Plasmon-sound hybridization in ionic crystals

TL;DR

This paper investigates the hybridization of plasmons, phonons, and electronic sound in ionic crystals using the Debye model, revealing how viscosity and Coulomb interactions influence collective mode behaviors.

Contribution

It provides explicit calculations of energies and damping of hybrid modes, highlighting the importance of Coulomb interactions for linear dispersion sound modes.

Findings

Increased viscosity causes plasmon modes to decay into incoherent backgrounds.

A phonon-like mode with linear dispersion remains sharp despite viscosity.

Coulomb interactions are essential for the existence of linear dispersion sound modes.

Abstract

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 find that with increasing viscosity a plasmon-like mode quickly decays into a broad, incoherent background, while a phonon-like mode with linear dispersion remains rather sharp. The quantitative behavior of the hybridized collective modes depends on the ratio of the electronic and the ionic plasma frequencies. We also show that the direct Coulomb interaction between the ions is essential to obtain a collective sound mode with linear dispersion.