Studies on magnetism in 2D van der Waals antiferromagnetic insulators using terahertz spectroscopy

Abstract

Revealing the spin excitations of complex quantum magnets is key to developing a minimal model that explains the underlying magnetic correlations in the ground state. In this thesis, we study the low-energy magnons in a-RuCl3 by combining timedomain terahertz spectroscopy under an external magnetic field and model Hamiltonian calculations. We observe two absorption peaks at 2.0 and 2.4 meV, which we attribute to zone-center spin waves. Using linear spin-wave theory with only nearest-neighbor terms of the exchange couplings, we calculate the antiferromagnetic resonance frequencies and reveal their dependence on an external field applied parallel to the nearest-neigbor Ru-Ru bonds. We find that the magnon behavior in an applied magnetic field can be understood only by including an off-diagonal exchange term to the minimal Heisenberg-Kitaev model. Such an anisotropic exchange interaction that manifests itself as a result of strong spin-orbit coupling can naturally account for the observed mixing of the modes at higher field strengths.