Substrate Effects on Spin Relaxation in 2D Dirac Materials from First-Principles Density-Matrix Dynamics

Effects of substrates are crucial for spin relaxation and transport in 2D Dirac Materials. Substrates provide support of samples and profoundly modify the physical properties of materials through proximity effect and interlayer couplings. However, in past theory substrate effects were often described by simple models. More importantly, the modifications of phonons and electron-phonon (e-ph) couplings by substrates were ignored or treated very approximately due to theoretical and computational difficulties. Such limitations have been overcome by our recently developed First-Principles Density-Matrix (FPDM) approach [1]. Here, through FPDM simulations with e-ph and spin-orbit couplings, we investigate the substrate effects on spin relaxation and transport properties of two 2D Dirac materials – graphene and germanene on different substrates. We find that for graphene, spin lifetime is strongly reduced by the substrate due to the induced "internal magnetic fields" which activate a new spin relaxation mechanism. For spin relaxation in germanene, spin relaxation becomes dominated by intervalley scattering at low temperatures if with the substrate. And the change of spin relaxation by a substrate is largely due to the substrate-induced modification of e-ph interactions.