Transport Mechanisms of Quantum Hall Supercurrents in Graphene Josephson Junctions
Supercurrents in high magnetic fields up to the quantum Hall (QH) regime have been observed in encapsulated graphene with superconducting contacts. The topological nature of the chiral QH edge states coupled with superconductivity may exhibit exotic topological states such as Majorana fermions. Here, I am reporting the current progress in our group at Duke University in understanding the transport mechanisms of the supercurrents. We propose that the supercurrents are mediated by Andreev bound states formed by electron-hole hybrid modes along each superconducting edge and chiral QH edge channels along the perimeter of the graphene. First, we compare a Josephson junction with one graphene edge extended beyond the superconducting molybdenum-rhenium contacts to a symmetric rectangular Josephson junction to verify this scenario. Second, a T-shaped Josephson junction with a ~100nm constriction between electrodes shows a robust supercurrent up to 2.5 T, well above the onset of the Hall quantization. We demonstrate that the supercurrent is carried by the tunneling states within the short and narrow channel confined on each side by the gapped graphene bulk. This supercurrent is due to the direct coupling between electron-hole hybrid modes via tunneling, which is different from our previous observation of supercurrent involving chiral QH edge channels.