Andreev-Klein reflection in graphene ferromagnet-superconductor junctions

TL;DR

This paper explores how Andreev reflection in ferromagnetic-superconductor graphene junctions differs from traditional types, revealing a Klein tunneling mechanism that enhances conductance and exhibits both retro and specular reflection modes.

Contribution

It introduces the concept of Andreev-Klein reflection in graphene F-S junctions, showing its impact on conductance and reflection types, which is a novel insight into graphene-based hybrid systems.

Findings

Enhanced subgap conductance due to Andreev-Klein process.

Identification of retro and specular Andreev reflection modes.

Conductance surpasses non-ferromagnetic junctions at certain exchange fields.

Abstract

We show that Andreev reflection in a junction between ferromagnetic (F) and superconducting (S) graphene regions is fundamentally different from the common FS junctions. For a weakly doped F graphene with an exchange field $h$ larger than its Fermi energy $E_{\rm F}$, Andreev reflection of massless Dirac fermions is associated with a Klein tunneling through an exchange field p-n barrier between two spin-split conduction and valence subbands. We find that this Andreev-Klein process results in an enhancement of the subgap conductance of a graphene FS junction by $h$ up to the point at which the conductance at low voltages $eV\ll \Delta$ is greater than its value for the corresponding non-ferromagnetic junction. We also demonstrate that the Andreev reflection can be of retro or specular types in both convergent and divergent ways with the reflection direction aligned, respectively, closer to and farther from the normal to the junction as compared to the incidence direction.