Neutron-scattering measurements of the spin excitations in LaFeAsO and Ba(Fe$_{0.953}$Co$_{0.047}$)$_{2}$As$_{2}$: Evidence for a sharp enhancement of spin fluctuations by nematic order

TL;DR

This study uses neutron scattering to show that electronic nematic order significantly enhances spin fluctuations and magnetic correlations in iron pnictides, highlighting its role in magnetic ordering.

Contribution

It provides direct experimental evidence linking nematic order to increased spin fluctuations and magnetic correlation length in LaFeAsO and Ba(Fe,Co)2As2.

Findings

Nematic order causes a sharp increase in spin fluctuation strength.

Structural transition at T_S influences magnetic spectra.

Nematic order promotes stripe antiferromagnetic order.

Abstract

Inelastic neutron scattering was employed to investigate the impact of electronic nematic order on the magnetic spectra of LaFeAsO and Ba(Fe$_{0.953}$Co$_{0.047}$)$_{2}$As$_{2}$. These materials are ideal to study the paramagnetic-nematic state, since the nematic order, signaled by the tetragonal-to-orthorhombic transition at $T_{{\rm S}}$, sets in well above the stripe antiferromagnetic ordering at $T_{{\rm N}}$. We find that the temperature-dependent dynamic susceptibility displays an anomaly at $T_{{\rm S}}$ followed by a sharp enhancement in the spin-spin correlation length, revealing a strong feedback effect of nematic order on the low-energy magnetic spectrum. Our findings can be consistently described by a model that attributes the structural/nematic transition to magnetic fluctuations, and unveils the key role played by nematic order in promoting the long-range stripe antiferromagnetic order in iron pnictides.