Self-Consistent Vertex Correction Analysis for Iron-Based Superconductors: Mechanism of Coulomb-Interaction-Driven Orbital Fluctuations

TL;DR

This paper develops a self-consistent vertex correction method to analyze orbital and spin fluctuations in iron-based superconductors, revealing their mutual enhancement and their role in superconductivity and structural transitions.

Contribution

The paper introduces a novel self-consistent vertex correction approach that captures multimode orbital and spin fluctuations beyond RPA, elucidating their interplay in iron-based superconductors.

Findings

Multiple orbital fluctuations are mutually enhanced by vertex corrections.

Both antiferro- and ferro-orbital fluctuations develop at specific interaction ratios.

Ferro-orbital fluctuations contribute to nematic order and structural transitions.

Abstract

We study the mechanism of orbital/spin fluctuations due to multiorbital Coulomb interaction in iron-based superconductors, going beyond the random-phase-approximation. For this purpose, we develop a self-consistent vertex correction (SC-VC) method, and find that multiple orbital fluctuations in addition to spin fluctuations are mutually emphasized by the "multimode interference effect" described by the VC. Then, both the antiferro-orbital and ferro-orbital (=nematic) fluctuations simultaneously develop for $J/U \sim 0.1$, both of which contribute to the s-wave superconductivity. Especially, the ferro-orbital fluctuations give the orthorhombic structure transition as well as the softening of shear modulus $C_{66}$.