Enhanced Specular Andreev reflection in bilayer graphene

TL;DR

This paper proposes a method to observe enhanced specular Andreev reflection in bilayer graphene at zero bias by applying a Zeeman field, enabling clearer experimental signatures of the retro to specular crossover.

Contribution

The study introduces a scheme involving Zeeman splitting to distinctly observe specular Andreev reflection in bilayer graphene, improving upon previous less pronounced experimental signatures.

Findings

Identification of well-defined regions of retro and specular Andreev reflection.

Demonstration of conductance features as a function of chemical potential and bias.

Analysis of the effects of a Zeeman field and work function mismatch on Andreev reflection.

Abstract

Andreev reflection in graphene is special since it can be of two types- retro or specular. Specular Andreev reflection (SAR) dominates when the position of the Fermi energy in graphene is comparable to or smaller than the superconducting gap. Bilayer graphene (BLG) is an ideal candidate to observe the crossover from retro to specular since the Fermi energy broadening near the Dirac point is much weaker compared to monolayer graphene. Recently, the observation of signatures of SAR in BLG have been reported experimentally by looking at the enhancement of conductance at finite bias near the Dirac point. However, the signatures were not very pronounced possibly due to the participation of normal quasi-particles at bias energies close to the superconducting gap. Here, we propose a scheme to observe the features of enhanced SAR even at zero bias at a normal metal (NM)-superconductor (SC) junction on BLG. Our scheme involves applying a Zeeman field to the NM side of the NM-SC junction on BLG (making the NM ferromagnetic), which energetically separates the Dirac points for up-spin and down-spin. We calculate the conductance as a function of chemical potential and bias within the superconducting gap and show that well-defined regions of specular- and retro-type Andreev reflection exist. We compare the results with and without superconductivity. We also investigate the possibility of the formation of a p-n junction at the interface between the NM and SC due to a work function mismatch.