Effect of nodes, ellipticity and impurities on the spin resonance in Iron-based superconductors

TL;DR

This paper investigates how doping, ellipticity, impurities, and gap anisotropy influence the spin resonance in iron-based superconductors, revealing shifts in resonance position, intensity, and broadening effects, with implications for understanding pairing symmetry.

Contribution

It provides a detailed analysis of the effects of ellipticity, impurities, and gap anisotropy on the spin resonance, highlighting differences between s+- and s++ pairing states.

Findings

Resonance shifts from commensurate to incommensurate with doping.

Impurities cause broadening and shift of the resonance.

Resonance is absent in s++ superconductors.

Abstract

We analyze doping dependence of the spin resonance of an s+- superconductor and its sensitivity to the ellipticity of electron pockets, to magnetic and non-magnetic impurities, and to the angle dependence of the superconducting gap along electron Fermi surfaces. We show that the maximum intensity of the resonance shifts from commensurate to incommensurate momentum above some critical doping which decreases with increasing ellipticity. Angle dependence of the gap and particularly the presence of accidental nodes lowers the overall intensity of the resonance peak and shifts its position towards the onset of the particle-hole continuum. Still, however, the resonance remains a true \delta-function in the clean limit. When non-magnetic or magnetic impurities are present, the resonance broadens and its position shifts. The shift depends on the type of impurities and on the ratio of intraband and interband scattering components. The ratio Omega_{res}/T_c increases almost linearly with the strength of the interband impurity scattering, in agreement with the experimental data. We also compare spin response of s+- and s++ superconductors. We show that there is no resonance for s++ gap, even when there is a finite mismatch between electron and hole Fermi surfaces shifted by the antiferromagnetic momentum.