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, ħω<0.5 meV Z(ω) was found to be of the form ${\mathrm{\omega }}^{\mathrm{p}}$ 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.

• Received 27 April 1987

DOI:https://doi.org/10.1103/PhysRevLett.59.1212