Quantum Materials Probed by Light and Electron Spectroscopy

Doctoral thesis, 2023

As new technologies rely heavily on materials’ properties, the quest for novel tunable electronic and magnetic materials occupies a large part of nowadays research efforts. Most promising materials are quantum materials, which exhibit a wide range of exotic electronic and magnetic phenomena and often show a great sensitivity to external perturbations such as temperature, pressure or chemical doping. At the origin of many of these phenomena lie strong many-body correlations and competition between different energy scales, from which rich phase diagrams with different structural, electronic and magnetic phases emerge. The understanding of the microscopic concepts in these materials relies on the knowledge of the relevant correlations and is one of the greatest efforts in the field of condensed matter physics. This thesis focuses on three different quantum material families that exhibit strong correlations and an unconventional superconducting phase. The experimental study investigates the electronic correlations in phases close to superconductivity, providing insight in energy scales that define the electronic ground state.
The first material family are the calcium ruthenates, an archetypal system for the study of spin-orbit coupling effects and the Mott metal-insulator transition. The competition of different energy scales leads to a non-trivial ground state, from which a complex excitation spectrum emerges. This work presents the study of the low-energy spin-orbital excitations in the single and bilayer compound, shedding light on the importance of the coupling between spin and orbital angular momentum.
The second system are the high-temperature superconducting La-based cuprates. Until now, the detailed connections between superconductivity and its surrounding phases remain elusive. The resonant inelastic x-ray scattering study presented here investigates the charge order phase and highlights the importance of an energy resolved technique to study weak charge correlations as a function of temperature and hole doping. The connection between the charge order and surrounding phases is investigated and the importance of a momentum dependent electron-phonon coupling is revealed.
The third topic is based on the spinel oxide superconductor LiTi2O4. New experimental insights have recently challenged the picture of a conventional s-wave superconductor and provided evidence for an anomalous pairing mechanism. The understanding has been impeded by the lack of direct measurements of the electronic band structure. This work presents an extensive angle-resolved photoemission spectroscopy study, revealing strong correlation effects at a low energy scale.