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$\alpha$-RuCl$_3$ has been studied extensively because of its proximity tothe Kitaev quantum-spin-liquid (QSL) phase and the possibility of approachingit by tuning the competing interactions. Here we present the first polarizedinelastic neutron scattering study on $\alpha$-RuCl$_3$ single crystals toexplore the scattering continuum around the $\Gamma$ point at the Brillouinzone center, which was hypothesized to be resulting from the Kitaev QSL statebut without concrete evidence. With polarization analyses, we find that whilethe spin-wave excitations around the M point vanish above the transitiontemperature $T_{\rm N}$, the pure magnetic continuous excitations around the$\Gamma$ point are robust against temperature. Furthermore, by calculating thedynamical spin-spin correlation function using the cluster perturbation theory,we derive magnetic dispersion spectra based on the $K$-$\Gamma$ model, whichinvolves with a ferromagnetic Kitaev interaction of -7.2 meV and anoff-diagonal interaction of 5.6 meV. We find this model can reproduce not onlythe spin-wave excitation spectra around the M point, but also the non-spin-wavecontinuous magnetic excitations around the $\Gamma$ point. These resultsprovide evidence for the existence of fractional excitations around the$\Gamma$ point originating from the Kitaev QSL state, and further support thevalidity of the $K$-$\Gamma$ model as the effective minimal spin model todescribe $\alpha$-RuCl$_3$.

Evidence for magnetic fractional excitations in a Kitaev quantum-spin-liquid candidate $\alpha$-RuCl$_3$