Structure of the pairing gap from orbital nematic fluctuations

TL;DR

This study investigates the structure of the superconducting gap influenced by orbital nematic fluctuations in a model relevant to iron-based superconductors, revealing how nematic phases affect pairing symmetry and gap anisotropy.

Contribution

It provides a detailed analysis of the momentum dependence of the pairing gap and the near-degeneracy of s-wave and d-wave symmetries in the nematic phase.

Findings

Weak momentum dependence of the gap in the tetragonal phase.

Enhanced anisotropy of the gap on hole Fermi surfaces in the nematic phase.

Near-degeneracy of s++ and d-wave pairing states in the tetragonal phase.

Abstract

We study superconducting instability from orbital nematic fluctuations in a minimal model consisting of the $d_{xz}$ and $d_{yz}$ orbitals, and choose model parameters which capture the typical Fermi surface geometry observed in iron-based superconductors. We solve the Eliashberg equations down to low temperatures with keeping the renormalization function and a full momentum dependence of the pairing gap. When superconductivity occurs in the tetragonal phase, we find that the pairing gap exhibits a weak momentum dependence over the Fermi surfaces. The superconducting instability occurs also inside the nematic phase. When the $d_{xz}$ orbital is occupied more than the $d_{yz}$ orbital in the nematic phase, a larger (smaller) gap is realized on the Fermi-surface parts, where the $d_{xz}$ ($d_{yz}$) orbital component is dominant, leading to a substantial momentum dependence of the pairing gap on the hole Fermi surfaces. On the other hand, the momentum dependence of the gap is weak on the electron Fermi surfaces. We also find that while the leading instability is the so-called $s_{++}$-wave symmetry, the second leading one is $d_{x^2-y^2}$-wave symmetry. In particular, these two states are nearly degenerate in the tetragonal phase whereas such quasi-degeneracy is lifted in the nematic phase and the $d_{x^2-y^2}$-wave symmetry changes to highly anisotropic $s$-wave symmetry.