Intrinsic spin-valley-coupled Dirac state in Janus functionalized β-BiAs monolayer

Recently, a new class of 2D Dirac materials, spin-valley-coupled Dirac semimetals (svc-DSMs), was proposed in strained SbAsX₂ monolayers (MLs) and transition metal dichalcogenide-supported graphene. Owing to the superb properties, including Dirac spin-valley Hall effect and dissipationless transport, svc-DSMs provide an ideal platform for exploring the integration of Dirac physics, spintronics and valleytronics. However, the predicted candidate materials are all extrinsic, requiring tensile strain or proximity effect. Using first-principles calculations, herein we identify that strain-free BrBiAsCl ML is an intrinsic svc-DSM that is located at the boundary between 2D trivial insulators and topological insulators owing to the balance between spin-orbit coupling (SOC) and the built-in polarized vertical electric field. Under inversion asymmetry, the strong SOC in BrBiAsCl ML induces giant spin-splittings in both the uppermost valence band and the lowermost conduction band, rendering a nearly closed bulk gap and the formation of a spin-valley-dependent Dirac cone. Remarkably, such an svc-DSM state can be well preserved in BrBiAsCl ML when supported on a proper substrate, which is indispensable for the application of svc-DSMs in devices.