Effects of spin-orbit coupling on the neutron spin resonance in iron-based superconductors

Abstract

The so-called neutron spin resonance consists of a prominent enhancement of the magnetic response at a particular energy and momentum transfer upon entering the superconducting state of unconventional superconductors. In the case of iron-based superconductors, the neutron resonance has been extensively studied experimentally, and a peculiar spin-space anisotropy has been identified by polarized inelastic neutron scattering experiments. Here we perform a theoretical study of the energy-and spin-resolved magnetic susceptibility in the superconducting state with s+−-wave order parameter, relevant to iron-pnictide and iron-chalcogenide superconductors. Our model is based on a realistic bandstructure including spin-orbit coupling with electronic Hubbard-Hund interactions included at the RPA level. Spin-orbit coupling is taken into account both in the generation of spin-fluctuation mediated pairing, as well as the numerical computation of the spin susceptibility in the superconducting state. We find that spin-orbit coupling and superconductivity in conjunction can reproduce the salient experimentally observed features of the magnetic anisotropy of the neutron resonance. This includes the possibility of a double resonance, the tendency for a c-axis polarized resonance, and the existence of enhanced magnetic anisotropy upon entering the superconducting phase.

Effects of spin-orbit coupling on spin-fluctuation induced pairing in iron-based superconductors

“…49 we focussed on the paramagnetic phase and found overall agreement between the theoretical results and the experimental data in terms of materialvariability, doping-, temperature-, and energy-dependence of the polarization of the low-energy magnetic fluctuations [50][51][52][53] . More recently, this was extended to the superconducting phase where the spin-space anisotropy of the neutron resonance was investigated, and again overall consistency was found to the experimental data 54 . These works give credence to the itinerant approach and the inclusion of SOC within our theoretical framework.…”

“…61 and employed in Ref. 54 tends to decrease the triplet component upon increasing the energy cutoff.…”

“…We are motivated partly by the existence of significant gap anisotropy in some of the FeSCs, as discussed above, and the question to which extent SOC may (or may not) play a role in generating such unusual gap details. Another motivation comes from our own earlier theoretical investigations of the effects of SOC on the magnetic fluctuations in FeSCs 49,54 . In Ref.…”