Detection of valley polarization in graphene by a superconducting contact

TL;DR

This paper proposes a method to detect valley polarization in graphene using superconducting contacts and Andreev reflection, linking conductance measurements to valley isospin angles.

Contribution

It introduces a novel approach to measure valley polarization in graphene via superconducting junction conductance related to valley isospin angles.

Findings

Conductance depends on the valley isospin angle Theta.

No current enters the superconductor when Theta=0.

Measurement of conductance yields intervalley relaxation time.

Abstract

Because the valleys in the band structure of graphene are related by time-reversal symmetry, electrons from one valley are reflected as holes from the other valley at the junction with a superconductor. We show how this Andreev reflection can be used to detect the valley polarization of edge states produced by a magnetic field. In the absence of intervalley relaxation, the conductance G_NS=2(e^2/h)(1-cos(Theta)) of the junction on the lowest quantum Hall plateau is entirely determined by the angle Theta between the valley isospins of the edge states approaching and leaving the superconductor. If the superconductor covers a single edge, Theta=0 and no current can enter the superconductor. A measurement of G_NS then determines the intervalley relaxation time.