Evidence of anomalous dispersion of the generalized sound velocity in glasses

TL;DR

This study measures the dynamic structure factor of vitreous silica using inelastic X-ray scattering, revealing for the first time a positive dispersion in the generalized sound velocity at specific wavevectors.

Contribution

It provides the first experimental evidence of anomalous dispersion of sound velocity in a glass, using high-precision inelastic X-ray scattering data.

Findings

Observation of two vibrational excitations in the acoustic region

Propagation of sound waves up to the pseudo-Brillouin zone edge

Generalized sound velocity increases from 6500 to 9000 m/s around Q=5 nm-1

Abstract

The dynamic structure factor, S(Q,w), of vitreous silica, has been measured by inelastic X-ray scattering in the exchanged wavevector (Q) region Q=4-16.5 nm-1 and up to energies hw=115 meV in the Stokes side. The unprecedented statistical accuracy in such an extended energy range allows to accurately determine the longitudinal current spectra, and the energies of the vibrational excitations. The simultaneous observation of two excitations in the acoustic region, and the persistence of propagating sound waves up to Q values comparable with the (pseudo-)Brillouin zone edge, allow to observe a positive dispersion in the generalized sound velocity that, around Q=5 nm-1, varies from 6500 to 9000 m/s: this phenomenon was never experimentally observed in a glass.