Magnetic excitations of the Kitaev model candidate RuBr3

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 RuBr3 with a layered honeycomb structure, which is isostructural to the low-temperature struc- ture of α-RuCl3. Strongly dispersive magnetic modes were observed near the centre of the antiferromagnetic zone below TN. With increasing temperature, the spectral weight remains near the centre of the antiferromagnetic zone just above TN, whereas it shifts to the Γ point at 3TN. Strong dispersion indicates that antiferromagnetic interactions that stabilise the zigzag antiferromagenetic order, nearest-neighbour symmetric off-diagonal or third-neighbour isotropic magnetic interactions, are comparable to or even stronger than ferromagnetic Kitaev inter- actions, and drive the ground state of RuBr3 away from the ideal ferromagnetic Kitaev spin liquid state.