Nematicity is ubiquitous in electronic phases of high-Tc superconductors, particularly in the Fe-based systems. We used inelastic x-ray scattering to extract the temperature-dependent nematic correlation length ζ from the anomalous softening of acoustic phonon modes in FeSe, underdoped Ba(Fe0.97Co0.03)2As2, and optimally doped Ba(Fe0.94Co0.06)2As2. In all cases, we find that ζ is well described by a power law (T-T0)-1/2 extending over a wide temperature range. Combined with the previously reported Curie-Weiss behavior of the nematic susceptibility, these results point to the mean-field character of the nematic transition, which we attribute to a sizable nematoelastic coupling that is likely detrimental to superconductivity.
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