Charge-spin interconversion in graphene-based systems from density functional theory

TL;DR

This paper introduces a first-principles density functional theory method to analyze charge-spin interconversion in graphene, revealing insights into spin Hall and Rashba-Edelstein effects and their interplay.

Contribution

It develops a novel DFT-based quantum transport approach to study charge-spin conversion mechanisms in 2D materials with spin-orbit coupling.

Findings

Flow of unpolarized current causes spin Hall effect signatures.

Spin accumulation indicates Rashba-Edelstein effect presence.

Unexpected competition between Rashba and intrinsic spin-orbit coupling.

Abstract

We present a methodology to address, from first principles, charge-spin interconversion in two-dimensional materials with spin-orbit coupling. Our study relies on an implementation of density functional theory based quantum transport formalism adapted to such purpose. We show how an analysis of the $k$-resolved spin polarization gives the necessary insight to understand the different charge-spin interconversion mechanisms. We have tested it in the simplest scenario of isolated graphene in a perpendicular electric field where effective tight-binding models are available to compare with. Our results show that the flow of an unpolarized current across a single layer of graphene produces, as expected, a spin separation perpendicular to the current for two of the three spin components (out-of-plane and longitudinal), which is the signature of the spin Hall effect. Additionally, it also yields an overall spin accumulation for the third spin component (perpendicular to the current), which is the signature of the Rashba-Edelstein effect. Even in this simple example, our results reveal an unexpected competition between the Rashba and the intrinsic spin-orbit coupling. Remarkably, the sign of the accumulated spin density does not depend on the electron or hole nature of the injected current for realistic values of the Rashba coupling.