MODELING EXCITATIONS IN QUANTUM MAGNETS

Abstract

The interactions and dynamics of different quasi-particles in magnetic materials have risen to the forefront of both fundamental and applied quantum materials research. Especially in quantum magnets, these many-body effects are complex and enriched due to strong spin-orbit interactions. This thesis reviews the theory of spin-phonon interaction and their consequences for experiments such as the thermal Hall effect in magnetic insulators from the first principles point of view. We first develop the generic approach to compute spin-phonon couplings for a material of interest and apply it to study a strongly correlated magnetic insulator. We demonstrate that strong spin-orbit coupling can give rise to finite phonon thermal Hall effect in magnetic insulator by introducing chirality into phonons through spin-phonon interactions. Alongside, the effects of structural distortions and disorder on the magnetic response are studied in the context of magnetic insulators. With honeycomb structure, we find that the spin Hamiltonian is sensitive to the details of structural perturbations and can significantly enhance and modify magnetic response. The importance of spin-orbit coupling on the ground state and excitations of 3d transition-metal-based compound with unquenched orbital moments is also discussed.