The crossover from propagating to strongly scattered acoustic modes of glasses observed in densified silica

TL;DR

This study investigates the transition from propagating to strongly scattered acoustic modes in densified silica, linking spectroscopic features to thermal properties and identifying a crossover frequency near the boson peak.

Contribution

It provides spectroscopic evidence of the crossover from propagating to strongly scattered modes in densified silica and relates this to the boson peak and local excitations.

Findings

Crossover frequency Omega_co nearly coincides with the boson peak center

Rapid increase in Brillouin linewidth marks the end of the acoustic branch

Presence of optic-like excitations related to SiO_4 librations

Abstract

Spectroscopic results on low frequency excitations of densified silica are presented and related to characteristic thermal properties of glasses. The end of the longitudinal acoustic branch is marked by a rapid increase of the Brillouin linewidth with the scattering vector. This rapid growth saturates at a crossover frequency Omega_co which nearly coincides with the center of the boson peak. The latter is clearly due to additional optic-like excitations related to nearly rigid SiO_4 librations as indicated by hyper-Raman scattering. Whether the onset of strong scattering is best described by hybridization of acoustic modes with these librations, by their elastic scattering (Rayleigh scattering) on the local excitations, or by soft potentials remains to be settled.