Engineering Superconducting Contacts Transparent to\na Bipolar Graphene

Graphene’s\nexceptional electronic mobility, gate-tunability,\nand contact transparency with superconducting materials make it ideal\nfor exploring the superconducting proximity effect. However, the work\nfunction difference between graphene and superconductors causes unavoidable\ndoping of graphene near contacts, forming a p–n junction in\nthe hole-doped regime and reducing the contact transparency. This\nchallenges the device implementation that exploits graphene’s\nbipolarity. To address this limitation, we developed a new fabrication\nscheme for two-dimensional superconducting contacts that allows independent\ncontrol over charge concentration and polarity for both the graphene\nin contact with superconductors and the graphene channel. Contact\ntransparency, conductance enhancement, and Josephson coupling were\nmeasured to confirm transparent contacts with both polarities of graphene.\nMoreover, we demonstrated the Andreev process in the quantum Hall\nedge state at a negative filling factor of ν = −2. This\nscheme will open up avenues for realizing various theoretical propositions\nusing the bipolarity of graphene combined with superconductivity.