How to Distinguish between Specular and Retroconfigurations for Andreev Reflection in Graphene Rings

TL;DR

This study numerically distinguishes between retro and specular Andreev reflections in graphene rings by analyzing conductance oscillation periods, providing a robust method to identify these processes experimentally.

Contribution

It introduces a numerical approach to differentiate Andreev reflection types in graphene based on conductance oscillation periods, confirming theoretical predictions.

Findings

Retroreflection shows $h/2e$ conductance oscillations.

Specular reflection exhibits $h/e$ oscillations.

The method is robust against disorder and system variations.

Abstract

We numerically investigate Andreev reflection in a graphene ring with one normal conducting and one superconducting lead by solving the Bogoliubov--de Gennes equation within the Landauer-B\"uttiker formalism. By tuning chemical potential and bias voltage, it is possible to switch between regimes where electron and hole originate from the same band (retroconfiguration) or from different bands (specular configuration) of the graphene dispersion, respectively. We find that the dominant contributions to the Aharonov-Bohm conductance oscillations in the subgap transport are of period $h/2e$ in retroconfiguration and of period $h/e$ in specular configuration, confirming the predictions obtained from a qualitative analysis of interfering scattering paths. Because of the robustness against disorder and moderate changes to the system, this provides a clear signature to distinguish both types of Andreev reflection processes in graphene.