Valley filter and valley valve in graphene

TL;DR

This paper demonstrates how to create and control valley polarization in graphene using ballistic point contacts with zigzag edges, enabling a valley valve device similar to a spin valve but without magnetic fields.

Contribution

It introduces a method to realize and invert valley polarization in graphene via local gating of ballistic point contacts, proposing a valley valve device.

Findings

Valley polarization can be achieved with zigzag edge contacts.

Gate voltage can invert the valley polarization.

A valley valve can be constructed with two series valley filters.

Abstract

It is known that the lowest propagating mode in a narrow ballistic ribbon of graphene may lack the twofold valley degeneracy of higher modes. Depending on the crystallographic orientation of the ribbon axis, the lowest mode mixes both valleys or lies predominantly in a single valley (chosen by the direction of propagation). We show, using a tight-binding model calculation, that a nonequilibrium valley polarization can be realized in a sheet of graphene, upon injection of current through a ballistic point contact with zigzag edges. The polarity can be inverted by local application of a gate voltage to the point contact region. Two valley filters in series may function as an electrostatically controlled ``valley valve'', representing a zero-magnetic-field counterpart to the familiar spin valve.