Orbital fluctuation theory in iron-based superconductors: s-wave superconductivity, structure transition, and impurity-induced nematic order

TL;DR

This paper presents a unified orbital fluctuation theory explaining key features of iron-based superconductors, including s-wave superconductivity, structural transition, and impurity-induced nematic order, emphasizing the role of orbital fluctuations.

Contribution

It introduces a model where small electron-phonon coupling induces large orbital fluctuations, leading to superconductivity, structural instability, and nematic order, providing a comprehensive explanation for observed phenomena.

Findings

Orbital fluctuations induce s-wave superconductivity without sign reversal.

Orbital fluctuations cause structural transition via two-orbiton process.

Impurities induce non-local orbital order explaining resistivity anisotropy.

Abstract

The main features in iron-based superconductors would be (i) the orthorhombic transition accompanied by remarkable softening of shear modulus, (ii) high-Tc superconductivity close to the orthorhombic phase, and (iii) nematic transition in the tetragonal phase. In this paper, we present a unified explanation for them, based on the orbital fluctuation theory, considering both the e-ph and the Coulomb interaction. It is found that a small e-ph coupling constant ($\lambda ~ 0.2$) is enough to produce large orbital (=charge quadrupole $O_{xz/yz}$) fluctuations, which causes the s-wave superconductivity without sign reversal ($s_{++}$-wave state). The derived orbital fluctuations also cause the instability toward the structure transition due to the bound state formation of two orbitons with opposite momenta, which is called the "two-orbiton process". Moreover, impurity-induced non-local orbital order with $C_2$-symmetry is obtained when the orbital fluctuations are strong. This "impurity-induced nematic state" explains the in-plane anisotropy of resistivity in detwinned samples. We stress that (i)-(iii) are reproducible only when orbital fluctuations with respect to $O_{xz}$ and $O_{yz}$ charge quadrupoles are the most divergent. This fact ensures the reliability of the present model Hamiltonian and calculation.