Spin-Wave Excitations Evidencing the Kitaev Interaction in Single Crystalline α-RuCl3

Kitaev interactions underlying a quantum spin liquid have long been sought, but experimental data from which their strengths can be determined directly, are still lacking. Here, by carrying out inelastic neutron scattering measurements on high-quality single crystals of alpha-RuCl3, we observe spin-wave spectra with a gap of similar to 2 meV around the M point of the two-dimensional Brillouin zone. We derive an effective-spin model in the strong-coupling limit based on energy bands obtained from first-principles calculations, and find that the anisotropic Kitaev interaction K term and the isotropic antiferromagnetic off-diagonal exchange interaction G term are significantly larger than the Heisenberg exchange coupling J term. Our experimental data can be well fit using an effective-spin model with K = -6.8 meV and Gamma = 9.5 meV. These results demonstrate explicitly that Kitaev physics is realized in real materials.