Phonon anomalies in FeS

Here, we present results from light scattering experiments on tetragonal FeS with the focus placed on lattice dynamics. We identify the Raman active A_1g and B_1g phonon modes, a second order scattering process involving two acoustic phonons, and contributions from potentially defect-induced scattering. The temperature dependence between 300 and 20 K of all observed phonon energies is governed by the lattice contraction. Below 20 K the phonon energies increase by 0.5–1 cm^-1 , thus indicating putative short range magnetic order. Additionally, along with the experiments we performed lattice-dynamical simulations and a symmetry analysis for the phonons and potential overtones and find good agreement with the experiments. In particular, we argue that the two-phonon excitation observed in a gap between the optical branches becomes observable due to significant electron-phonon interaction.