▎ 摘　　要

A graphene sheet partially covered with a bulk superconductor serves as a normal conductor-superconductor (NS) junction, in which electron transport is mainly governed by Andreev reflection (AR). As excess carriers induced over the covered region penetrate into the uncovered region over a screening length, the charge neutrality point (CNP) in the uncovered region shifts only near the NS interface. We theoretically study the electron transport in a bilayer graphene junction taking account of such spatial variation of the CNP in the electron-doped case. When the Fermi level is close to the CNP away from the NS interface, the AR takes place in a specular manner owing to the diffraction of a reflected hole occurring at a pn junction, which is naturally formed in the uncovered region. It is shown that the differential conductance shows an unusual asymmetric behavior as a function of bias voltage under the influence of the pn junction. It is also shown that, if the Fermi level is located below the CNP, the pn junction gives rise to quasi-bound states near the NS interface, leading to the appearance of resonant peaks in the differential conductance.