Magnetic field-dependent low-energy magnon dynamics in $\alpha$-RuCl3

TL;DR

This study investigates low-energy magnon excitations in $ ext{RuCl}_3$ using terahertz spectroscopy and modeling, revealing the necessity of including off-diagonal exchange interactions to explain magnetic field effects.

Contribution

It demonstrates that off-diagonal $ ext{Gamma}$ exchange terms are essential to accurately model magnon behavior in $ ext{RuCl}_3$ under magnetic fields, extending the minimal Heisenberg-Kitaev model.

Findings

Observation of two magnon absorption peaks at 2.0 and 2.4 meV.

Magnon modes depend on magnetic field orientation and strength.

Off-diagonal $ ext{Gamma}$ exchange explains mode mixing at higher fields.

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. We investigate the low-energy magnons in $\alpha$-RuCl$_3$ by combining time-domain terahertz spectroscopy under an external magnetic field and model Hamiltonian calculations. We observe two absorption peaks around 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-neighbor Ru-Ru bonds. We find that the magnon behavior in an applied magnetic field can be understood only by including an off-diagonal $\Gamma$ 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 fields strengths.