Masters defense: Emil Hellebek

Titel: Charge order in kagome metal

Abstract:

The discovery of coexisting time reversal symmetry breaking and superconductivity in the kagome metals AV_3Sb_5 (A = K, Cs, Rb) have made the scientific community pour huge efforts into understanding the nature of the materials, and a flurry of papers has been written in the last two years. Concurrent with the onset of the time reversal symmetry breaking, a charge ordering phase has been found in the materials that break translational symmetry. The phase is believed to form a Tri-Hexagonal or Star-of-David bond order in the kagome plane. In this thesis we investigate the new kagome metals starting from two tight-binding models: One considering the bare kagome lattice, the other using a materially specific model fitted to CsV_3Sb_5. We introduce interactions and decouple them using the unrestricted Hartree-Fock approximation, such that we can minimize the free energy using a self-consistency algorithm. In the phase diagrams for the bare kagome lattice, we find a large region where the Tri-Hexagonal bond order is the ground state, and regions with coexisting Tri-Hexagonal, charge density wave and spin density wave phase. Curiously, the latter of the two phases conserves the symmetries that AV_3Sb_5 breaks at the onset of the charge ordering phase. In the phase diagrams for the materially specific tight-binding model, we find a coexisting ferromagnet, imaginary spin density wave and imaginary charge density wave phase, with a loop current configuration that matches a previous proposal to the time reversal symmetry breaking order.

With this study we have contributed to the current discussion on how the charge bond order emerges and how time reversal symmetry breaking might be linked to the charge bond order.