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 Γ 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 (π,0) and that scattering at other momentum transfers can be useful in distinguishing between gap structures.