Strongly enhanced Andreev reflection in ultra-transparent NbSe₂/ bilayer graphene junctions

Van der Waals (vdW) layered materials with diverse properties provide an alternative route to construct future generations of miniaturized electronic devices. The prospect of constructing superconducting circuits from all vdW materials is attractive from many different viewpoints. Making transparent superconductor to normal conductor (S-N) junctions is a critical step in this endeavor. Here we present measurement and simulations of Andreev reflection (AR) in ultra-transparent NbSe₂/ bilayer grapehene (BLG) S-N junctions constructed via vdW stacking. The resistance across the junction is as low as 13 Ω, indicating a highly transparent NbSe₂/ BLG interface. In these devices, the proximity effect penetrates to the BLG side for at least 500 nm. We observe strongly enhanced AR in the differential conductance measurement across the junction. The zero-bias conductance peak reaches up to 1.7 times the normal value. We present a comprehensive study of the AR by systematically varying the carrier density and temperature in different devices. A model is constructed to capture the rich features of the experimental data. Important factors include the spatial decay of the superconducting gap, the effect of the carrier density profile near the interface and the interface transparency.