On the origin of the electronic anisotropy in iron pnicitde   superconductors
  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. 
W.-L. Zhang, P. Richard, H. Ding, Athena S. Sefat, J. Gillett,   Suchitra E. Sebastian, M. Khodas, and G. Blumberg
cond-mat.supr-con cond-mat.str-el