Andreev Reflections in NbN/graphene Junctions under Large Magnetic Fields

TL;DR

This study explores how high magnetic fields influence Andreev reflections in NbN/graphene junctions, revealing tunable quantum effects and conductance oscillations related to Landau levels and edge states.

Contribution

It demonstrates superconducting NbN/graphene interfaces remain functional under high magnetic fields and reveals the tunability of Andreev reflections via magnetic field orientation.

Findings

Superconducting NbN/graphene junctions withstand magnetic fields over 18 T.

Magnetic fields can switch between specular and retro Andreev reflections.

Oscillatory conductance suppression observed at the $ u=4$ Landau level.

Abstract

Hybrid superconductor/graphene (SC/g) junctions are excellent candidates for investigating correlations between Cooper pairs and quantum Hall (QH) edge modes. Experimental studies are challenging as Andreev reflections are extremely sensitive to junction disorder and high magnetic fields are required to form QH edge states. We fabricated low-resistance SC/g interfaces, composed of graphene edge contacted with NbN with a barrier strength of $Z\approx 0.4$, that remain superconducting under magnetic fields larger than $18$ T. We establish the role of graphene's Dirac band structure on zero-field Andreev reflections and demonstrate dynamic tunability of the Andreev reflection spectrum by moving the boundary between specular and retro Andreev reflections with parallel magnetic fields. Through the application of perpendicular magnetic fields, we observe an oscillatory suppression of the 2-probe conductance in the $\nu = 4$ Landau level attributed to the reduced efficiency of Andreev processes at the NbN/g interface, consistent with theoretical predictions.