Minigap and Andreev bound states in ballistic graphene

TL;DR

This study reports the direct observation of a minigap and Andreev bound states in ballistic graphene coupled to superconducting leads, confirming theoretical predictions through spectroscopy and modeling.

Contribution

It provides experimental evidence of the minigap and Andreev bound states in ballistic graphene, aligning with theoretical models and calculations.

Findings

Observation of a minigap in ballistic graphene with superconducting contacts

Detection of Andreev bound states near the superconductor-graphene interface

Quantitative agreement with theoretical models under magnetic fields

Abstract

A finite-size normal conductor, proximity-coupled to a superconductor has been predicted to exhibit a so-called minigap, in which quasiparticle excitations are prohibited. Here, we report on the direct observation of such a minigap in ballistic graphene, coupled to superconducting MoRe leads. The minigap is probed by finite bias spectroscopy through a weakly coupled junction in the graphene region and its value is given by the dimensions of the device. Besides the minigap, we observe a distinct peak in the differential resistance, which we attribute to weakly coupled Andreev bound states (ABS) located near the superconductor-graphene interface. For weak magnetic fields, the phase accumulated in the normal-conducting region shifts the ABS in quantitative agreement with predictions from tight-binding calculations based on the Bogolioubov-de Gennes equation as well as with an analytical semiclassical model.