Andreev Bound States as a Phase-Sensitive Probe of the Pairing Symmetry of the Iron Pnictide 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 phase-sensitive probe that is also momentum selective. We show that the sign structure of s(+/-) pairing can lead to surface Andreev bound states (ABS) 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 ABS, 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. In both cases, surface ABS are obtained.