Crossovers in the density of states of fractal silica aerogels

Neutron and Raman spectroscopies are used to identify a succession of regimes in the vibrational dynamics of two aerogels of different microstructures. Aside from the phonon-fracton and fracton-particle mode crossovers, an additional crossover is found which suggests two distinct elastic regimes in the fracton range, associated with bending and stretching. Greatly different Debye-Waller factors are found in the two regimes, distinguishing them unambiguously in neutron spectra. (Author)