Low-symmetry topological materials for large charge-to-spin interconversion: The case of transition metal dichalcogenide monolayers

TL;DR

This paper predicts that low-symmetry transition metal dichalcogenide monolayers exhibit a giant, gate-tunable spin Hall effect with high charge-to-spin conversion efficiency, driven by their unique electronic topology and symmetry properties.

Contribution

It introduces a theoretical prediction of a large, tunable spin Hall effect in MoTe2 and WTe2 monolayers, highlighting the role of reduced symmetry and electronic topology in enhancing spintronic functionalities.

Findings

Predicted large spin Hall effect figure of merit (λsθxy ~ 1-50 nm) in MoTe2 and WTe2 monolayers.

Identified long spin diffusion lengths and high charge-to-spin conversion efficiency (~80%).

Linked the effects to momentum-invariant spin textures and large spin Berry curvature.

Abstract

The spin polarization induced by the spin Hall effect (SHE) in thin films typically points out of the plane. This is rooted on the specific symmetries of traditionally studied systems, not in a fundamental constraint. Recently, experiments on few-layer ${\rm MoTe}_2$ and ${\rm WTe}_2$ showed that the reduced symmetry of these strong spin-orbit coupling materials enables a new form of {\it canted} spin Hall effect, characterized by concurrent in-plane and out-of-plane spin polarizations. Here, through quantum transport calculations on realistic device geometries, including disorder, we predict a very large gate-tunable SHE figure of merit $\lambda_s\theta_{xy}\sim 1\text{--}50$ nm in ${\rm MoTe}_2$ and ${\rm WTe}_2$ monolayers that significantly exceeds values of conventional SHE materials. This stems from a concurrent long spin diffusion length ($\lambda_s$) and charge-to-spin interconversion efficiency as large as $\theta_{xy} \approx 80$\%, originating from momentum-invariant (persistent) spin textures together with large spin Berry curvature along the Fermi contour, respectively. Generalization to other materials and specific guidelines for unambiguous experimental confirmation are proposed, paving the way towards exploiting such phenomena in spintronic devices. These findings vividly emphasize how crystal symmetry and electronic topology can govern the intrinsic SHE and spin relaxation, and how they may be exploited to broaden the range and efficiency of spintronic materials and functionalities.