Valley and spin pump by scattering at non-magnetic disorders

TL;DR

This paper reveals that non-magnetic disorder scattering in 2D TMDs can induce valley and spin pumping, offering a new approach for valleytronics and spintronics by turning disorder into a resource.

Contribution

It demonstrates that valley and spin pumping can be achieved through scattering at non-magnetic disorders, independent of specific material properties.

Findings

Valley pump effect driven by charge current in TMD nanoribbons.

Spin pump effect for spin-valley locked holes due to non-magnetic disorder scattering.

Disorders can be harnessed as a resource for valley and spin polarization.

Abstract

In solid, the crystalline structure can endow electron an internal degree of freedom known as valley, which characterizes the degenerate energy minima in momentum space. The recent success in optical pumping of valley polarization in 2D transition metal dichalcogenides (TMDs) has greatly promoted the concept of valley-based informatics and electronics. However, between the demonstrated valley polarization of transient electron-hole pair excitations and practical valleytronic operations, there exist obvious gaps to fill, among which is the valley pump of long-lived charge carriers. Here we discover that the quested valley pump of electrons or holes can be realized simply by scattering at the ubiquitous nonmagnetic disorders, not relying on any specific material property. The mechanism is rooted in the nature of valley as a momentum space index: the intervalley backscattering in general has valley contrasted rate due to the distinct momentum transfers, causing a net transfer of population from one valley to another. As examples, we numerically demonstrate the sizable valley pump effects driven by charge current in nanoribbons of monolayer TMDs, where the spin-orbit scattering by non-magnetic disorders also realizes spin pump for the spin-valley locked holes. Our finding points to an unexpected new opportunity towards valley-spintronics, turning disorders from a deleterious factor to a resource of valley and spin polarization.