High-frequency vibrational dynamics in glasses

In this paper we present a review of the recent inelastic x-ray scattering (IXS) studies on the high-frequency collective dynamics in topologically disordered systems. At mesoscopic exchanged momenta (Q approximate to 1-10 mn(-1)), these IXS. studies allowed experimental demonstration of the existence in glasses of propagating acoustic phonon-like excitations whose characteristics are qualitatively independent from the specific system investigated. These universal features are as follows: (i) on the small-Q side of the momentum region examined, the existence of a linear dispersion for the excitation frequency of the longitudinal excitations; this dispersion starts to bend downward with increasing Q-value; (ii) a quadratic Q-dependence of the excitation width, Gamma (Q), in the Q-region where Omega (Q) is linear; (iii) temperature independence of this excitation; and (iv) the appearance-with increasing Q-value-of the signature of the collective transverse dynamics. In this paper, using the case of vitreous silica as a representative example, we present the IXS. evidence for these universal features and we compare them with the outcome of computer simulations. Finally, we will show that all the observed characteristics of the high-frequency vibrations can be retrieved within the 'harmonic' approximation of the glass dynamics.