The effects of Coulomb interactions on the superconducting gaps in iron-based superconductors

TL;DR

This paper demonstrates that strong interband Coulomb interactions in a multiband model can explain the observed superconducting gaps and their robustness in LiFeAs, revealing an unconventional s± gap symmetry.

Contribution

It introduces a five-band model with Coulomb interactions explaining superconducting gaps and symmetry in LiFeAs, differing from conventional BCS theory.

Findings

Reproduces the superconducting transition temperature and five gaps of LiFeAs.

Shows the gap on the shallow band is robust due to energy-independent Coulomb interactions.

Predicts an unconventional s± gap symmetry with a sign change between bands.

Abstract

Recent angle-resolved photoemission spectroscopy measurements of Co-doped LiFeAs report a large and robust superconducting gap on the $\Gamma$-centered hole band that lies 8 meV below the Fermi level. We show that, unlike a conventional superconductor described by BCS theory, a multiband system with strong interband Coulomb interactions can explain these observations. We model LiFeAs with a five-band model in which the shallow hole band is coupled with the other bands by only Coulomb interactions. Using Eliashberg theory, we find reasonable interaction parameters that reproduce the $T_c$ and all five gaps of LiFeAs. The energy independence of the Coulomb interactions then ensures the robustness of the gap induced on the shallow band. Furthermore, due to the repulsive nature of the Coulomb interactions, the gap changes sign between the shallow band and the other hole pockets, corresponding to an unconventional $s_{\pm}$ gap symmetry. Unlike other families of iron-based superconductors, the gap symmetry of LiFeAs has not been ascertained experimentally. The experimental implications of this sign-changing state are discussed.