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

A leading contender for the pairing symmetry in the Fe-pnictide high temperature superconductors is extended s-wave $s_-$, a nodeless state in which the pairing changes sign between Fermi surfaces. Verifying such a pairing symmetry requires a special probe that is sensitive to both phase and magnitude of the order parameter. We show that the sign structure of $s_-$ pairing leads to Andreev bound states at the edge. In the clean limit they only occur when the edge is along the Fe-Fe bond, but not for a diagonal edge. 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.