Theory of light scattering from soft modes in IV-VI compounds

We present a theoretical description of the Raman-Brillouin spectra from the coupled soft optical phonon and acoustical modes in IV-VI compounds. The spectra are studied above and below the cubic to rhombohedral transition temperature $T_c$. A mean-field description of the sublattice distortion and static strains below $T_c$ is also given, supposing electrostrictive coupling as well as anharmonic phonon-phonon interactions are present. The theoretical results are compared with data on SnTe. We predict strong interference between the Brillouin and second-order Raman spectra above $T_c$. A different class of interference phenomena is present below $T_c$. We illustrate these points through model calculations on SnTe.