Giant Spin Lifetime Anisotropy and Spin-Valley Locking in Silicene and Germanene from First-Principles Density-Matrix Dynamics

TL;DR

This study uses first-principles simulations to explore spin relaxation in silicene and germanene, revealing giant anisotropy and spin-valley locking, with germanene showing exceptionally long spin lifetimes suitable for spintronic applications.

Contribution

It introduces a first-principles density-matrix dynamics approach to analyze spin relaxation mechanisms in silicene and germanene, highlighting novel spin lifetime anisotropy and spin-valley locking effects.

Findings

Germanene exhibits giant spin lifetime anisotropy.

Germanene has extremely long spin lifetime (~100 ns at 50 K).

The electric field dependence of spin relaxation varies between silicene and germanene.

Abstract

Through First-Principles real-time Density-Matrix (FPDM) dynamics simulations, we investigate spin relaxation due to electron-phonon and electron-impurity scatterings with spin-orbit coupling in two-dimensional Dirac materials - silicene and germanene, at finite temperatures and under external fields. We discussed the applicability of conventional descriptions of spin relaxation mechanisms by Elliott-Yafet (EY) and D'yakonov-Perel' (DP) compared to our FPDM method, which is determined by a complex interplay of intrinsic spin-orbit coupling, external fields, and electron-phonon coupling strength, beyond crystal symmetry. For example, the electric field dependence of spin relaxation time is close to DP mechanism for silicene at room temperature, but rather similar to EY mechanism for germanene. Due to its stronger spin-orbit coupling strength and buckled structure in sharp contrast to graphene, germanene has a giant spin lifetime anisotropy and spin valley locking effect under nonzero Ez and relatively low temperature. More importantly, germanene has extremely long spin lifetime (~100 ns at 50 K) and ultrahigh carrier mobility, which makes it advantageous for spin-valleytronic applications.