Fast relaxation in disordered systems: From a double well to a cage

An overview of light scattering data measured in broad frequency (0.5GH_z<ν<10TH_z) and temperature (10K<T<1.2T_g) ranges are presented for a number of glass-forming systems. The data are compared with two basic approaches: the model of asymmetric double-well potentials (ADWPs) and the mode-coupling theory. It is shown that the ADWP model provides a reasonable description of the data at low temperatures, much below the glass transition temperature T_g. It fails, however, at higher temperatures. Two possible explanations are discussed: firstly, the appearance of a new relaxation process with a near-constant-loss spectral shape and, secondly, an increase in coupling between different relaxation modes with temperature increase.