Thermal expansion of LaFeAsO: Evidence for high-temperature fluctuations

TL;DR

This study investigates the thermal expansion behavior of LaFeAsO with fluorine doping, revealing high-temperature fluctuations and a first-order suppression of magnetic and structural transitions as doping induces superconductivity.

Contribution

It provides experimental evidence linking high-temperature fluctuations to structural and magnetic transitions in LaFeAsO and shows their abrupt suppression with doping.

Findings

Enhanced thermal expansion above structural transition due to fluctuations

Absence of fluctuations in superconducting samples

First-order suppression of magnetic and structural transitions with doping

Abstract

We present measurements of the thermal expansion coefficient alpha of polycrystalline LaFeAsO1-xFx (x <= 0.1). The magnetic and structural transitions of the samples with x <= 0.04 give rise to large anomalies in alpha(T), while the onset of superconductivity in the crystals with x >= 0.05 is not resolved. Above the structural transition, the thermal expansion coefficient of LaFeAsO is significantly enhanced. This is attributed to fluctuations, which also affect the electrical transport properties of the compound. The complete absence of these fluctuations in the superconducting samples even for x = 0.05 is taken as evidence for an abrupt first-order type of suppression of the structural and magnetic transitions upon F doping.