Nematic correlation length in hole-doped and isovalent-dopedsuperconductors

Abundant evidence indicates that nematic fluctuations are closely related to unconventional superconductivity (SC). For carrier-doped iron-based superconductors, the underdoped phase is widely inhabited by intertwined nematic and magnetic orders. In isovalent-doped samples, in which chemical disorder plays a less role, optimal SC appears near a putative nematic quantum critical point. The strength of the nematic fluctuations, characterized by the nematic susceptibility χ, reveals a marked enhancement upon cooling. However, the spatial dependence of the nematic fluctuations, namely the nematic correlation length ξ  has only recently been measured by Fernandes and Reznick through the in-plane transverse acoustic phonon (IPTA). We studied the IPTA phonon via inelastic X-ray,  and observed a softening of phonons in Sr_1-xNa_xFe₂As₂  samples. Building on earlier work, we extract both χ and ξ simultaneously by an exact mean field method of analysis without additional inputs. We find a large nematic susceptibility persisting in the AFM-T phase, and a short nematic correlation length ξ ~ 10 Å that is much smaller than that on the electron-doped side. Finally, we will present recent measurements on isovalent-doped BaFe₂(As_1-xP_x)₂ superconductors. These results indicate that short-range nematic fluctuations may be an important ingredient favorable for superconductivity.