  In the presence of spin-orbit coupling (SOC), achieving both spin and valley
polarized Dirac state is significant to promote the fantastic integration of
Dirac physics, spintronics and valleytronics. Based on ab initio calculations,
here we demonstrate that a class of spin-valley-coupled Dirac semimetals
(svc-DSMs) in the functionalized SbAs monolayers (MLs) can host such desired
state. Distinguished from the graphene-like 2D Dirac materials, the Dirac cones
in svc-DSMs hold giant spin-splitting induced by strong SOC under inversion
symmetry breaking. In the 2.3% strained SbAsH2 ML, the Dirac fermions in
inequivalent valleys have opposite Berry curvature and spin moment, giving rise
to Dirac spin-valley Hall effect with constant spin Hall conductivity as well
as massless and dissipationless transport. Topological analysis reveals that
the svc-DSM emerges at the boundary between trivial and 2D topological
insulators, which provides a promising platform for realizing the flexible and
controllable tuning among different quantum states.
