Andreev Bound states as a phase sensitive probe of the pairing symmetry of the iron pnictide superconductors

TL;DR

This paper proposes that surface Andreev bound states, which depend on edge orientation, can serve as phase-sensitive probes to identify the extended s-wave $s_$ pairing symmetry in iron pnictide superconductors, aiding in understanding their pairing mechanism.

Contribution

It demonstrates that edge-dependent Andreev bound states can distinguish $s_$ pairing symmetry, providing a novel phase-sensitive experimental approach.

Findings

Andreev bound states occur at edges along Fe-Fe bonds in $s_$ superconductors.

These states are not at zero energy and do not produce zero bias peaks.

Surface orientation critically affects the presence of bound states.

Abstract

A leading contender for the pairing symmetry in the Fe-pnictide high temperature superconductors is extended s-wave $s_\pm$, a nodeless state in which the pairing changes sign between Fermi surfaces. Verifying such a pairing symmetry requires a special phase sensitive probe that is also momentum selective. We show that the sign structure of $s_\pm$ pairing leads to surface Andreev bound states at the sample edge. In the clean limit they only occur when the edge is along the nearest neighbor Fe-Fe bond, but not for a diagonal edge or a surface orthogonal to the c-axis. In contrast to d-wave Andreev bound states, they are not at zero energy and, in general, do not produce a zero bias tunneling peak. Consequences for tunneling measurements are derived, within a simplified two band model and also for a more realistic five band model.