The glass transition: General scenario and crossover temperature

Analysis of light and neutron scattering spectra of both fragile and non-fragile glass forming liquids shows that the glass transition is not a monotonie process, qualitative changes in the dynamics occur at a crossover temperature, T_x. The crossover temperature shows up in many processes and seems to occur for different liquids at approximately the same viscosity (or structural relaxation time). A general scenario for the dynamics of strong and fragile glass forming liquids is discussed. It is shown that at T > T_x it follows, the qualitative scenario suggested by mode-coupling theory (MCT) for the higher temperature region. However, below T_x it does not agree with the asymptotic MCT predictions, not even qualitatively. The importance of a potential energy landscape for the dynamics in this low temperature range (T < T^x) is discussed.