I present experimental evidence for the violation of symmetry constraints in the Raman spectra of rock-salt Lead-Chalcogenides.
The theoretical framework describing Raman scattering relies almost entirely on the harmonic approximation. In the standard (harmonic) Raman picture an incident photon inelastically scatters by either absorbing or emitting a quantum of energy into a vibrational normal mode excitation, or phonon. The phonon itself is a construct of the harmonic approximation. Given well-defned normal modes, group-theory translates symmetry considerations into selection rules for the kind of allowed light-matter scattering events. In some crystal structures, like rock-salt, symmetry forbids any single phonon Raman scattering.

Using polarization dependent measurements, we observe two new low-frequency modes consistent with a reduced effective symmetry. These modes persist across the Lead-Chalcogenides series, following a number of curious spectroscopic trends. I discuss this anomalous Raman activity as the result of anharmonic temperature activated symmetry breaking.