Andreev reflection in a proximity junction of graphene: Influence of a naturally formed pn junction

Abstract

We theoretically study the electron transport in a proximity junction made by partially covering a graphene sheet with a bulk superconductor. Such a system serves as a normal conductor–superconductor (NS) junction and its electron transport properties are mainly governed by Andreev reflection (AR). In this NS junction, the charge neutrality point (CNP) of graphene must spatially vary as a result of the penetration of excess carriers induced from the superconductor. By taking this into account in the electron-doped case, we calculate the differential conductance GNS of the graphene NS junction. Owing to the variation of the CNP in the uncovered region, a smooth pn junction is naturally formed at the point where the Fermi energy crosses the CNP. The resulting pn junction causes unusual behavior of GNS. For example, a resonant peak structure appears in GNS reflecting the fact that quasi-bound states are created by the pn junction.