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Measurements of phase coherent transport in graphene have so far given unexpected results. For instance, the magnitude of weak localization appears to be sample dependent and it is often suppressed or even absent. Another phenomena, that like weak-localization depends on the presence of phase coherence and of time reversal symmetry, is Josephson supercurrent. Here, I report the observation of supercurrent in graphene Josephson junctions. Contrary to weak localization the phenomenon is robust. We have observed supercurrent in all the devices that we have fabricated -approximately 15-, four of which could be unambiguously proven from quantum Hall measurements to consist of single-layer graphene. Using a gate electrode, we have also measured the dependence of the supercurrent on carrier density. We find that supercurrent is present irrespective of gate voltage, both when the Fermi level is located in the conduction or in the valence band of graphene. The devices act therefore as ambipolar Josephson field-effect transistors. The supercurrent remains finite (although somewhat suppressed) even when the Fermi level is swept across the Dirac point where the valence and the conduction bands touch, a result that may be relevant for the understanding of the minimum conductivity previously reported in graphene. I will conclude by emphasizing the conceptual difference between the microscopic interference processes responsible for the presence of supercurrent and of weak-localization.
This work has been done in collaboration with H. Heersche, P. Jarillo-Herrero, J. Oostinga, and L. Vandersypen |
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