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 α-RuCl3 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 Γ 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.
@article{arxiv.1909.00462,
title = {Magnetic field-dependent low-energy magnon dynamics in $\alpha$-RuCl3},
author = {Ilkem Ozge Ozel and Carina A. Belvin and Edoardo Baldini and Itamar Kimchi and Seunghwan Do and Kwang-Yong Choi and Nuh Gedik},
journal= {arXiv preprint arXiv:1909.00462},
year = {2019}
}