Spin Relaxation and Diffusion in Monolayer 1T'-WTe₂ from First-Principles

Understanding spin relaxation in topological systems such as quantum spin-hall (QSH) insulator is critical for realizing coherent transport at high temperature. WTe₂, known as a QSH insulator with a high transition temperature of 100K, is an important test-bed of unveiling spin relaxation mechanism in topological materials. In this work, we employ our recently-developed ab initio density-matrix dynamics approach to investigate spin relaxation mechanism, and calculate spin lifetime and diffusion length of monolayer 1T'-WTe₂, at finite temperature under an external electric field. We found the spin lifetime of electrons have the largest anisotropy when measuring along the canted-spin-texture direction. Moreover, we found an opposite trend between spin and carrier relaxation against applied electric field. Most importantly, the relaxation mechanism under intermediate electric field around 1V/nm can not be explained by either Eillot-Yafet or Dyakonov-Perel models, which highlights the generality of our ab initio density-matrix framework. We then proposed analytical models to explain its mechanism and compare well with ab initio results at small and large electric field. We predict that spin lifetime and spin diffusion length of bulk-state electrons are ~1 ps and ~30 nm at room temperature respectively, suggesting its promise for spintronic applications.