Probing nematic fluctuations in iron-based superconductors with pair distribution function analysis*

The origin and implications of nematicity in iron-based superconductors remain among the most pressing questions in the field. Recent efforts to address this topic have focused not only on the nematic phase itself, but also on the nematic fluctuations that exist outside the region of static nematicity. Pair distribution function (PDF) analysis of x-ray and neutron total scattering data is a proven method of studying local, short-range structural correlations that deviate from the average structure, such as the orthorhombic distortions associated with nematic fluctuations in the high-temperature tetragonal phase of iron-based superconductors. Focusing primarily on the representative hole-doped system (Sr,Na)Fe₂As₂, we present PDF analysis that reveals a remarkably large region of nanometer-scale local orthorhombic distortions in temperature-composition space, reaching up to approximately 500 K for the parent compound and extending to doping levels beyond the C₂ dome near optimal superconductivity. These results offer a rich and detailed view of nematic fluctuations in iron-based superconductors and should be helpful in guiding future experimental and theoretical work.