Unified Framework for Charge-Spin Interconversion in Spin-Orbit Materials

TL;DR

This paper introduces a unified physical model for charge-spin interconversion in spin-orbit materials, linking experimental data across various materials through a simple inverse relationship involving the density of states.

Contribution

It provides a unifying framework that explains charge-spin conversion efficiency based on density of states and introduces scaling laws for spin-orbit torque efficiency.

Findings

Experimental data follow the inverse relationship predicted by the model.

The model's figure-of-merits align well with diverse experimental results.

A scaling law for SOT efficiency with carrier concentration is identified.

Abstract

Materials with spin-orbit coupling are of great interest for various spintronics applications due to the efficient electrical generation and detection of spin-polarized electrons. Over the past decade, many materials have been studied, including topological insulators, transition metals, Kondo insulators, semimetals, semiconductors, and oxides; however, there is no unifying physical framework for understanding the physics and therefore designing a material system and devices with the desired properties. We present a model that binds together the experimental data observed on the wide variety of materials in a unified manner. We show that in a material with a given spin-momentum locking, the density of states plays a crucial role in determining the charge-spin interconversion efficiency, and a simple inverse relationship can be obtained. Remarkably, experimental data obtained over the last decade on many different materials closely follow such an inverse relationship. We further deduce two figure-of-merits of great current interest: the spin-orbit torque (SOT) efficiency (for the direct effect) and the inverse Rashba-Edelstein effect length (for the inverse effect), which statistically show good agreement with the existing experimental data on wide varieties of materials. Especially, we identify a scaling law for the SOT efficiency with respect to the carrier concentration in the sample, which agrees with existing data. Such an agreement is intriguing since our transport model includes only Fermi surface contributions and fundamentally different from the conventional views of the SOT efficiency that includes contributions from all the occupied states.