Functional renormalization group study of the pairing symmetry and pairing mechanism in iron-selenide superconductors

TL;DR

This study uses functional renormalization group analysis to reveal an in-phase S-wave pairing mechanism in iron-selenide superconductors, driven by competing spin fluctuations, aligning with experimental observations.

Contribution

It demonstrates that an in-phase S-wave pairing on electron pockets arises from competing spin fluctuations, challenging previous quasi-nesting theories.

Findings

Identifies $S^{++}_{ee}$ pairing as dominant in iron-selenide superconductors.

Shows spin fluctuations drive pairing via Cooperon excitations.

Proposes experimental probes to distinguish pairing phases.

Abstract

In iron selenide superconductors only electron-like Fermi pockets survive, challenging the $S^{\pm}$ pairing based on the quasi-nesting between the electron and hole Fermi pockets (as in iron arsenides). By functional renormalization group study we show that an in-phase $S$-wave pairing on the electron pockets ($S^{++}_{ee}$) is realized. The pairing mechanism involves two competing driving forces: The strong C-type spin fluctuations cause attractive pair scattering between and within electron pockets via Cooperon excitations on the virtual hole pockets, while the G-type spin fluctuations cause repulsive pair scattering. The latter effect is however weakened by the hybridization splitting of the electron pockets. The resulting $S^{++}_{ee}$-wave pairing symmetry is consistent with experiments. We further propose that the quasiparticle interference pattern in scanning tunneling microscopy and the Andreev reflection in out-of-plane contact tunneling are efficient probes of in-phase versus anti-phase $S$-wave pairing on the electron pockets.