Indications for a change of diffusion mechanism in supercooled liquids

Indications for a crossover between two distinct dynamical regimes at a temperature ${\mathit{T}}_{\mathit{c}}$ above the glass-transition temperature ${\mathit{T}}_{\mathit{g}}$ 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/Dη is observed, demonstrating the failure of the Stokes-Einstein relations for T<${\mathit{T}}_{\mathit{c}}$. (ii) Plotting NMR and dielectric correlation times on a reduced scale ${\mathit{T}}_{\mathit{g}}$/T shows that a bifurcation of the two relevant processes in a supercooled liquid (α and β processes) appears for T<${\mathit{T}}_{\mathit{c}}$. ${\mathit{T}}_{\mathit{c}}$ is found at T/${\mathit{T}}_{\mathit{g}}$=1.18–1.28.