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 J = 0 --> 1 propagating excitation in doped Sm1-xYxS. 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 J = 1 triplet into a m(i) . J = 0 level with energy Delta(1) and a m(i) . J = +/-1 doublet with energy Delta(2)>Delta(1). A model with a randomly oriented quantization axis m(i) fits not only the observed mode frequencies but also their intensities, which strongly depends on the wave vector.