Inelastic neutron and x-ray scattering as probes of the sign structure of the superconducting gap in iron pnictides

Neutron spin-flip scattering observations of a resonance in the superconducting state is often taken as evidence of an unconventional superconducting state in which the gap changes sign Δ(k+Q)=-Δ(k) for momentum transfers Q which play an important role in the pairing. Recently questions regarding this identification for the Fe-pnictide superconductors have been raised and it has been suggested that Δ(k+Q)=Δ(k). Here we propose that inelastic neutron or x-ray scattering measurements of the spectral weight of a phonon of momentum Q can distinguish between these two pairing scenarios.