Elastoresistance measurements on CaKFe$_4$As$_4$ and KCa$_2$Fe$_4$As$_4$F$_2$ with the Fe site of $C_{2v}$ symmetry

TL;DR

This study compares elastoresistance in iron-based superconductors with different Fe-site symmetries, revealing temperature-dependent behaviors that challenge existing interpretations of nematic fluctuations.

Contribution

It provides new measurements of elastoresistance in compounds with distinct Fe-site symmetries, highlighting their similar temperature dependence and implications for nematicity.

Findings

Elastoresistance increases with decreasing temperature in all compounds.

Fe-site symmetry does not significantly alter the temperature dependence of elastoresistance.

Results suggest a need to reconsider the role of nematic fluctuations in resistivity anisotropy.

Abstract

We report resistance and elastoresistance measurements on (Ba$_{0.5}$K$_{0.5}$)Fe$_2$As$_2$, CaKFe$_4$As$_4$, and KCa$_2$Fe$_4$As$_4$F$_2$. The Fe-site symmetry is $D_{2d}$ in the first compound but $C_{2v}$ in the latter two, which lifts the degeneracy of the Fe $d_{xz}$ and $d_{yz}$ orbitals. The temperature dependence of the resistance and elastoresistance is similar between the three compounds. Especially, the [110] elastoresistance is enhanced with decreasing temperature irrespective of the Fe-site symmetry. This appears to be in conflict with recent Raman scattering studies on CaKFe$_4$As$_4$, which suggest the absence of nematic fluctuations. We consider possible ways of reconciliation and suggest that the present result is important in elucidating the origin of in-plane resistivity anisotropy in iron-based superconductors.