Proximity superconductivity in ballistic graphene at high magnetic fields

We present measurements of the superconducting proximity effect in graphene-based Josephson junctions with a mean free path of several microns, which exceeds the junctions' length [1]. The junctions exhibit low contact resistance and large supercurrents. We observe Fabry-P\'{e}rot oscillations in the normal-state resistance and the critical current of the junctions. The proximity effect is mostly suppressed in magnetic fields of \textless 10 mT showing the conventional Fraunhofer interference pattern; however, unexpectedly, a weak proximity effect survives in magnetic fields as high as 1 T. Superconducting states randomly appear and disappear as a function of field and carrier concentration, and each exhibits a supercurrent carrying capacity close to the universal limit of e$\Delta $/h where $\Delta $ is the superconducting gap of the contacts. We attribute the high-field supercurrent to mesoscopic Andreev states that persist near graphene edges. Our work reveals new proximity regimes that can be controlled by quantum confinement and cyclotron motion. [1] Ben Shalom et al., arXiv:1504.03286 (2015)