Andreev reflection in 2D relativistic materials with realistic tunneling transparency in normal-metal-superconductor junctions

TL;DR

This paper theoretically investigates Andreev reflection in 2D relativistic materials, highlighting how local strain influences junction transparency and conductance, with implications for topological phase transitions and potential applications in doped graphene.

Contribution

It introduces the role of local strain as an effective gauge field affecting Andreev conductance in relativistic 2D materials and explores topological phase transitions in doped Kane-Mele models.

Findings

Conductance suppression increases with gauge field strength.

Strain-induced gauge fields significantly alter Andreev reflection.

Distinct conductance behaviors across topological phase transition.

Abstract

The Andreev conductance across 2d normal metal (N)/superconductor (SC) junctions with relativistic Dirac spectrum is investigated theoretically in the Blonder-Tinkham-Klapwijk formalism. It is shown that for relativistic materials, due to the Klein tunneling instead of impurity potentials, the local strain in the junction is the key factor that determines the transparency of the junction. The local strain is shown to generate an effective Dirac $\delta$-gauge field. A remarkable suppression of the conductance are observed as the strength of the gauge field increases. The behaviors of the conductance are in well agreement with the results obtained in the case of 1d N/SC junction. We also study the Andreev reflection in a topological material near the chiral-to-helical phase transition in the presence of a local strain. The N side of the N/SC junction is modeled by the doped Kane-Mele (KM) model. The SC region is a doped correlated KM t-J (KMtJ) model, which has been shown to feature d+id'-wave spin-singlet pairing. With increasing intrinsic spin-orbit (SO) coupling, the doped KMtJ system undergoes a topological phase transition from the chiral d-wave superconductivity to the spin-Chern superconducting phase with helical Majorana fermions at edges. We explore the Andreev conductance at the two inequivalent Dirac points, respectively and predict the distinctive behaviors for the Andreev conductance across the topological phase transition. Relevance of our results for the adatom-doped graphene is discussed.