# Spin-orbit coupling in elemental two-dimensional materials

## TL;DR

This study investigates spin-orbit coupling and spin mixing in various two-dimensional elemental materials using first-principles calculations, revealing how these effects depend on atomic number and can be tuned by Fermi level adjustments.

## Contribution

It provides detailed first-principles analysis of spin-orbit effects in 2D materials, including spin mixing, anisotropy, and their dependence on atomic number and symmetry.

## Key findings

- Spin-orbit coupling scales as Z^2, with spin admixture b^2 scaling as Z^4.
- Non-zero spin mixing exists in graphene despite mirror symmetry.
- Spin-mixing anisotropy can be controlled by Fermi level tuning.

## Abstract

The fundamental spin-orbit coupling and spin mixing in graphene and rippled honeycomb lattice materials silicene, germanene, stanene, blue phosphorene, arsenene, antimonene, and bismuthene is investigated from first principles. The intrinsic spin-orbit coupling in graphene is revisited using multi-band $k\cdot p$ theory, showing the presence of non-zero spin mixing in graphene despite the mirror symmetry. However, the spin mixing itself does not lead to the the Elliott-Yafet spin relaxation mechanism, unless the mirror symmetry is broken by external factors. For other aforementioned elemental materials we present the spin-orbit splittings at relevant symmetry points, as well as the spin admixture $b^2$ as a function of energy close to the band extrema or Fermi levels. We find that spin-orbit coupling scales as the square of the atomic number Z, as expected for valence electrons in atoms. For isolated bands, it is found that $b^2\sim Z^4$. The spin-mixing parameter also exhibits giant anisotropy which, to a large extent, can be controlled by tuning the Fermi level. Our results for $b^2$ can be directly transferred to spin relaxation time due to the Elliott-Yafet mechanism, and therefore provide an estimate of the upper limit for spin lifetimes in materials with space inversion center.