Indications for a change of diffusion mechanism in supercooled liquids

Indications for a crossover between two distinct dynamical regimes at a temperature Tc above the glass-transition temperature Tg are presented for simple supercooled liquids. (i) Relating diffusion coefficient D and reorientational correlation times {τ} with viscosity {η}, a discontinuity for the temperature coefficient of {τ}/{η} or 1/D{η} is observed, demonstrating the failure of the Stokes-Einstein relations for TTc. (ii) Plotting NMR and dielectric correlation times on a reduced scale Tg/T shows that a bifurcation of the two relevant processes in a supercooled liquid ({α} and {β} processes) appears for TTc. Tc is found at T/Tg=1.18--1.28.