1. Kavli Institute of Nanoscience, Delft University of Technology, PO Box 5046, 2600 GA, Delft, The Netherlands
2. These authors contributed equally to this work.

Correspondence to: Hubert B. Heersche^1,2Pablo Jarillo-Herrero^1,2 Correspondence and requests for materials should be addressed to H.B.H. (Email: h.b.heersche@tudelft.nl) or P.J.-H. (Email: P.D.Jarillo-Herrero@tudelft.nl).

Abstract

Graphene—a recently discovered form of graphite only one atomic layer thick¹—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^{2, 3}, which is now understood theoretically^{4, 5}, 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 4e²/h (where e is the electron charge and h is Planck's constant) at the charge neutrality (or Dirac) point², have come as a surprise and remain to be explained^{5, 6, 7, 8, 9, 10, 11, 12, 13}. Here we experimentally study the Josephson effect¹⁴ in mesoscopic junctions consisting of a graphene layer contacted by two closely spaced superconducting electrodes¹⁵. The charge density in the graphene layer 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 demonstrate phase coherent electronic transport at the Dirac point.

1. Kavli Institute of Nanoscience, Delft University of Technology, PO Box 5046, 2600 GA, Delft, The Netherlands
2. These authors contributed equally to this work.

Correspondence to: Hubert B. Heersche^1,2Pablo Jarillo-Herrero^1,2 Correspondence and requests for materials should be addressed to H.B.H. (Email: h.b.heersche@tudelft.nl) or P.J.-H. (Email: P.D.Jarillo-Herrero@tudelft.nl).

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