Transparent superconducting contact to both polarities of graphene

Graphene has been a favorable platform for studying the superconducting proximity effect thanks to its high electronic mobility and high transparency with superconducting contacts. It is known that many kinds of superconducting materials such as aluminium, niobium, tantalum and molybdenum-rhenium alloys, induce negative doping on graphene in contact with superconducting electrodes due to their work function difference. Thus, when the graphene channel is positively doped, p-n junction is developed near the superconducting contacts, which largely limits the various device scheme utilizing the bipolarity of graphene. Overcoming such issue, we fabricate two-dimensional superconducting contact which is highly transparent to both polarities of graphene. In this structure, the charge concentration of graphene beneath the superconductor and that of graphene channel are independently controlled to minimize the potential barrier between them. Measured contact transparency as well as the magnitude of Josephson critical current at zero magnetic field demonstrate the realization of transparent contact to both polarities of graphene. The contact transparency reaches about 0.6 for n-doped and 0.45 for p-doped regime. Our result paves a promising way to realize various theoretical propositions utilizing the bipolarity of graphene with superconductivity.