Evidence for Magnetic Fractional Excitations in a Kitaev Quantum-Spin-Liquid Candidate $\alpha$-RuCl$_3$
Kejing Ran1,2†, Jinghui Wang1,2†, Song Bao2, Zhengwei Cai2, Yanyan Shangguan2, Zhen Ma2,3, Wei Wang4, Zhao-Yang Dong5, P. Čermák6,7, A. Schneidewind6, Siqin Meng8,9, Zhilun Lu8,10, Shun-Li Yu2,11*, Jian-Xin Li2,11*, and Jinsheng Wen2,11*
1School of Physical Science and Technology, and ShanghaiTech Laboratory for Topological Physics, ShanghaiTech University, Shanghai 200031, China 2National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University, Nanjing 210093, China 3Institute for Advanced Materials, Hubei Normal University, Huangshi 435002, China 4School of Science, Nanjing University of Posts and Telecommunications, Nanjing 210023, China 5Department of Applied Physics, Nanjing University of Science and Technology, Nanjing 210094, China 6Jülich Centre for Neutron Science (JCNS) at Heinz Maier-Leibnitz Zentrum (MLZ), Forschungszentrum Jülich GmbH, Lichtenbergstr. 1, 85748 Garching, Germany 7Charles University, Faculty of Mathematics and Physics, Department of Condensed Matter Physics, Ke Karlovu 5, 121 16, Praha, Czech Republic 8Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Hahn-Meitner-Platz 1D-14109, Berlin, Germany 9China Institute of Atomic Energy (CIAE), Beijing 102413, China 10The Henry Royce Institute and Department of Materials Science and Engineering, The University of Sheffield, Sir Robert Hadfield Building, Sheffield, S1 3JD, United Kingdom 11Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
Abstract：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$_3$.
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2022, Vol. 39 
