DO HIGH-FREQUENCY ACOUSTIC VIBRATIONS PROPAGATE IN STRUCTURALLY DISORDERED SOLIDS

Glasses exhibit well known and apparently universal anomalies in their thermal properties. Their excess specific heat poses the question of the nature of the modes that contribute to the density of vibrational states and to the plateau in their thermal conductivity of the propaga tion of acoustic phonons. The vibrational modes involved in both anoma lies must have frequencies well above 100 GHz, reaching into the terah ertz range. This is a difficult region for direct spectroscopic observ ation of acoustic excitations. New small-angle inelastic neutron and i nelastic X-ray scattering spectroscopies now give access to the struct ure factor S(Q,omega) in this region. Recent results on vitreous silic a are discussed in the light of other well established spectroscopic i nformation. We show in some detail that an elementary model, which con sists in fitting inelastic spectra simply with a damped harmonic oscil lator response, gives the impression that a mode continues to exist at frequencies omega above the strong phonon-scattering cross-over. It s hows then, over some range of omega, an apparently constant velocity a nd a damping proportional to Q(2). Our present understanding, instead, is that the boson peak observed in S(Q,omega) relates to this strong scattering of the phonons by the structural disorder. The latter produ ces a cross-over from plane waves to non-propagating excitations which agrees with all the available information.