Spin Relaxation and Dephasing in Solids from Ab Initio Density-matrix Dynamics*

Designing new quantum materials with long-lived electron spin states is in urgent need of a general theoretical formalism and computational technique to reliably predict spin lifetimes. We present a new, universal first-principles methodology based on density matrix (DM) dynamics for open quantum systems to calculate the spin relaxation and decoherence time of solids with arbitrary spin mixing and crystal symmetry. In particular, this method describes contributions of the Elliott-Yatfet (EY) and D’yakonov-Perel (DP) mechanisms to spin relaxation, on an equal footing[1]. Our ab initio predictions are in excellent agreement with experimental spin lifetime for a broad range of materials, such as Si, bcc Fe, MoS₂, graphene as well as GaAs.

We then discuss our implementation of real-time DM dynamics for time-resolved Kerr rotation and spin dynamics under external electric and magnetic field [2]. Through the complete theoretical descriptions of pump, probe and scattering processes including electron-phonon, electron-impurity and electron-electron scattering with spin-orbit coupling, our method can directly simulate the time resolved measurements for coupled spin and electron dynamics when varying temperatures and doping levels. We use this method to simulate spin dynamics of several 3D and 2D systems and obtain excellent agreement with experiments. It is found that the relative contributions of different scattering mechanisms and phonon modes vary considerably between spin and carrier relaxation processes. We further apply our method to investigate the condition of realizing long spin lifetime and spin-valley locking of 2D Dirac materials such as germanene under electric field, and understand substrate effects on spin relaxation in graphene and germanene, resolving the contribution of electron-phonon coupling and proximity effect on spin-orbit coupling. Finally we show our recent implementation of ab-initio Land´e g-factor for solids and the important effect of g factor inhomogeneity on spin dephasing under an external magnetic field in halide perovskites. Our work provides important insights and guidelines for materials design in spintronics and spin-based quantum information science.