Multiple Dirac cones and spontaneous quantum anomalous Hall state in monolayer transition metal trichalcogenides

Atomically-thin magnets have attracted much attention for potential application to next-generation devices. Among many van der Waals materials, the transition metal trichalcogenides (TMTs) have gained great interest since the discovery of ferromagnetism in the few-layer forms. Although the interplay between electron correlations and the spin-orbit coupling (SOC) in TMTs would play a key role to realize the exotic phase of matters in the purely 2D systems, their potentials have not been fully explored. Here, using the ab initio calculations, we investigate the electronic properties of the group 10 TMTs in the monolayer form [1]. We find that e_g-orbital bands possess multiple Dirac cones near the Fermi level. The origin of the multiple Dirac cones is ascribed to a honeycomb superstructure composed of the third-neighbor hopping enhanced by the spatial symmetry of orbitals and crystal structure of the monolayer TMTs. We also clarify that the SOC gaps out these Dirac cones and the mass gap as well as the valley structure can be tuned by the trigonal distortion. Furthermore, we elucidate that electron correlations and the SOC may turn the multiple-Dirac semimetal to a quantum anomalous Hall insulator with a high Chern number. [1] Y. Sugita, T. Miyake, and Y. Motome, arXiv:1704.00318.