The relationship between static and dynamic nematic susceptibility studied by low-energy Raman spectroscopy

The critical nematic fluctuations are widely observed in many iron-based superconductors and are related to the superconductivity [1]. Both Raman susceptibility and elastic shear modulus $C_{66}$ show evidence of XY quadrupolar nematic fluctuations near the nematic instability. However, while the dynamic Raman response and the static $C_{66}$ data exhibit apparent different temperature dependence, similar critical temperatures were derived from both measurements [2,3]. Here we measure and analyze ultra-low frequency Raman susceptibility in BaFe$_2$As$_2$, which we compare to the $C_{66}$, and demonstrate consistency between the dynamic and static nematic susceptibilities, with accounting for the ultra-low frequency spectral weight in the Raman response, which was missed in the prior studies. [1] H.-H.Kuo \textit{et al}., Science \textbf{352}, 958 (2016). [2] Y.Gallais \textit{et al}., Phys.Rev.Lett. \textbf{111}, 267001 (2013). [3] A.E.Bohmer \textit{et al}., Phys.Rev.Lett. \textbf{112}, 047001 (2014).