Andreev Reflection and Klein Tunneling in High-Temperature Superconductor/Graphene Junctions

TL;DR

This study demonstrates the coexistence of Andreev reflection and Klein tunneling in high-temperature superconductor/graphene junctions, revealing hybrid resonances that could influence graphene-based superconducting devices.

Contribution

It introduces the observation of combined Andreev and Klein tunneling resonances in graphene/superconductor interfaces, supported by experimental data and quantum transport calculations.

Findings

Resonances due to simultaneous Andreev reflection and Klein tunneling.

Resonances persist across different gating configurations.

Suppression of oscillations indicates Andreev reflection contribution.

Abstract

Scattering processes in quantum materials emerge as resonances in electronic transport, including confined modes, Andreev states, and Yu-Shiba-Rusinov states. However, in most instances, these resonances are driven by a single scattering mechanism. Here we show the appearance of resonances due to the combination of two simultaneous scattering mechanisms, one from superconductivity and the other from graphene p-n junctions. These resonances stem from Andreev reflection and Klein tunneling that occur at two different interfaces of a hole-doped region of graphene formed at the boundary with superconducting graphene due to proximity effects from Bi$_2$Sr$_2$Ca$_1$Cu$_2$O$_{8+\delta}$. The resonances persist with gating the from p$^+$-p and p-n configurations. The suppression of the oscillation amplitude above the bias energy comparable to the induced superconducting gap indicates the contribution from Andreev reflection. Our experimental measurements are supported by quantum transport calculations in such interfaces, leading to analogous resonances. Our results put forward a hybrid scattering mechanism in graphene/high-temperature superconductor heterojunctions of potential impact for graphene-based Josephson junctions.