Neutron scattering resonance and the iron-pnictide superconducting gap

The existence of a neutron scattering resonance at a wave vector q(*) implies a sign change of the gap between two Fermi-surface regions separated by wave vector q(*). For the Fe pnictides, a resonance has been observed for a wave vector q(*) which connects a hole Fermi surface around the Gamma point with an electron Fermi surface around the X or Y points of the 1 Fe/unit cell Brillouin zone. Here we study the neutron scattering resonance for a five-orbital model within a random-phase approximation-BCS approximation. Our results show that both an isotropic sign-switched and an anisotropic extended s-wave gap are consistent with the present data for q(*) near (pi,0) and that scattering at other momentum transfers can be useful in distinguishing between gap structures.