Fractons observed

Fractons are strongly localized vibrational excitations of self-similar fractals. These can be visualized in simulations, and also observed in real fractal materials. It is shown that Brillouin, Raman and incoherent neutron scattering on silica aerogels, as well as low-temperature measurements, all find a consistent explanation in terms of fractons. The fractal dimension D, the spectral dimension d ̄, an internal length exponent σ and the density of vibrational states have been determined in these experiments. © 1989.