Effect of disorder on the competition between nematic and superconducting order in FeSe

TL;DR

This paper investigates how disorder affects the competition between nematic order and superconductivity in FeSe, showing that impurities can enhance superconductivity by suppressing nematic order, especially in multiband systems.

Contribution

It introduces a theoretical model combining nematic and superconducting orders with impurity effects, explaining experimental observations in FeSe.

Findings

Disorder can enhance $T_c$ in FeSe.

Impurities suppress nematic order.

Multiband effects are crucial for the observed phenomena.

Abstract

Crystalline FeSe is known to display strong nematic order below a weak tetragonal-orthorhombic structural transition around $T_s\sim 90$K, and a superconducting transition at $T_c\sim 9K$. Recently, it was shown that electron irradiation, which creates pointlike potential scattering defects, has the surprising effect of enhancing $T_c$ while suppressing $T_s$. Here we discuss a possible scenario for such an effect, which postulates a competition between $s_\pm$ superconductivity and nematic order. The transition to the nematic state is modeled by a mean field theory of a $d$-wave Pomeranchuk instability, together with a Cooper pairing interaction in both one- and multiband models. The effect of nonmagnetic impurities on both orders is treated on equal footing within the Born approximation. We find evidence that disorder can indeed enhance $T_c$ while suppressing the competition with nematic order, but only in a multiband situation. We discuss our results in the context of experimental data on FeSe crystals.