Apply neutron spin-echo spectroscopy, we investigate low-energy vibrational excitations in a series of silica aerogels of varying connectivity. Connectivity is adjusted both by precursor solution chemistry and by heat treatment. Polymeric aerogels show fractonlike densities of states, whereas colloidal materials exhibit a peak in the {μ}ev regime. On sintering, the low-frequency excitations disappear. We conclude that neutron spin-echo spectroscopy provides a measure of the topological aspects of disordered materials.
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Schaefer et al. (1990) studied this question.
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