Quantum waveguide theory of Andreev reflection in multiband superconductors: the case of Fe-pnictides

TL;DR

This paper develops a quantum waveguide theory to analyze Andreev reflection in multiband superconductors, specifically Fe-pnictides, revealing interference effects and resonant states affecting conductance.

Contribution

It extends quantum waveguide theory to multiband superconductors and applies it to Fe-pnictides, uncovering interference phenomena and conductance zeros at nonzero bias.

Findings

Resonant transmission through surface Andreev bound states.

Destructive interference causes zeros in conductance at nonzero bias.

Interference effects depend on multiband structure and pairing symmetry.

Abstract

The problem of Andreev reflection between a normal metal and a multiband superconductor is addressed. The appropriate matching conditions for the wave function at the interface are established on the basis of an extension of quantum waveguide theory to these systems. Interference effects between different bands of the superconductor manifest themselves in the conductance and the case of FeAs superconductors is specifically considered, in the framework of a recently proposed effective two-band model, in the sign-reversed s-wave pairing scenario. Resonant transmission through surface Andreev bound states is found as well as destructive interference effects that produce zeros in the conductance at normal incidence. Both these effects occur at nonzero bias voltage.