  Electrons incident from a normal metal onto a superconductor are reflected
back as holes - a process called Andreev reflection. In a normal metal where
the Fermi energy is much larger than a typical superconducting gap, the
reflected hole retraces the path taken by the incident electron. In graphene
with ultra low disorder, however, the Fermi energy can be tuned to be smaller
than the superconducting gap. In this unusual limit, the holes are expected to
be reflected specularly at the superconductor-graphene interface due to the
onset of interband Andreev processes, where the effective mass of the reflected
holes change sign. Here we present measurements of gate modulated Andreev
reflections across the low disorder van der Waals interface formed between
graphene and the superconducting NbSe2. We find that the conductance across the
graphene-superconductor interface exhibits a characteristic suppression when
the Fermi energy is tuned to values smaller than the superconducting gap, a
hallmark for the transition between intraband retro- and interband specular-
Andreev reflections.
