and their adiabatic evolution across the Brillouin zone. It has revolutionized condensed matter physics and material science by explaining quantum Hall effects, establishing the modern theory of polarization, and enabling a systematic search for topological materials on a large scale. Yet, despite these advances, we lack a framework for leveraging quantum geometry in the strongly interacting regime. Bridging this gap is critical: if we can harness geometric responses in correlated metals, we stand to engineer desirable transport and optical properties using existing material platforms, thus bypassing the complex task of designing materials with tailored quantum geometry from scratch. In this thesis, we take steps toward such a framework. First, we analyze the resilience of topological boundary modes in the presence of electronic correlations, identifying when interactions preserve, diminish, or destroy boundary modes. Second, we reveal geometric fingerprints of fluctuations in magnetically ordered systems, tying the electric quantum metric to the formation of instabilities and chiral quasi-particle excitations. Third, we generalize quantum geometry to describe families of many-body wave functions, providing a new algorithm to compute state-manifold curvatures suited to interacting phases. Our approach combines analytical theory with numerical methods, including density functional theory and tensor-network simulations, and is supported by open-source software developed during the PhD. Together, these results advance the topological classification of interacting phases and extend quantum geometry from single-particle bands to correlation functions, providing tools to design materials and devices with targeted geometric responses.Item Open Access Exploring strong correlations in the calcium ruthenates using spectroscopic techniques(2025) Suen, Cissy T.; Keimer, Bernhard (Prof. Dr. rer. nat.) and Damascelli, Andrea (Prof. Dr.)The rich multiband physics of the calcium ruthenates gives rise to a complex free-energy landscape shaped by the strong interplay among spin-orbit coupling effects, electronic interactions, lattice distortions, and spin correlations. This competition produces a variety of emergent quantum phenomena with promise for future applications. For instance, beyond its Mott-insulating ground state, the single-layer Ca2RuO4 hosts exotic fluctuation modes of the antiferromagnetic state and an unconventional non-equilibrium metallic state induced upon application of an electrical current. Meanwhile, the bilayer Ca3Ru2O7 exhibits pseudogap behaviour and an incommensurate cycloidal magnetic phase arising uniquely from the Dzyaloshinskii–Moriya interaction. By simultaneously employing transport measurements with angle-resolved photoemission spectroscopy (ARPES), this work reveals how the electronic band structure evolves across the current-induced insulator-to-metal transition in Ca2RuO4. A reduction of the insulating band gap and a modification of the Ru bands are observed in the current-induced L phase. Landau–Ginzburg free-energy analysis indicates that current flow imposes a directional anisotropy that breaks the symmetry between the out-of-plane orbitals. Thus, while the insulating and metallic phases are thermodynamically equivalent, they host distinct orbital populations. Understanding the differences and triggers for each state provides new avenues for Mottronic devices, such as energy-efficient electronic switches. Historically, ARPES measurements have avoided electromagnetic fields due to their effect on the outgoing trajectory of the emitted photoelectrons. This manifests as a shift in energy and momentum and a broadening of the recorded spectra. While technological developments such as nano-ARPES mitigate the latter issue, transport-ARPES has been confined to the study of materials with clearly defined features in energy, such as the d-wave node in superconducting cuprates or the Dirac point in graphene. By using core level spectra as the energy reference, not only can transport-ARPES be extended to any ARPES suitable material, careful observations of the core level energies and widths can provide further information about the material under current. Meanwhile, Raman measurements in an external magnetic field reveal an unusually large field-induced energy shift of a phonon mode in Ca3Ru2O7, pointing to exceptionally strong spin–phonon coupling in a regime where the Dzyaloshinskii–Moriya interaction plays a dominant role. While even larger couplings have been observed in some 5d compounds such as iridates and osmates, this is the largest reported value among the ruthenates, and appears to be strongly influenced by the incommensurate cycloidal magnetic phase. Additionally, the strength of this coupling underscores the delicate interplay between the different degrees of freedom in the system, making these materials highly tunable by small perturbations. For instance, substituting 1% Ti switches the magnetic ground state from A-type to G-type antiferromagnetic order and applying < 1% compressive strain can shift the insulator-to-metal transition by more than 70K. This thesis therefore integrates external electromagnetic fields with advanced spectroscopic probes in order to probe regions of the ruthenate free-energy landscape not previously explored by conventional techniques. A complex landscape is uncovered, illustrating how these materials can be finely tuned with current, field, doping, or strain, offering new insights into the underlying physics of 4d systems and opening opportunities for their incorporation into future quantum devices. Additionally, with the developments in transport-ARPES presented by this work, this thesis builds upon our ever growing capability to look at strongly correlated electron systems in operando.Item Open Access Quantum phases of topological many-body systems(2025) Bollmann, Steffen; Metzner, Walter (Prof. Dr.)In this thesis, we study the interplay between different topological phases, including both symmetry-protected and