Spin-valley-coupled quantum spin Hall insulator with topological Rashba-splitting edge states in Janus monolayer...

Achieving combination of spin and valley polarized states with topological insulating phase is pregnant to promote the fantastic integration of topological physics, spintronics and valleytronics. In this work, a spin-valley-coupled quantum spin Hall insulator (svc-QSHI) is predicted in Janus monolayer CSb_1.5Bi_1.5with dynamic, mechanical and thermal stabilities. Calculated results show that the CSb_1.5Bi_1.5is a direct band gap semiconductor with and without spin-orbit coupling, and the conduction-band minimum and valence-band maximum are at valley point. The inequivalent valleys have opposite Berry curvature and spin moment, which can produce a spin-valley Hall effect. In the center of Brillouin zone, a Rashba-type spin splitting can be observed due to missing horizontal mirror symmetry. The topological characteristic of CSb_1.5Bi_1.5is confirmed by theZ₂invariant and topological protected conducting helical edge states. Moreover, the CSb_1.5Bi_1.5shows unique Rashba-splitting edge states. Both energy band gap and spin-splitting at the valley point are larger than the thermal energy of room temperature (25 meV) with generalized gradient approximation level, which is very important at room temperature for device applications. It is proved that the spin-valley-coupling and nontrivial quantum spin Hall state are robust again biaxial strain. Our work may provide a new platform to achieve integration of topological physics, spintronics and valleytronics.