Emergence of a spin-valley Dirac semimetal in a strained group-VA monolayer.

The combination of Dirac and Valley physics in one single-layer system is a very interesting topic and has received widespread attention in materials science and condensed matter physics. Using density-functional theoretical calculations, we predict that a two-dimensional (2D) cyanided group-VA monolayer, MAs(CN) (M = Sb, Bi), can turn into the spin-valley Dirac point (svDP) state under external strains. In sharp contrast to the symmetry protected 2D Dirac semimetal (DSM), the Dirac Fermions in svDP materials are spin non-degenerate due to strong spin-splitting under SOC. Remarkably, the Dirac fermions in inequivalent valleys can host opposite Berry curvature and spin moment, leading to the Dirac spin-valley Hall effect with dissipationless transport. We also find that the svDP of MAs(CN) is a critical state of topological phase transition between the trivial and nontrivial states. An effective tight-binding model is used to unveil the physics of svDP and topological phase transition under strain. These results will provide a route towards the integration of spin-valley indexes in 2D Dirac materials and design multipurpose and controllable devices in valleytronics.