Low-Energy Vibration Excess in Silica Xerogels

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