The Raman coupling function in amorphous silica and the nature of the long wavelength excitations in disordered systems

TL;DR

This study investigates the vibrational modes in amorphous silica using Raman and neutron scattering data combined with simulations, revealing that long-wavelength excitations are influenced by local disorder, affecting their atomic displacement patterns.

Contribution

It provides new insights into the Raman coupling function at low frequencies and the nature of long-wavelength excitations in disordered systems through combined experimental and simulation approaches.

Findings

C(ω) extrapolates to a non-zero value as ω → 0

Long-wavelength modes exhibit non-regular atomic displacements

Local disorder influences vibrational mode characteristics

Abstract

New Raman and incoherent neutron scattering data at various temperatures and molecular dynamic simulations in amorphous silica, are compared to obtain the Raman coupling coefficient $C(\omega)$ and, in particular, its low frequency limit. This study indicates that in the $\omega \to 0$ limit $C(\omega)$ extrapolates to a non vanishing value, giving important indications on the characteristics of the vibrational modes in disordered materials; in particular our results indicate that even in the limit of very long wavelength the local disorder implies non-regular local atomic displacements.