SCALING BEHAVIOR OF STRUCTURAL RELAXATION NEAR THE GLASS-TRANSITION - A CRITICAL ANALYSIS

Inelastic neutron scattering proved to be a powerful tool for exploring the characteristic dynamics in glassy systems on the atomic time and length scale. Results obtained in a number of chemically very different sample systems of the fragile glass category reveal a catalogue of common features including a complex structural relaxation pattern. This relaxation pattern displays a two-step nature, tentatively identified with the alpha- and beta-relaxation processes, as predicted by recent mode coupling theories. The data also show a remarkable compatibility with the theoretically predicted scaling features, which partially correspond to the so-called time-temperature superposition property. In this respect, there is a contradiction between the neutron scattering results and macroscopic type relaxation experiments. A critical analysis of extended new neutron data in an archetypical ionic glassy system reveals that in a broader temperature range deviations from time-temperature superposition scaling become evident and, in addition, considering worst case errors, there remains no hard evidence for the asymptotic validity of scaling near a critical temperature. Thus, even if the compatibility of neutron scattering and mode coupling results is impressively broad, in the light of an advocatus diaboli approach only the remarkable two-step character of the structural relaxation remains as real and arguably decisive evidence in favour of mode coupling theories.