Nodes in the order parameter of superconducting iron pnictides investigated by infrared spectroscopy

The temperature and frequency dependences of the conductivity derived from optical reflection and transmission measurements of electron-doped BaFe2As2 crystals and films are analyzed according to gap nodes or possibly a very small gap, or in the crossover region between these two possibilities. This can arise when one of the several pockets known to exist in these systems has extended s-wave gap symmetry with an anisotropic piece cancelling or nearly so the isotropic part in some momentum direction. Alternatively, a node can be lifted by impurity scattering which reduces anisotropy. We find that the smaller gap on the hole pocket at the Gamma point in the Brillouin zone is isotropic s wave while the electron pocket at the M point has a larger gap which is anisotropic and falls in the crossover region.