Engineering superconducting contacts transparent to a bipolar graphene

TL;DR

This paper presents a new fabrication method for superconducting graphene contacts that independently controls charge polarity, enabling transparent contacts for both electron and hole doping, and demonstrates the Andreev process in quantum Hall edge states.

Contribution

Developed a novel fabrication scheme for bipolar superconducting contacts on graphene, allowing independent control of charge polarity and transparency for both doping regimes.

Findings

Achieved transparent superconducting contacts for both electron and hole doping in graphene.

Measured conductance enhancement and Josephson coupling confirming contact transparency.

Demonstrated Andreev process in quantum Hall edge states at negative filling factor.

Abstract

Graphene's exceptional electronic mobility, gate-tunability, and contact transparency with superconducting materials make it ideal for exploring the superconducting proximity effect. However, the work function difference between graphene and superconductors causes unavoidable doping of graphene near contacts, forming a p-n junction in the hole-doped regime and reducing contact transparency. This challenges the device implementation that exploits graphene's bipolarity. To address this limitation, we developed a new fabrication scheme for two-dimensional superconducting contacts that allows independent control over charge concentration and polarity for both the graphene in contact with superconductors and the graphene channel. Contact transparency, conductance enhancement, and Josephson coupling were measured to confirm transparent contacts to both polarities of graphene. Moreover, we demonstrated the Andreev process in the quantum Hall edge state at a negative filling factor of {\nu} = -2. This scheme will open avenues for realizing various theoretical propositions utilizing the bipolarity of graphene combined with superconductivity.