Density of states in fractal silica smoke-particle aggregates

The low-frequency vibrational density of states, Z({ω}), 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, {}{ω}<0.5 meV Z({ω}) was found to be of the form ωᵖ with p=0.8±{}0.1 at T=136 K and p=1.1±{}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({ω}) indicates an important role of anharmonicity.