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

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 (Δcos(kx) cos(ky)) scenario resonant transmission through surface Andreev bound states (ABS) at non-zero energy is found as well as destructive interference effects that produce zeros in the conductance; in the extended s-wave (Δ[cos(kx) + cos(ky)]) scenario no ABS at finite energy are found.