Density of states in fractal silica smoke-particle aggregates

Abstract

The low-frequency vibrational density of states, Z(\ensuremath{\omega}), in fractal silica aggregates has been measured by inelastic, incoherent neutron scattering. The samples were compressed powders of a hydroxylated, silica smoke-particle product, Cab-O-Sil. In the regime, \ensuremath{\Elzxh}\ensuremath{\omega}<0.5 meV Z(\ensuremath{\omega}) was found to be of the form ${\mathrm{\ensuremath{\omega}}}^{\mathrm{p}}$ with p=0.8\ifmmode\pm\else\textpm\fi{}0.1 at T=136 K and p=1.1\ifmmode\pm\else\textpm\fi{}0.05 at T=265 K. The values of p are larger than the current theoretical predictions for excitations on fractal networks. The observed temperature dependence of Z(\ensuremath{\omega}) indicates an important role of anharmonicity.