Unconventional pairing originating from disconnected Fermi surfaces in the iron-based superconductor

TL;DR

This study develops a five-band model for LaFeAsO$_{1-x}$F$_x$ to explore its superconducting mechanism, revealing that spin fluctuations from disconnected Fermi surfaces promote an extended s-wave pairing with sign-changing gaps.

Contribution

It introduces a minimal five-band model and demonstrates that spin fluctuations from disconnected Fermi pockets induce unconventional s-wave pairing in iron-based superconductors.

Findings

Spin fluctuation modes from Fermi surface nesting drive superconductivity.

The pairing symmetry is extended s-wave with sign change across nesting vectors.

All five Fe d-bands are involved in the superconducting pairing mechanism.

Abstract

For the iron-based high $T_c$ superconductor LaFeAsO$_{1-x}$F$_x$, we construct a minimal model, where all of the five Fe $d$ bands turn out to be involved. We then investigate the origin of superconductivity with a five-band random-phase approximation by solving the Eliashberg equation. We conclude that the spin fluctuation modes arising from the nesting between the disconnected Fermi pockets realise, basically, an extended s-wave pairing, where the gap changes sign across the nesting vector.