Widespread nematic fluctuations in the (Sr,Na)Fe$_2$As$_2$ family of superconductors

Author(s): Frandsen, BA; Taddei, KM; Bugaris, DE; Stadel, R; Yi, M; Acharya, A; Osborn, R; Rosenkranz, S; Chmaissem, O; Birgeneau, RJ | Abstract: We report comprehensive pair distribution function measurements of the hole-doped iron-based superconductor system Sr$_{1-x}$Na$_{x}$Fe$_2$As$_2$. Structural refinements performed as a function of temperature and length scale reveal orthorhombic distortions of the instantaneous local structure across a large region of the phase diagram possessing average tetragonal symmetry, indicative of fluctuating nematicity. These nematic fluctuations are present up to high doping levels ($x \gtrsim 0.48$, near optimal superconductivity) and high temperatures (above room temperature for $x = 0$, decreasing to 150~K for $x = 0.48$), with a typical length scale of 1--3~nm. This work highlights the ubiquity of nematic fluctuations in a representative iron-based superconductor and provides important details about the evolution of these fluctuations across the phase diagram.