Quantum transport through a graphene nanoribbon-superconductor junction

TL;DR

This paper investigates electron transport in graphene nanoribbon-superconductor junctions, revealing conductance plateaus influenced by magnetic field, disorder, and ribbon width, with consistent Andreev reflection at 0.5 after saturation.

Contribution

It introduces a detailed analysis of transport properties in graphene nanoribbon-superconductor junctions considering magnetic field and disorder effects, highlighting conductance quantization and Andreev reflection behavior.

Findings

Conductance exhibits plateau structures under magnetic field and disorder.

Plateau values scale with the ribbon width and filling factor.

Andreev reflection coefficient stabilizes at 0.5 after saturation.

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 the magnetic field and disorder on the transport property are investigated. By using the tight-binding model and the non-equilibrium Green's function method, the expressions of the current, conductance, normal tunneling coefficient, and Andreev reflection coefficient are obtained. For a clean system and at zero magnetic field, the linear conductance increases approximatively in a linear fashion with the on-site energy. In the presence of a magnetic field and a moderate disorder, the linear conductance exhibits plateau structures for both armchair and zigzag edges. The plateau values increase with the width of the graphene ribbon. With a wide sample width, a saturated plateau value of $|\nu|e^2/h$ emerges at the filling factor $\nu$. 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 reach the saturated value. In particular, the Andreev reflection coefficient is always at 0.5 after reaching the saturated value, independent of any system parameters. In addition, we also consider the finite bias case, in which the Andreev reflection coefficient and normal tunneling coefficient are studied.