Browsing by Author "Daghofer, Maria (Prof. Dr.)"

Now showing 1 - 7 of 7

Open Access
Collective modes of the superconducting condensate
(2023) Haenel, Rafael; Daghofer, Maria (Prof. Dr.)
When a continuous symmetry is spontaneously broken, collective modes emerge. Usually, their spectrum is dominated by the low-energy physics of massless Goldstone modes. Superconductors, that break U(1) symmetry, are different. Here, the Goldstone boson is gapped out due to the Anderson-Higgs mechanism. The superconducting condensate can therefore host a zoo of massive collective excitations that are stable for lack of a gapless decay channel. The most prominent of them is the Higgs mode. Spectroscopy of collective modes can serve as a probe to reveal the nature of the superconducting state. In this thesis, we study the signatures of collective modes in nonlinear optical experiments. We explore the theoretical description of a new spectroscopic excitation scheme. We show how impurity scattering significantly enhances the optical Higgs mode response. We apply group theoretical methods to multi-order-parameter theories and investigate microscopic signatures of coupled modes in third harmonic generation experiments. We study the phenomenology and collective mode spectrum of an exotic system of twisted cuprate bilayers that supports topological superconductivity. Finally, we propose a novel device implementation of the superconducting diode effect. These results contribute to the emerging field of collective mode spectroscopy.
Open Access
Effective Kugel-Khomskii type models for d4 and d5 materials
(2023) Strobel, Pascal; Daghofer, Maria (Prof. Dr.)
Open Access
Exciton Bose-Einstein condensation and topology in Van Vleck-type Mott insulators
(2024) Aust, Friedemann; Daghofer, Maria (Prof. Dr.)
Open Access
Excitonic antiferromagnetism in two-dimensional t4 2g systems
(2020) Feldmaier, Teresa; Daghofer, Maria (Prof. Dr.)
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.
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.)
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.