Neutron inelastic scattering peak by dissipationless mechanism in the s++-wave state in iron-based superconductors

We investigate the neutron scattering spectrum in iron pnictides based on the random-phase approximation in the five-orbital model with a realistic superconducting (SC) gap, Delta = 5 meV. In the normal state, the neutron spectrum is suppressed by large inelastic quasiparticle (QP) scattering rate. gamma* similar to Delta. In the fully-gapped s-wave state without sign reversal (s(++)), a hump-shaped enhancement appears in the neutron spectrum just above 2 Delta, since the inelastic QP scattering is prohibited by the SC gap. That is, the hump structure is produced by the dissipationless QPs for QP energy E-k < 3 Delta. The obtained result is more consistent with experimental spectra, compared to the results of our previous paper with Delta = 50 meV. On the other hand, both height and weight of the resonance peak in the fully-gapped s-wave states with sign reversal (s(+/-)) are much larger than those observed in experiments. We conclude that the experimentally observed broad spectral peak in iron pnictides is created by the present dissipationless mechanism in the s(++)-wave state.