Andreev spectra and subgap bound states in multiband superconductors

TL;DR

This paper develops a theory for Andreev conductance in junctions with multiband superconductors, revealing unique signatures like suppressed Andreev reflection and surface bound states that can help identify nodeless extended s±-wave pairing.

Contribution

It introduces a theoretical framework for Andreev conductance in multiband superconductors, highlighting effects of interband interference and surface bound states specific to s±-wave symmetry.

Findings

Interband interference suppresses Andreev reflection in highly transparent interfaces.

Surface bound states can appear at zero and non-zero energies.

Conductance spectra are strongly modified by interband sign reversal.

Abstract

The theory of Andreev conductance is formulated for junctions involving normal metals (N) and multiband superconductors (S) and applied to the case of superconductors with nodeless extended $s_{\pm}$-wave order parameter symmetry, as possibly realized in the recently discovered ferro pnictides. We find qualitative differences from tunneling into s-wave or d-wave superconductors that may help to identify such a state. First, interband interference leads to a suppression of Andreev reflection in the case of a highly transparent N/S interface and to a current deficit in the tunneling regime. Second, surface bound states may appear, both at zero and at non-zero energies. These effects do not occur in multiband superconductors without interband sign reversal, though the interference can still strongly modify the conductance spectra.