Local Symmetry Breaking by Impurities and Mode Splitting in Doped SmS

We introduce the idea of local symmetry breaking by impurities to explain the recently observed splitting of the [Formula presented] propagating excitation in doped [Formula presented]. While preserving the global cubic symmetry of the crystal, Y impurities change the local crystal-field environment of each Sm ion from cubic to tetragonal, thereby splitting the [Formula presented] triplet into a [Formula presented] level with energy [Formula presented] and a [Formula presented] doublet with energy [Formula presented]. A model with a randomly oriented quantization axis [Formula presented] fits not only the observed mode frequencies but also their intensities, which strongly depends on the wave vector.