On the Stokes-Einstein relation in glass forming liquids

Nowadays, the experimental evidence of a crossover between two dynamical re gimes for supercooled glass-forming liquids at a temperature T-c, T-c > T-g , where T-g is the glass transition temperature, has been widely reported i n the literature. On one hand, it is a well-known fact that in the region T -g < T < T-c, fragile glass forming liquids do not follow the Vogel-Fulcher -Tammann law. On the other hand, in the same region the Stokes-Einstein rel ation between the viscosity and the translational diffusion coefficient bre aks down. Using the temperature derivative analysis, we present, for three glass-forming liquids, an empirical law for log(10) f, where f is the frequ ency, and we exhibit the break down of the Stokes-Einstein relation. (C) 20 00 Published by Elsevier Science B.V. All rights reserved.