Spin resonance peak in Fe-based superconductors with unequal gaps

Abstract

We study the spin resonance in superconducting state of iron-based materials within multiband models with two unequal gaps, ${\Delta }_L$ and ${\Delta }_S$, 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 $\mathbf{Q}$ the total gap $\tilde{\Delta }$ in the bare susceptibility is determined by the sum of gaps on two different Fermi surface sheets connected by $\mathbf{Q}$. For the Fermi surface geometry characteristic to the most of iron pnictides and chalcogenides, the indirect gap is either $\tilde{\Delta }={\Delta }_L+{\Delta }_S$ or $\tilde{\Delta }=2{\Delta }_L$. In the $s_{++}$ state, spin excitations below $\tilde{\Delta }$ are absent unless additional scattering mechanisms are assumed. The spin resonance appears in the $s_{\pm }$ superconducting state at frequency ${\omega }_R\le \tilde{\Delta }$. 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.

arXiv commentPublished version, 6 pages, 3 figures, includes Supplemental material (4 pages) after the main text

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