Plasmon-sound hybridization in ionic crystals

We study the hybridization between plasmons, phonons, and electronic sound in ionic crystals using the Debye model, where the ionic background is modeled as a homogeneous, isotropic, elastic medium. We explicitly obtain the energies and the damping of the hybrid plasmon-sound modes in the hydrodynamic regime and calculate the corresponding dynamic structure factor. We show that the direct Coulomb interaction between the ions is essential to obtain a collective sound mode with linear dispersion.