topologically ordered states, and how they respond to interaction as well as fluctuations in different proximitized quantum phases. First, it is demonstrated how spinful Majorana edge modes in the presence of charging effects give rise to a novel Kondo effect when coupled to metallic leads. This is followed up by a study on phases of parafermions in fractional quantum (anomalous) Hall - superconductor heterostructures. Finally, it is investigated how the coupling of a Chern insulator to the topologically ordered Toric Code realizes topological Green's function zeros and how they can be used to classify interacting topological systems.Item Open Access Spectroscopic study of electronic and lattice collective excitations in Ta2NiSe5 and Ta2NiSe7(2025) Shi, Xiaotong; Keimer, Bernhard (Prof. Dr.)Item Open Access Transport phenomena in fractionalized quantum materials(2025) Mazzilli, Raffaele; Metzner, Walter (Prof. Dr.)The quantum effects in a many-body system can induce novel behavior on a system with fractionalized degrees of freedom, whose study is not only interesting from the perspective of fundamental research but also in view of potential applications to quantum technologies such as quantum computers. In this regard, quantum spin liquids are systems of particular interest. They are an exotic phase of matter characterized by the presence of fractionalized excitation (spinons) and emergent gauge fields. The efforts in the community have not yet succeeded in asserting the existence of quantum spin liquids beyond any reasonable doubt. The technological advances in material synthesis provide us with two-dimensional samples of candidate quantum spin liquid materials like 1T-TaS2, 1T-TaSe2, and αRuCl3, which might avoid the problem of the disruptive effects of the interlayer interactions in candidate materials. Experimental techniques like neutron scattering, aimed at measuring the bulk properties of a sample, are not applicable in the case of two-dimensional samples. One of the difficulties in probing experimentally a QSL phase comes from the fact that the spinons do not carry an electric charge, ruling out the possibility of using conventional electrical probes. Going beyond conventional transport, we propose two setups of electric probes to characterize a QSL phase. First, we analyze a setup in which a QSL layer is interposed between two metallic layers. In this setup, we apply a current in the first metallic layer and measure the induced voltage on the second one. The momentum transfer is affected by the non-trivial behavior of momentum-carrying spinons and results in a response that carries information about the dynamic of the spinons and will potentially be helpful for the future characterization of candidate QSL materials. The second probe we propose is a scanning tunneling microscopy (STM) experiment on a Kondo lattice with the addition of an antiferromagnetic interaction between the localized magnetic moments. We calculate the STM response in each of the phase configurations of this system allowing also for the possibility for the conduction electrons and for the spinons to form a superconducting phase and present our derivation of the mean field equations in a Kondo lattice system. This last setup might find a concrete realization in materials such as TaS2, TaSe2, and NbSe2 in the 1T, 2H, and in the 4Hb crystallographic phases.Item Open Access Development of the transcorrelated full configuration interaction quantum Monte Carlo method(2025) Haupt, Jacobus Philip; Alavi, Ali (Prof. Dr.)The transcorrelated (TC) method is a technique in electronic structure theory that has recently been gaining momentum. In it, a similarity transformation is applied to the electronic Hamiltonian to capture effects of electron correlation, particularly dynamical correlation, by explicit treatment of analytically-known properties of the Hamiltonian near coalescence points. This has already been combined with the full configuration interaction quantum Monte Carlo (FCIQMC) method, an efficient stochastic approach to solving the electronic Schrödinger equation and calculating physical observables. This dissertation further explores these methods, particularly TC. After a recapitulation of the core concepts of wave function and quantum Monte Carlo methods, we introduce the use of flexible Jastrow factors familiar in the variational Monte Carlo (VMC) literature to minimise the variance of the TC-reference energy. This is shown to result in a rapid basis-set convergence, reaching accuracy for which conventional FCIQMC would require much larger basis sets. Moreover, this minimisation procedure is shown to also compactify the wave function, allowing for more efficient sampling in FCIQMC. We next extend the methodology for problems of strongly multireference character, notably using the dissociation of the nitrogen dimer as a stress test. We illustrate the need for a multireference Jastrow-factor ansatz, and hence minimise the variance of a multireference state. This is shown to recover favourable, size-consistent energies while maintaining the rapid basis-set convergence that comes with TC. As a multireference technique (FCIQMC) is used after optimisation, it moreover does not increase the computational scaling of the method to use the conventional (non-TC) form of the same multireference technique as the TC ansatz. Finally, we explore the possibility of constructing simplified Jastrow factors in order to improve or possibly wholly bypass the optimisation procedure so far for the TC method, which can be computationally expensive. We show that parameter-free Jastrow factors can result in poor absolute energies, but favourable relative energies thanks to error cancellation, whereas Jastrow factors optimised for atoms and reused for molecules result in both accurate absolute and relative energies. This opens the possibility of optimising Jastrow factors for atoms across the periodic table and storing them in a database, which can be queried for larger molecules, thereby aiding the scalability of the method.