Neutron inelastic scattering measurements have been performed on samples of fumed silica (``Cab-O-Sil'') as a function of energy, momentum-transfer vector Q, and temperature T, and compared with analogous results for amorphous quartz. No evidence is seen for a sharp crossover in the vibrational density of states of Cab-O-Sil between phonon and fracton regimes, but the inelastic scattering shows anomalous behavior in several respects. Neither the T dependence nor the Q dependence nor the absolute intensity of the scattering from Cab-O-Sil is in accord with simple phonon models.
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Page et al. (1989) studied this question.
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