Superconductivity in FeSe: the role of nematic order

TL;DR

This paper develops a microscopic model explaining how nematic order influences superconductivity in FeSe, showing it causes gap anisotropy without significantly affecting the critical temperature, and challenges the orbital-selective pairing concept.

Contribution

It introduces a model incorporating s- and d-wave mixing and orbital spectral weight changes to explain nematic superconductivity in FeSe.

Findings

Nematicity causes a cos2θ gap variation consistent with experimental data.

Nematic order alone explains gap anisotropy without changing T_c.

Self-energy effects reduce the contribution of d_xz orbitals to pairing.

Abstract

Bulk FeSe is a special iron-based material in which superconductivity emerges inside a well-developed nematic phase. We present a microscopic model for this nematic superconducting state, which takes into account the mixing between $s-$wave and $d-$wave pairing channels and the changes in the orbital spectral weight promoted by the sign-changing nematic order parameter. We show that nematicity gives rise to a $\cos2\theta$ variation of the pairing gap on the hole pocket that agrees with ARPES and STM data for experimentally-extracted Fermi surface parameters. We further argue that, in BCS theory, $d_{xz}$ and $d_{yz}$ orbitals give nearly equal contributions to the pairing glue, i.e. nematic order alone accounts for the gap anisotropy, but has little effect on $T_{c}$. This result questions the validity of the concept of orbital-selective pairing. Self-energy corrections, however, make $d_{xz}$ orbital more incoherent and reduce its contribution to pairing.