Direct observation of distinct bulk and edge nonequilibrium spin accumulation in ultrathin MoTe$_2$

TL;DR

This study directly measures and distinguishes bulk and edge spin accumulations in ultrathin MoTe$_2$, revealing unique spin behaviors linked to crystal symmetry and thickness, with implications for spintronic device design.

Contribution

It provides the first direct spatially resolved measurements of spin accumulation in ultrathin MoTe$_2$, clarifying the roles of different charge-to-spin conversion mechanisms and their dependence on material thickness.

Findings

Disentangled spin Hall and Rashba-Edelstein contributions.

Uncovered unconventional spin polarizations at edges and bulk.

Linked spin behaviors to crystal symmetry and layer thickness.

Abstract

Low-symmetry two-dimensional (2D) topological materials such as MoTe$_2$ host efficient charge-to-spin conversion (CSC) mechanisms that can be harnessed for novel electronic and spintronic devices. However, the nature of the various CSC mechanisms and their correlation with underlying crystal symmetries remain unsettled. In this work, we use local spin-sensitive electrochemical potential measurements to directly probe the spatially dependent nonequilibrium spin accumulation in MoTe$_2$ flakes down to four atomic layers. We are able to clearly disentangle contributions originating from the spin Hall and Rashba-Edelstein effects and uncover an abundance of unconventional spin polarizations that develop uniquely in the sample bulk and edges with decreasing thickness. Using ab-initio calculations, we construct a unified understanding of all the observed CSC components in relation to the material dimensionality and stacking arrangement. Our findings not only illuminate previous CSC results on MoTe$_2$ but also have important ramifications for future devices that can exploit the local and layer-dependent spin properties of this 2D topological material.