Graphene -a recently discovered one-atom-thick layer of graphite- constitutesa new model system in condensed matter physics, because it is the firstmaterial in which charge carriers behave as massless chiral relativisticparticles. The anomalous quantization of the Hall conductance, which is nowunderstood theoretically, is one of the experimental signatures of the peculiartransport properties of relativistic electrons in graphene. Other unusualphenomena, like the finite conductivity of order 4e^2/h at the chargeneutrality (or Dirac) point, have come as a surprise and remain to beexplained. Here, we study the Josephson effect in graphene. Our experimentsrely on mesoscopic superconducting junctions consisting of a graphene layercontacted by two closely spaced superconducting electrodes, where the chargedensity can be controlled by means of a gate electrode. We observe asupercurrent that, depending on the gate voltage, is carried by eitherelectrons in the conduction band or by holes in the valence band. Moreimportantly, we find that not only the normal state conductance of graphene isfinite, but also a finite supercurrent can flow at zero charge density. Ourobservations shed light on the special role of time reversal symmetry ingraphene and constitute the first demonstration of phase coherent electronictransport at the Dirac point.
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