Progressive slowing down of spin fluctuations in underdoped LaFeAsO$_{1-x}$F$_x$

TL;DR

This study investigates how low-energy spin fluctuations in LaFeAsO$_{1-x}$F$_x$ evolve with doping, temperature, and magnetic field, revealing a gradual slowdown of spin dynamics near the superconducting transition using NMR techniques.

Contribution

It introduces a BPP-type model to describe the slowing down of spin fluctuations, improving upon previous Curie-Weiss fits across a broad temperature range.

Findings

Underdoped samples show strong antiferromagnetic spin fluctuations.

Optimally and overdoped samples lack significant spin fluctuations.

A peak in 1/T1T indicates progressive slowing down of spins before superconductivity.

Abstract

The evolution of low-energy spin dynamics in the iron-based superconductor LaFeAsO$_{1-x}$F$_x$ was studied over a broad doping, temperature, and magnetic field range (x = 0 - 0.15, T up to 480K, H up to 30T) by means of As nuclear magnetic resonance (NMR). An enhanced spin-lattice relaxation rate divided by temperature, 1/T1T, in underdoped superconducting samples (x = 0.045, 0.05 and 0.075) suggests the presence of antiferromagnetic spin fluctuations, which are strongly reduced in optimally-doped (x = 0.10) and completely absent in overdoped (x = 0.15) samples. In contrast to previous analysis, Curie-Weiss fits are shown to be insufficient to describe the data over the whole temperature range. Instead, a BPP-type model is used to describe the occurrence of a peak in 1/T1T clearly above the superconducting transition, reflecting a progressive slowing down of the spin fluctuations down to the superconducting phase transition.