Momentum dependence and nodes of the superconducting gap in the iron pnictides

Using general symmetry arguments and model calculations we analyze the superconducting gap in materials with multiple Fermi-surface pockets, with applications to iron pnictides. We show that the gap in the pnictides has an extended s-wave symmetry but is either nodeless or has nodes, depending on the interplay between intraband and interband interactions. We argue that the nodes in the gap emerge without a phase transition as the tendency toward a spin-density-wave order gets weaker. These findings provide a way to reconcile seemingly conflicting results of numerical and experimental studies of the pnictides. In particular, we argue that isovalent-P doped BaFe$_2$As$_{2}$ superconductors likely has an extended s-wave gap with nodes. On the other hand, electron or hole doped BaFe$_2$As$_{2}$ superconductors have a nodeless gap. Still even these materials exhibit a gapless behavior at strong disorder.