Quantum transport through a graphene nanoribbon–superconductor junction

Abstract

We study the electron transport through a graphene nanoribbon–superconductor junction.Both zigzag and armchair edge graphene nanoribbons are considered, and the effects of themagnetic field and disorder on the transport property are investigated. By using thetight-binding model and the non-equilibrium Green’s function method, the expressions ofthe current, conductance, normal tunneling coefficient and Andreev reflection coefficient areobtained. For a clean system and at zero magnetic field, the linear conductance increasesapproximately in a linear fashion with the on-site energy. In the presence of a magneticfield and a moderate disorder, the linear conductance exhibits plateau structures forboth armchair and zigzag edges. The plateau values increase with the width ofthe graphene ribbon. With a wide sample width, a saturated plateau value of|ν|e2/h emerges at thefilling factor ν. For a small filling factor, the conductance can reach the saturated value at a small width,but for a high filling factor it requires to have a quite wide sample width to reachthe saturated value. In particular, the Andreev reflection coefficient is always at0.5 after reaching the saturated value, independent of any system parameters. In addition, wealso consider the finite bias case, in which the Andreev reflection coefficient and normaltunneling coefficient are studied.