Low-Energy Vibration Excess in Silica Xerogels

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