Crossovers in the density of states of fractal silica aerogels

Combining neutron and Raman spectroscopies, we 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, we find an additional crossover that suggests two distinct elastic regimes in the fracton range, which we associate with bending and stretching. Greatly different Debye-Waller factors are found in the two regimes, distinguishing them unambiguously in neutron spectra.