Fast dynamics perspective on the breakdown of the Stokes-Einstein law in fragile glassformers

The breakdown of the Stokes-Einstein (SE) law in fragile glassformers is examined by Molecular-Dynamics simulations of atomic liquids and polymers and consideration of the experimental data concerning the archetypical OTP glassformer. All the four systems comply with the universal scaling between the viscosity (or the structural relaxation) and the Debye-Waller factor $\langle u^2\rangle$, the mean square amplitude of the particle rattling in the cage formed by the surrounding neighbors. It is found that the SE breakdown is scaled in a master curve by a reduced $\langle u^2\rangle$. Two approximated expressions of the latter, with no and one adjustable parameters respectively, are derived.