Toward a general description of the dynamics of glass formers

Recent results from low-frequency Raman scattering experiments on different types of glass formers are discussed. In particular, fast relaxations above T_g in the frequency range 10¹⁰ Hz < v < 10¹³Hz are described. Both fragile and non-fragile glass formers exhibit similar dynamics which qualitatively can be reproduced by mode coupling the-ory (MCT). A crossover temperature above T_g can be identified which marks a transition from liquid- to solid-like behavior in the supercooled, highly viscous liquid. Quantitatively, the deviations from the asymptotic laws of MCT are the stronger the less fragile the glass former is. One reason for the deviations stems from low-frequency vibrations which also contribute to the dynamic structure factor. This so-called boson peak is small for fragile and large for strong glasses. To account for both the boson peak and the fast relaxation, a model of damped vibrations is discussed which can consistently describe the Raman and neutron scattering spectra and the acoustic attenuation in the glass. A change of the dynamics at a well defined temperature is also shown by a slow secondary relaxation process at the crossover temperature upon cooling and by the break down of rotational translational coupling below the crossover which has been found in simple liquids. Viscosity analysis yields a master plot for different kinds of glasses below the crossover temperature whereas deviations occur above this point. The experiments support the idea of a change of transport mechanism in both fragile and strong glasses at a characteristic temperature well above T_g.