Tracer diffusion in glassforming liquids

in the last decades, a wide collection of experimental evidence has been found in the study of supercooled glassformers on the existence of a crossover between two dynamical regimes at a temperature T-c. We discuss the validity of the Vogel-Fulcher-Tammann in both regions. The breakdown of the Stokes-Einstein relation below T-c is presented, indicating that the diffusion coefficient of a tracer becomes decoupled from the viscosity through an exponent xi, and the diffusion process is intensified. We verify that a temperature shift on the diffusion coefficient introduces the same effect as the Stokes-Einstein breakdown equation. We present the dependence of this exponent on the ratio between the radii of the tracer and the host liquid molecule. (C) 2008 Elsevier B.V. All rights reserved