Current-phase relation of encapsulated graphene Josephson junctions

In the past few years there has been remarkable progress in the study of graphene-superconductor hybrids. This surge in interest has primarily been driven by the ability to combine high-quality graphene with superconductors via clean interfaces. We use such ballistic graphene Josephson junctions to create a superconducting quantum interference device (SQUID) which can be tuned continuously from a symmetric to asymmetric configuration. The symmetric SQUID produces typical flux-periodic oscillations in the critical current with a large modulation amplitude. More interestingly, we show that the highly asymmetric configuration allows one to directly obtain the current-phase relation (CPR) of these ballistic graphene JJs. The CPR is found to be skewed, deviating significantly from a sinusoidal form. The skewness can be tuned with the gate voltage and shows correlations with Fabry-Perot oscillations in the ballistic cavity. We compare our experiments with tight-binding calculations which include realistic graphene-superconductor interfaces and find a good qualitative agreement.