Effect of pairing fluctuations on the spin resonance in Fe-based superconductors

Abstract

The spin resonance observed in the inelastic neutron scattering data on Fe-based superconductors has played a prominent role in the quest for determining the symmetry of the order parameter in these compounds. Most theoretical studies of the resonance employ an RPA-type approach in the particle-hole channel and associate the resonance in the spin susceptibility χS(q, ω) at momentum Q = (π, π) with the spin-exciton of an s +− superconductor, pulled below 2∆ by residual attraction associated with the sign change of the gap between Fermi points connected by Q. Here we explore the effect of fluctuations in the particle-particle channel on the spin resonance. Particle-particle and particle-hole channels are coupled in a superconductor and to what extent the spin resonance can be viewed as a particle-hole exciton needs to be addressed. In the case of purely repulsive interactions we find that the particle-particle channel at total momentum Q (the πchannel) contributes little to the resonance. However, if the interband density-density interaction is attractive and the π−resonance is possible on its own, along with spin-exciton, we find a much stronger shift of the resonance frequency from the position of the would-be spin-exciton resonance. We also show that the expected doublepeak structure in this situation does not appear because of the strong coupling between particle-hole and particle-particle channels, and ImχS(Q, ω) displays only a single peak.

Strong electron-boson coupling in the iron-based superconductor BaFe1.9Pt0.1As2<

“…The appearance of the resonance implies a sign change of superconducting gap(s) between either different patches of the Fermi surface or different Fermi pockets connected by a resonance mode at momentum q (see Ref. [51] and references therein). From the modeling of our infrared absorption data, the large sharp peak in the Eliashberg function of BaFe1.9Pt0.1As2 is centered at 5.1 ± 0.6 meV (41 ± 5 cm -1 ), with a full width at half maximum of 1 meV, and is only present in the superconducting state.…”

Renormalized parameters and perturbation theory in dynamical mean-field theory for the Hubbard model

“…is the dynamic susceptibility of the free electrons. The RPA approximation has been used recently, for example, to estimate the effective electron interaction due to spin fluctuations 45 . This calculation is based on a perturbation expansion in powers of the bare interaction U .…”

Raman resonance in iron-based superconductors: The magnetic scenario

“…In principle, in evaluating Π s (Q, ν) one has to include also G s F s terms and combine renormalizations in the particle-hole and the particle-particle channels because in the superconducting state particles and holes are mixed [47]. Previous work on the subject [48], however, has shown that as long as all the interactions are repulsive, the effect of inclusion of these extra terms is minimal in the case of Fe pnictides and merely shifts the resonance frequency (see below) down by a few percentage points.…”