Orbital Fluctuation Mediated Superconductivity in Iron Pnictides: Analysis of Five Orbital Hubbard-Holstein Model

TL;DR

This paper demonstrates that moderate electron-phonon interactions in iron pnictides can induce orbital fluctuations that promote s-wave superconductivity, explaining experimental robustness and crossover behaviors.

Contribution

It reveals that Fe-ion oscillations induce orbital fluctuations leading to s-wave pairing, providing a new understanding of superconductivity mechanisms in iron pnictides.

Findings

Orbital fluctuations induce s_{++}-wave superconductivity.

Superconductivity remains robust against impurities.

Crossover from s_{±} to s_{++} state with impurity increase.

Abstract

In iron pnictides, we find that the moderate electron-phonon interaction due to the Fe-ion oscillation can induce the critical d-orbital fluctuations, without being prohibited by the Coulomb interaction. These fluctuations give rise to the strong pairing interaction for the s-wave superconducting (SC) state without sign reversal ($s_{++}$-wave state), which is consistent with experimentally observed robustness of superconductivity against impurities. When the magnetic fluctuations due to Coulomb interaction are also strong, the SC state shows a smooth crossover from the s-wave state with sign reversal ($s_\pm$-wave state) to the $s_{++}$-wave state as impurity concentration increases.