On the origin of the electronic anisotropy in iron pnicitde superconductors

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We use polarization-resolved Raman spectroscopy to study the anisotropy of the electronic characteristics of the iron-pnictide parent compounds AFe₂As₂ (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.