Anisotropy of the pairing gap of FeAs-based superconductors induced by spin fluctuations

We determine the anisotropy of the spin-fluctuation-induced pairing gap on the Fermi surface of the FeAs-based superconductors as function of the exchange and Hund's coupling J_H. We find that for sufficiently large J_H, nearly commensurate magnetic fluctuations yield a fully gapped s^±-pairing state with small anisotropy of the gap amplitude on each Fermi-surface sheet, but significant variations of the gap amplitude for different sheets of the Fermi surface. In particular, we obtain the large variation of the gap amplitude on different Fermi-surface sheets, as seen in angular resolved photoemission spectroscopy experiments. For smaller values of Hund's coupling incommensurate magnetic fluctuations yield an s^±-pairing state with line nodes. Such a state is also possible once the anisotropy of the material is reduced and three-dimensional effects come into play.