Enhancement of subgap conductance in a graphene superconductor junction by valley polarization**Project supported by the National Natural Science Foundation of China (Grant Nos. 11274059 and 11074233).

Abstract

We theoretically study the differential conductance of a graphene/graphene superconductor junction, where the valley polarization of Dirac electrons is considered in the nonsuperconducting region. It is shown that the subgap conductance will increase monotonically with the valley-polarization strength when the chemical potential µ is near the Dirac point µ ≤ 3Δ (Δ is the superconducting gap), whereas it will decrease monotonically when µ is far away from the Dirac point, µ ≥ 5Δ. The former case is induced by the specular Andreev reflection while the retro-reflection accounts for the later result. Our findings may shed light on the control of conductance of a graphene superconductor junction by valley polarization.