Theory of Andreev reflection in a two-orbital model of iron-pnictide superconductors

TL;DR

This paper develops a quantum waveguide-based theory for Andreev reflection in multiband iron-pnictide superconductors, revealing interference effects and conductance features for different gap symmetries.

Contribution

It extends waveguide theory to multiband superconductors, deriving matching conditions and analyzing conductance with interference effects for specific gap symmetries.

Findings

Resonant transmission through surface Andreev bound states in sign-reversed s-wave case.

Zeros in conductance due to destructive interference effects.

No finite-energy ABS in extended s-wave scenario.

Abstract

A recently developed theory for the problem of Andreev reflection between a normal metal (N) and a multiband superconductor (MBS) assumes that the incident wave from the normal metal is coherently transmitted through several bands inside the superconductor. Such splitting of the probability amplitude into several channels is the analogue of a quantum waveguide. Thus, the appropriate matching conditions for the wave function at the N/MBS interface are derived from an extension of quantum waveguide theory. Interference effects between the transmitted waves inside the superconductor manifest themselves in the conductance. We provide results for a FeAs superconductor, in the framework of a recently proposed effective two-band model and two recently proposed gap symmetries: in the sign-reversed s-wave ($\Delta\cos(k_x)\cos(k_y)$) scenario resonant transmission through surface Andreev bound states (ABS) at nonzero energy is found as well as destructive interference effects that produce zeros in the conductance; in the extended s-wave ($\Delta[\cos(k_x)+\cos(k_y)]$) scenario no ABS at finite energy are found.