Tunnelling spectroscopy of Andreev states in graphene

Abstract

A normal conductor placed in good contact with a superconductor can inherit its remarkable electronic properties1,2. This proximity effect microscopically originates from the formation in the conductor of entangled electron-hole states, called Andreev states3-8. Spectroscopic studies of Andreev states have been performed in just a handful of systems9-13. The unique geometry, electronic structure and high mobility of graphene14,15 make it a novel platform for studying Andreev physics in two dimensions. Here we use a full van der Waals heterostructure to perform tunnelling spectroscopy measurements of the proximity effect in superconductor-graphene- superconductorjunctions.Themeasuredenergyspectra,which depend on the phase difference between the superconductors, reveal the presence of a continuum of Andreev bound states. Moreover, our device heterostructure geometry and materials enable us to measure the Andreev spectrum as a function of the graphene Fermi energy, showing a transition between different mesoscopic regimes. Furthermore, by experimentally introducing a novel concept, the supercurrent spectral density, we determine the supercurrent-phase relation in a tunnelling experiment,thusestablishingtheconnectionbetweenAndreev physics at finite energy and the Josephson effect. This work opens up new avenues for probing exotic topological phases of matter in hybrid superconducting Dirac materials 16-18 .