Realization of valley and spin pumps by scattering at nonmagnetic disorders

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 nu-
merically 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 a new opportunity towards valley-spintronics,
turning disorders from a deleterious factor to a resource of valley and spin polarization.