Abstract
Spintronics is one of the most promising applications of the two-dimensional material graphene. Although pristine graphene has negligible spin-orbit coupling (SOC), both theory and experiment suggest that SOC in graphene can be enhanced by extrinsic means, such as functionalization by adatom impurities. Using the semiclassical Boltzmann transport equation, I will first discuss the spin-charge coupled dynamics and the ubiquitous spin Hall effect in adatoms functionalized graphene. Next, I will go beyond the semiclassical theory and discuss a quantum Boltzmann theory that accounts for the spin-coherent dynamics of the carriers. This theory predicts a novel “anisotropic spin precession” (ASP) scattering process in graphene, which contributes to a large current-induced spin polarization and modifies the standard spin Hall effect. More importantly, the ASP scattering couples the electric current directly to the spin density in the spin-continuity equation without any constitutive relationships. Hence, it is a form of direct mangetoelectric coupling. Next, I will show that ASP scattering can also arise in two-dimensional electron gases lacking inversion symmetry. Therefore, ASP scattering mechanism and the associated direct magnetoelectric coupling is a universal spin transport phenomena that is independent of the microscopic details of the disorder potential. Direct mangetoelectric coupling gives rise to two anomalous features in the nonlocal transport behavior of two-dimensional metallic materials. Firstly, the nonlocal resistance can have negative values and oscillate with distance, even in the absence of a magnetic field. Secondly, the oscillations of the nonlocal resistance under an applied in-plane magnetic field (Hanle effect) can be asymmetric under field reversal. Our study provides theoretical foundations for designing future graphene-based integrated spintronic devices.