using resonant Raman scattering*

Iron-based materials are characterized by the competition of various phases including magnetism, nematicity, and superconductivity. The interrelation of these instabilities is a key question of both materials science and theoretical modelling. In this study we present light scattering data on twin-free BaFe₂As₂ as a function of the energy of the incident photons. The phonon intensities display an anisotropy for the polarizations oriented along either the ferro- or the antiferromagnetically ordered direction. This anisotropy makes the fully symmetric As phonon to appear in crossed polarizations. The anomalous intensity and the anisotropy of the spectral weight increase substantially in the blue-green spectral range. The experimental results are analyzed using density functional theory. Qualitative agreement between model calculations and experiment is found if the magnetic order is properly taken into account. The anomalies are fully developed only in the presence of long range magnetic order and are not directly linked to nematicity. Rather they are a high-energy phenomenon indicating that magnetism and nematicity are interrelated.