Van der Waals heterostructures assembled from atomically thin crystals are ideal model systems to study spin-orbital coupled transport because they exhibit a strong interplay between spin, lattice, and valley degrees of freedom that can be manipulated by strain, electric bias, and proximity effects. The recently predicted spin- helical regime in graphene on transition metal dichalcogenides, in which spin and pseudospin degrees of freedom are locked together [Offidani et al., Phys. Rev. Lett. 119, 196801 (2017)] suggests their potential application in spintronics. Here, by deriving an Eilenberger equation for the quasiclassical Green’s function of two-dimensional Dirac fermions in the presence of spin-orbit coupling (SOC) and scalar disorder, we obtain analytical expressions for the dc spin galvanic susceptibility and spin Hall conductivity in the spin-helical regime. Our results disclose a sign change in the spin Hall angle (SHA) when the Fermi energy relative to the Dirac point matches the Bychkov-Rashba energy scale, irrespective of the magnitude of the spin-valley interaction imprinted on the graphene layer. The behavior of the SHA is connected to a reversal of the total internal angular momentum of Bloch electrons that reflects the spin-pseudospin entanglement induced by SOC. We also show that the charge- to-spin conversion efficiency reaches a maximum when the Fermi level lies at the edge of the spin-minority band in agreement with previous findings. Both features are fingerprints of spin-helical Dirac fermions and suggest a direct way to estimate the strength of proximity-induced SOC from transport data. The relevance of these findings for interpreting recent spin-charge conversion measurements in nonlocal spin-valve geometry is also discussed.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)