On the origin of the electronic anisotropy in iron pnicitde superconductors

TL;DR

This study uses polarization-resolved Raman spectroscopy to investigate the electronic anisotropy in iron-pnictide parent compounds, revealing a critical collective mode and its interplay with structural transitions that influence anisotropic electronic states.

Contribution

It uncovers the role of a $xy$-symmetry collective mode in governing electronic anisotropy and its interaction with structural phase transitions in iron-pnictide superconductors.

Findings

Identification of a $xy$-symmetry collective mode above the structural transition.

Evidence of a spontaneous symmetry breaking at temperature $T^{*}$.

Structural transition at $T_s$ interrupts the mode's critical slowing down.

Abstract

We use polarization-resolved Raman spectroscopy to study the anisotropy of the electronic characteristics of the iron-pnictide parent compounds $A$Fe$_{2}$As$_{2}$ ($A$~=~Eu, Sr). We demonstrate that above the structural phase transition at Ts the dynamical anisotropic properties of the 122 compounds are governed by the emergence of $xy$-symmetry critical collective mode foretelling a condensation into a state with spontaneously broken four-fold symmetry at a temperature $T^{*}$. However, the mode's critical slowing down is intervened by a structural transition at Ts, about 80~K above $T^{*}$, resulting in an anisotropic density wave state.