The inelastic light scattering of crystals at finite temperatures and the correct tensor to describe it

The microscopic manifestation of temperature in lattice dynamics is generally understood through the harmonic approximation, where atomic motion is mapped onto a set of vibrational normal modes (i.e., phonons). Modern studies into lattice dynamics of solids significantly expand on this view to account for anharmonic phenomena such a thermal expansion and phase transitions. In contrast, studies into the electronic properties of solids rarely go beyond a quasi-harmonic treatment.

We combine ab-initio simulations and Raman scattering measurements to demonstrate explicit anharmonic effects in the temperature dependent dielectric response of a NaCl single crystal. Measuring the temperature evolution of its Raman spectrum demonstrates the necessity of including anharmonic lattice dynamics to explain the dielectric response of NaCl, as it is manifested in Raman scattering.

We discuss how considering an experimentally accessible tensor structure for inelastic light scattering can account for anharmonic spectra, while also directly incorporating all relevant symmetry constraints.