Nonequilibrium valley polarization in graphene nanoconstrictions

TL;DR

This paper demonstrates that nonequilibrium valley polarization in graphene nanoconstrictions is robust against orientation changes and can be manipulated to direct valley-polarized currents, enabling potential valleytronic devices.

Contribution

It shows the robustness of valley polarization in graphene filters against orientation changes and proposes a method to control valley currents in bent graphene structures.

Findings

Valley polarization persists unless the filter has perfect armchair edges.

Polarization inverts when crossing the armchair orientation.

Valley-polarized current can be directed to one edge in bent graphene.

Abstract

We recently shown, using tight-binding calculations, that nonequilibrium valley polarization can be realized in graphene, when the current is injected through "valley filter": a ballistic point contact with zigzag edges. Here we demonstrate, that the effect is surprisingly robust against changing the crystallographic orientation of the filter axis. Namely, the output current remains polarized unless a point contact has perfect armchair edges, at which two subblattices are equally represented. The polarization is inverted when the filter orientation crosses the amchair line and, subsequently, dominating subblattice index of terminal atoms changes. In a bended graphene strip, the valley-polarized current can be directed towards one edge providing a theoretical possibility to observe a zero-magnetic-field analogue of the well-known Hall effect. For the valley valve, build of two valley filters in series and controlled elecrostatically by a gate voltage, the conductance-to-gate characteristic is inverted when $\pi/3$ vertex is placed between two filters.