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

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.