Low-Energy Vibration Excess in Silica Xerogels

Abstract

Specific heat (1.5--25 K) and Raman scattering (below $100{\mathrm{cm}}^{\ensuremath{-}1}$) in silica xerogels permit one to assess the frequency dependence of the coupling coefficient $C(\ensuremath{\omega})$ and the low-energy density of states $g(\ensuremath{\omega})$. Different network connectivity does not change the shape of the quasielastic scattering, but alters the spectral shape of $C(\ensuremath{\omega})$. The results permit one to ascribe the frequency shifts of the boson peak to the contribution of $C(\ensuremath{\omega})$ and to the Debye density of states ${g}_{D}(\ensuremath{\omega})$. Furthermore, the different magnitudes of $g(\ensuremath{\omega})$ appear to be a manifestation of changes in the Debye-like phonon contributions arising from connectivity changes.