Fe-As bond fluctuations in a double-well potential in LaFeAsO

TL;DR

This study investigates local Fe-As bond dynamics in LaFeAsO using EXAFS, revealing double-well potential oscillations and their temperature dependence, which relate to structural and electronic phase transitions in iron-based superconductors.

Contribution

It provides detailed analysis of Fe-As bond fluctuations and characterizes the double-well potential parameters using polarized EXAFS data, highlighting local structural dynamics.

Findings

Fe-As pairs oscillate in a double-well potential in LaFeAsO.

Structural phase transition at 150 K softens Fe-As oscillations.

Superconducting doping shows weak softening related to charge density waves.

Abstract

While for a long time iron based superconductors have been investigated looking at the average crystalline structure there is now growing interest on the divergence of local from the average structure. Here we use advanced data analysis of EXAFS, a fast and local probe, which has key advantages compared to crystallographic measurements, in unveiling the dynamics of the local structure. The results show that the Fe-As pair oscillates in the double-well potential both in undoped and in cobalt doped LaFeAsO crystals. The parameters and the characteristics of the double-well potential are determined by curve fits of the polarized EXAFS data. Temperature dependent Fe-As pair distribution and tunneling frequency are derived from selected model of two potential wells. We observe the softening of the Fe-As oscillations at the tetragonal to orthorhombic structural phase transition at 150 K in LaFeAsO. A weak broad softening is observed while decreasing the temperature from 70 to 40 K in superconducting LaFe0.89Co0.11AsO which could be correlated with the onset of short range charge density wave order.