Spin resonance peak in Fe-based superconductors with unequal gaps

Abstract

We study the spin resonance in the superconducting state of iron-based materials within multiband models with two unequal gaps, ΔL and ΔS, on different Fermi-surface pockets. We show that, due to the indirect nature of the gap entering the spin susceptibility at the nesting wave vector Q, the total gap Δ˜ in the bare susceptibility is determined by the sum of gaps on two different Fermi-surface sheets connected by Q. For the Fermi-surface geometry characteristic of most iron pnictides and chalcogenides, the indirect gap is either Δ˜=ΔL+ΔS or Δ˜=2ΔL. In the s++ state, spin excitations below Δ˜ are absent unless additional scattering mechanisms are assumed. The spin resonance appears in the s± superconducting state at frequency ωR≤Δ˜. Comparison with available inelastic neutron-scattering data confirms that what is seen is the true spin resonance and not a peak inherent to the s++ state.