Current-Phase Relation of Ballistic Graphene Josephson Junctions
G. Nanda (Kavli institute of nanoscience Delft, TU Delft - QN/Kavli Nanolab Delft)
J.L. Aguilera Servin (TU Delft - QCD/Vandersypen Lab, Kavli institute of nanoscience Delft, Institute of Science and Technology Austria)
P. Rakyta (Eötvös University)
A. Kormányos (Universität Konstanz)
Reinhold Kleiner (Eberhard Karls Universität Tübingen)
Dieter Koelle (Eberhard Karls Universität Tübingen)
K. Watanabe (National Institute for Materials Science)
T Taniguchi (National Institute for Materials Science)
Lieven M K Vandersypen (TU Delft - QuTech Advanced Research Centre, TU Delft - QN/Vandersypen Lab, TU Delft - QCD/Vandersypen Lab, Kavli institute of nanoscience Delft)
S. Goswami (TU Delft - QRD/Kouwenhoven Lab, Kavli institute of nanoscience Delft, TU Delft - QuTech Advanced Research Centre, TU Delft - QN/Quantum Transport)
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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 ultraclean 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 antiphase with Fabry-Pérot resistance oscillations of the ballistic graphene cavity. We compare our experiments with tight-binding calculations that include realistic graphene-superconductor interfaces and find a good qualitative agreement.
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