Title: Tunnelling Spectroscopy of Andreev States in Graphene,
Abstract: A normal conductor placed in good contact with a superconductor can inherit
its remarkable electronic properties. This proximity effect microscopically
originates from the formation in the conductor of entangled electron-hole
states, called Andreev states. Spectroscopic studies of Andreev states have
been performed in just a handful of systems. The unique geometry, electronic
structure and high mobility of graphene 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-superconductor junctions. The
measured energy spectra, 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, thus
establishing the connection between Andreev 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.