Current-phase relation of ballistic graphene Josephson junctions

TL;DR

This study measures the current-phase relation of ballistic graphene Josephson junctions using a gate-tunable SQUID, revealing a tunable skewed CPR that aligns well with theoretical models, advancing understanding of graphene-based superconducting devices.

Contribution

It introduces a method to directly measure the CPR of ballistic graphene JJs and demonstrates its tunability and deviation from sinusoidal behavior.

Findings

CPR is skewed and tunable with gate voltage

Skewness oscillates with Fabry-Pérot resistance

Good qualitative agreement with tight-binding calculations

Abstract

The current-phase relation (CPR) of a Josephson junction (JJ) determines how the supercurrent evolves with the superconducting phase difference across the junction. Knowledge of the CPR is essential in order to understand the response of a JJ to various external parameters. Despite the rising interest in ultra-clean encapsulated graphene JJs, the CPR of such junctions remains unknown. Here, we use a fully gate-tunable graphene superconducting quantum intereference device (SQUID) to determine the CPR of ballistic graphene JJs. Each of the two JJs in the SQUID is made with graphene encapsulated in hexagonal boron nitride. By independently controlling the critical current of the JJs, we can operate the SQUID either in a symmetric or asymmetric configuration. The highly asymmetric SQUID allows us to phase-bias one of the JJs and thereby directly obtain its CPR. The CPR is found to be skewed, deviating significantly from a sinusoidal form. The skewness can be tuned with the gate voltage and oscillates in anti-phase with Fabry-P\'{e}rot resistance oscillations of the ballistic graphene cavity. We compare our experiments with tight-binding calculations which include realistic graphene-superconductor interfaces and find a good qualitative agreement.