Effect of nematic order on the spin fluctuation spectrum of LaFeAsO*

Inelastic neutron scattering measurements on the LaFeAsO antiferromagnetic (AFM) system reveal distinct temperature-dependent behavior in the low-energy spin dynamics. As expected, the dynamic susceptibility at the AFM wavevector peaks at the AFM transition temperature $T_{\mathrm{N}}$, but also displays an anomaly at the orthorhombic-to-tetragonal transition temperature $T_{\mathrm{S}}$. The spin-spin correlation length increases rapidly below $T_{\mathrm{S}}$ once long-range nematic order sets in. The sharp changes in both the dynamic susceptibility and the spin-spin correlation length at $T_{\mathrm{S}}$ evidence a strong effect of nematic order on the magnetic spectrum, in agreement with models that attribute the structural transition to an electronic nematic phase driven by spin fluctuations.