Transport Through Andreev Bound States in a Graphene Quantum Dot

TL;DR

This paper reports on the direct measurement of gate-tunable Andreev bound states in a graphene-based superconductor-quantum dot system, revealing sharp, controllable spectral features that advance understanding of superconducting transport in graphene.

Contribution

It demonstrates the formation and tunability of discrete Andreev bound states in a graphene quantum dot system, providing new insights into superconducting transport phenomena.

Findings

Sharp, gate-tunable ABS observed in graphene quantum dot

ABS spectra can be tuned to zero energy with gate voltage

Transport measurements reveal distinctive patterns of ABS in graphene

Abstract

Andreev reflection-where an electron in a normal metal backscatters off a superconductor into a hole-forms the basis of low energy transport through superconducting junctions. Andreev reflection in confined regions gives rise to discrete Andreev bound states (ABS), which can carry a supercurrent and have recently been proposed as the basis of qubits [1-3]. Although signatures of Andreev reflection and bound states in conductance have been widely reported [4], it has been difficult to directly probe individual ABS. Here, we report transport measurements of sharp, gate-tunable ABS formed in a superconductor-quantum dot (QD)-normal system, which incorporates graphene. The QD exists in the graphene under the superconducting contact, due to a work-function mismatch [5, 6]. The ABS form when the discrete QD levels are proximity coupled to the superconducting contact. Due to the low density of states of graphene and the sensitivity of the QD levels to an applied gate voltage, the ABS spectra are narrow, can be tuned to zero energy via gate voltage, and show a striking pattern in transport measurements.