Spin Fluctuations, Interband Coupling, and Unconventional Pairing in Iron-based Superconductors

TL;DR

This study uses a two-band model and FLEX approach to analyze spin fluctuations and their role in unconventional superconductivity in Fe-based superconductors, highlighting the importance of interband coupling.

Contribution

It reveals the distinct roles of interband and intraband AF spin fluctuations and their influence on pairing symmetry in Fe-based superconductors.

Findings

Interband AF spin fluctuation arises from Coulomb repulsion.

Fermi-surface topology favors extended s-wave pairing.

Interband coupling significantly affects intraband pairing.

Abstract

Based on an effective two-band model and using the fluctuation-exchange (FLEX) approach, we explore spin fluctuations and unconventional superconducting pairing in Fe-based layer superconductors. It is elaborated that one type of interband antiferromagnetic (AF) spin fluctuation stems from the interband Coulomb repulsion, while the other type of intraband AF spin fluctuation originates from the intraband Coulomb repulsion. Due to the Fermi-surface topology, a spin-singlet extended s-wave superconducting state is more favorable than the nodal $d_{XY}$-wave state if the interband AF spin fluctuation is more significant than the intraband one, otherwise vice versa. It is also revealed that the effective interband coupling plays an important role in the intraband pairings, which is a distinct feature of the present two-band system.