Evidence for Magnetic Fractional Excitations in a Kitaev Quantum-Spin-Liquid Candidate \alpha-RuCl_3

It is known that \alpha-RuCl_3 has been studied extensively because of its proximity to the Kitaev quantum-spin-liquid (QSL) phase and the possibility of approaching it by tuning the competing interactions. Here we present the first polarized inelastic neutron scattering study on \alpha-RuCl_3 single crystals to explore the scattering continuum around the \varGamma point at the Brillouin zone center, which was hypothesized to be resulting from the Kitaev QSL state but without concrete evidence. With polarization analyses, we find that, while the spin-wave excitations around the M point vanish above the transition temperature T_\rm N, the pure magnetic continuous excitations around the \varGamma point are robust against temperature. Furthermore, by calculating the dynamical spin-spin correlation function using the cluster perturbation theory, we derive magnetic dispersion spectra based on the K–\varGamma model, which involves with a ferromagnetic Kitaev interaction of -7.2 meV and an off-diagonal interaction of 5.6 meV. We find this model can reproduce not only the spin-wave excitation spectra around the M point, but also the non-spin-wave continuous magnetic excitations around the \varGamma point. These results provide evidence for the existence of fractional excitations around the \varGamma point originating from the Kitaev QSL state, and further support the validity of the K–\varGamma model as the effective minimal spin model to describe \alpha-RuCl_3.