Short-range nematic fluctuations in Sr1-xNaxFe2As2*

Abundant evidence indicates that nematic fluctuations are closely related to unconventional superconductivity. For both hole- and electron-doped iron-based superconductors (FeSCs), the underdoped phase space is widely inhabited by intertwined nematic and magnetic orders. Hole-doped FeSCs, such as Sr1-xNaxFe2As2, uniquely exhibit a re-entrant tetragonal magnetic (AFM-T) phase before entering the superconducting phase in the underdoped (UD) regime. By using inelastic x-ray scattering, we observed softened transverse acoustic phonons in the UD sample that harbors the AFM-T phase, and the optimal-doped (OP) sample. By extracting both the nematic susceptibility and correlation length, 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. The OP sample manifests weaker phonon softening effect, indicative of reduced fluctuations. This result indicates that short-range nematic fluctuations may be an important ingredient favorable for superconductivity.