Magnetic excitations of the Kitaev model candidate RuBr3

Abstract The Kitaev model, which consists of bond-dependent Ising interactions on the honeycomb plane, is known as a promising model for realising a spin liquid in the exact ground state. Since bond-dependent Ising interactions are induced by orbital exchange processes via ligand ions, it is important to reveal the effect of ligand ions on the formation of bond-dependent Ising interactions experimentally. In this study, we performed powder inelastic neutron scattering on RuBr 3 with a layered honeycomb structure, which is isostructural to the low-temperature structure of α -RuCl 3 . Strongly dispersive magnetic modes were observed near the centre of the antiferromagnetic zone below T N . With increasing temperature, the spectral weight remains near the centre of the antiferromagnetic zone just above T N , whereas it shifts to the Γ point at 3 T N . Strong dispersion indicates that antiferromagnetic interactions that stabilise the zigzag antiferromagnetic order, namely, nearest-neighbour symmetric off-diagonal or third-neighbour isotropic magnetic interactions, are comparable to or even stronger than ferromagnetic Kitaev interactions, and drive the ground state of RuBr 3 away from the ideal ferromagnetic Kitaev spin liquid state.