Nonlinear valley and spin currents from Fermi pocket anisotropy in 2D crystals

TL;DR

This paper reveals that anisotropic Fermi pockets in 2D crystals enable nonlinear generation of pure spin and valley currents via electric bias or temperature gradients, offering new avenues for spintronic and valleytronic devices.

Contribution

It introduces a universal mechanism for nonlinear spin and valley current generation in 2D materials based on Fermi pocket anisotropy, including second-order responses and thermoelectric effects.

Findings

Nonlinear spin and valley currents can be generated without net charge flow.

Pure spin and valley flows are achievable using AC bias or inhomogeneous temperature gradients.

Significant currents are predicted in graphene, TMDs, and gallium selenide monolayers.

Abstract

Controlled flow of spin and valley pseudospin is key to future electronics exploiting these internal degrees of freedom of carriers. Here we discover a universal possibility for generating spin and valley currents by electric bias or temperature gradient only, which arises from the anisotropy of Fermi pockets in crystalline solids. We find spin and valley currents to the second order in the electric field, as well as their thermoelectric counterparts, i.e. the nonlinear spin and valley Seebeck effects. These second-order nonlinear responses allow two unprecedented possibilities to generate pure spin and valley flows without net charge current: (i) by an AC bias; or (ii) by an arbitrary inhomogeneous temperature distribution. As examples, we predict appreciable nonlinear spin and valley currents in two-dimensional (2D) crystals including graphene, monolayer and trilayer transition metal dichalcogenides, and monolayer gallium selenide. Our finding points to a new route towards electrical and thermal generations of spin and valley currents for spintronic and valleytronic applications based on 2D quantum materials.