# Quantitative characterization of short-range orthorhombic fluctuations in FeSe through pair distribution function analysis

## TL;DR

This study uses pair distribution function analysis of neutron and x-ray scattering data to reveal persistent short-range orthorhombic fluctuations in FeSe, providing insights into nematicity in this superconductor.

## Contribution

It quantitatively characterizes short-range orthorhombic fluctuations in FeSe, linking local distortions to nematic fluctuations across a wide temperature range.

## Key findings

- Orthorhombic fluctuations persist up to at least 300 K.
- Correlation length of fluctuations increases as temperature decreases.
- Enhanced local orthorhombic distortion in the nematic state.

## Abstract

Neutron and x-ray total scattering measurements have been performed on powder samples of the iron chalcogenide superconductor FeSe. Using pair distribution function (PDF) analysis of the total scattering data to investigate short-range atomic correlations, we establish the existence of an instantaneous, local orthorhombic structural distortion attributable to nematic fluctuations that persists well into the high-temperature tetragonal phase, at least up to 300 K and likely to significantly higher temperatures. This short-range orthorhombic distortion is correlated over a length scale of about 1 nm at 300 K and grows to several nm as the temperature is lowered toward the long-range structural transition temperature. In the low-temperature nematic state, the local instantaneous structure exhibits an enhanced orthorhombic distortion relative to the average structure with a typical relaxation length of 3 nm. The quantitative characterization of these orthorhombic fluctuations sheds light on nematicity in this canonical iron-based superconductor.