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 ${\gamma }^{*}\sim \Delta$. 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 $E_{\mathit{k}}<3\Delta$. 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 ($s_{\pm }$) 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.

• Received 6 June 2011

DOI:https://doi.org/10.1103/PhysRevB.84.144518