The dynamics of strong and fragile glass formers

We review recent results from Raman scattering experiments on different types of glass formers. The fast dynamics is studied in the frequency range 1010Hz < v < 1013Hz and at temperatures near and above the glass transition temperature Tg. The Raman spectra are analysed in the spirit of mode coupling theory (MCT) which is briefly summarized. Both fragile and non-fragile glass formers exhibit similar high-temperature dynamics which qualitatively can be reproduced by MCT. Quantitatively, systematic deviations from MCT predictions are found. They are stronger the less fragile the glass former is. One possible reason for the deviations is the presence of low-frequency vibrations which also contribute to the dynamic structure factor. This boson peak is small for fragile and large for strong glasses. A change of the dynamics at a well-distinguished temperature Tc with c > Tg is observed in all cases. This crossover is also indicated by the emergence of a slow secondary relaxation process at T ≈ Tc upon cooling and by the breakdown of rotational translational coupling for T < Tc which has been found in simple liquids. Also, a viscosity analysis yields a master plot at T < Tc. All in all, the experiments support the idea of a change of transport mechanism in fragile and strong glasses at a characteristic temperature well above Tg.