Theoretical investigation of magnetic dynamics in α-RuCl3

We study spin-wave excitations in alpha-RuCl3 by the spin-wave theory. Starting from the five-orbital Hubbard model and the perturbation theory, we derive an effective isospin-1/2 model in the large Hubbard (U) limit. Based on the energy-band structure calculated from the first-principles method, we find that the effective model can be further reduced to the K-Gamma model containing a ferromagnetic nearest-neighbor (NN) Kitaev interaction (K) and a NN off-diagonal exchange interaction (Gamma). With the spin-wave theory, we find that the K-Gamma model can give magnetic excitations, which is consistent with recent neutron-scattering experiments.