%A Kejing Ran, Jinghui Wang, Song Bao, Zhengwei Cai, Yanyan Shangguan, Zhen Ma, Wei Wang, Zhao-Yang Dong, P. Čermák, A. Schneidewind, Siqin Meng, Zhilun Lu, Shun-Li Yu, Jian-Xin Li, and Jinsheng Wen
%T Evidence for Magnetic Fractional Excitations in a Kitaev Quantum-Spin-Liquid Candidate $\alpha$-RuCl$_3$
%0 Journal Article
%D 2022
%J Chin. Phys. Lett.
%R 10.1088/0256-307X/39/2/027501
%P 027501%V 39
%N 2
%U {https://cpl.iphy.ac.cn/CN/abstract/article_116057.shtml}
%8 2021-12-27
%X 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$_3$.