Elastic coupling and spin-driven nematicity in iron-based superconductors

TL;DR

This paper develops a theoretical framework showing that elastic coupling in iron-based superconductors transforms the nematic transition into a mean-field type, reconciling magnetic fluctuations with observed nematic behavior.

Contribution

The authors introduce a $ ext{phi}^4$ theory demonstrating that elastic strain coupling changes the nematic transition to mean-field without suppressing magnetic fluctuations.

Findings

Elastic coupling transforms the nematic transition into mean-field behavior.

The mean-field behavior persists across the temperature range with shear modulus softening.

Magnetic fluctuations responsible for nematic order are not suppressed by elastic coupling.

Abstract

Spin-driven nematic order that has been proposed for iron-based superconductors is generated by pronounced fluctuations of a striped density wave state. On the other hand it is a well known fact that nematic order parameter couples bilinearly to the strain, which supresses the fluctuations of the nematic order parameter itself and lowers the upper critical dimension, yielding mean-field behaviour of the nematic degrees of freedom for $d>2$. This is consistent with the measured Currie-Weiss behaviour of the nematic susceptibility. Here we reconcile this apparent contradiction between pronounced magnetic fluctuations and mean-field behaviour of the nematic degrees of freedom. We show, by developing a $\varphi^4$ theory for the nematic degrees of freedom, that the coupling to elastic strain does not suppress the fluctuations that cause the nematic order in the first place (magnetic fluctuations), yet it does transform the Ising-nematic transition into a mean-field transition. In addition, we demonstrate that the mean field behavior occurs in the entire temperature regime where a softening of the shear modulus is observed.