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

Inelastic neutron scattering has proved to be a powerful tool for exploring the characteristic dynamics in glassy systems on the time and length scale of atomic motions. 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 corresponds 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 new, extended neutron data in an archetypical ionic glassy system reveals that in a broader temperature range deviations from the 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 the remarkable two step character of the structural relaxation and the roughly wavenumber independent dynamics only remain as real and arguably decisive evidence in favour of mode coupling theories.