On superconducting mechanism in the iron-based layered superconductors

TL;DR

This paper proposes a superconducting mechanism for doped Fe-based compounds involving both electron-electron and electron-hole pairing, explaining high Tc and coexistence of superconductivity with spin-density wave order.

Contribution

It introduces a model combining electron-phonon and Coulomb interactions to explain the high Tc and coexistence phenomena in Fe-based superconductors, contrasting with prior theories.

Findings

Higher Tc explained by (e-e)-pairing near pseudogap edges.

Doping dependence shows maximum Tc at optimal doping.

Coexistence of SC and SDW states with incommensurate SDW.

Abstract

It is demonstrated that SC mechanism of doped Fe-based compounds is characteristic for itinerant electron systems with coexistence of both (e-e)- and (e-h)-pairing arising due to electron-phonon and Coulomb interactions, respectively. The higher Tc of the SC transition here (as compared with conventional (LTSC) BCS-systems without (e-h)-pairing) is a natural consequence of (e-e)-pairing at the background of high density of states which arises in the narrow energy range near dielectric-(SDW)-gap (pseudogap) edges due to removing of electronic states from the energy region of dielectric (SDW) gap (pseudogap) (already formed at the part of the Fermi surface in the normal state with onset temperature T* (near the same for corresponding structural transition) due to (e-h)-pairing). Below Tc the system enters the coexistence (SC+SDW) state. The SDW formed is incommensurate with lattice and dynamic in character. The phase diagram for such system is determined: the doping dependence of the SC gap (and Tc) has a maximum (optimal doping) while the dielectric (SDW) gap (pseudogap) (and T*) is a decreasing function of doping. These conclusions follow from detailed analysis of available resistivity and another data for Fe-based superconductors on the basis of model with partial dielectrization of electron energy spectra. The picture obtained and manifestation of two order parameters (SC and SDW) in experiments, first of all, in threshold phenomena are discussed. The comparison with the case of cuprates is performed.