  Despite extensive search for about a decade, specular Andreev reflection is
only recently realized in bilayer graphene-superconductor interface. However,
the evolution from the typical retro type Andreev reflection to the unique
specular Andreev reflection in single layer graphene has not yet been observed.
We investigate this transition by measuring the differential conductance at the
van der Walls interface of single layer graphene and NbSe2 superconductor. We
find that the normalized conductance becomes suppressed as we pass through the
Dirac cone via tuning the Fermi level and bias energy, which manifests the
transition from retro to non-retro type Andreev reflection. The suppression
indicates the blockage of Andreev reflection beyond a critical angle of the
incident electron with respect to the normal between the single layer graphene
and the superconductor junction. The results are compared with a theoretical
model of the corresponding setup.
