Andreev-Enhanced Conductance in a Bilayer Graphene Quantum Point Contact

Abstract

Quantum point contacts provide a platform for probing mode-dependent Andreev reflection. Here, we demonstrate Andreev-enhanced conductance in a gate-defined bilayer graphene quantum point contact coupled to a superconducting aluminum electrode. Electrostatic gating enables sequential mode occupation, verified by normal-state conductance quantization. In the superconducting state, the conductance is enhanced within the conductance plateaus, while the enhancement decreases between plateaus, consistent with the nonlinear transmission dependence of Andreev reflection. The enhancement yields effective transmission probabilities up to 0.9, and extends over a bias window of approximately 140 $μ$V, which we associate with a proximity-induced superconducting gap, close to the gap of the aluminum film. For in-plane fields, the enhancement persists up to the critical field, and the bias window follows the spectral gap predicted by Abrikosov-Gor'kov theory. In contrast, perpendicular fields suppress the enhancement approximately linearly from zero field, deviating from homogeneous pair breaking and indicative of flux penetration.