Bipolar supercurrent in graphene

TL;DR

This paper demonstrates phase-coherent supercurrent flow in graphene at the Dirac point, revealing unique transport properties and the influence of time reversal symmetry in a material where charge carriers behave as relativistic particles.

Contribution

It provides the first experimental evidence of a supercurrent in graphene at zero charge density, highlighting the role of relativistic charge carriers in superconducting transport.

Findings

Finite supercurrent observed at the Dirac point

Supercurrent carried by electrons or holes depending on gate voltage

Phase coherence demonstrated in graphene at charge neutrality

Abstract

Graphene -a recently discovered one-atom-thick layer of graphite- constitutes a new model system in condensed matter physics, because it is the first material in which charge carriers behave as massless chiral relativistic particles. The anomalous quantization of the Hall conductance, which is now understood theoretically, is one of the experimental signatures of the peculiar transport properties of relativistic electrons in graphene. Other unusual phenomena, like the finite conductivity of order 4e^2/h at the charge neutrality (or Dirac) point, have come as a surprise and remain to be explained. Here, we study the Josephson effect in graphene. Our experiments rely on mesoscopic superconducting junctions consisting of a graphene layer contacted by two closely spaced superconducting electrodes, where the charge density can be controlled by means of a gate electrode. We observe a supercurrent that, depending on the gate voltage, is carried by either electrons in the conduction band or by holes in the valence band. More importantly, we find that not only the normal state conductance of graphene is finite, but also a finite supercurrent can flow at zero charge density. Our observations shed light on the special role of time reversal symmetry in graphene and constitute the first demonstration of phase coherent electronic transport at the Dirac point.