Heterogeneous diffusion in highly supercooled liquids

The diffusivity of tagged particles is demonstrated to be heterogeneous on time scales comparable to or less than the stress relaxation time congruent to tau(alpha) in a highly supercooled model liquid. The particle motions in the relatively active regions dominantly contribute to the mean square displacement, giving rise to a diffusion constant larger than the Stokes-Einstein value. The van Hove self-correlation function G(s)(r, t) is shown to have a large r tail which can be scaled in terms of r/t(1/2) for t less than or similar to 3 tau(alpha). Its presence indicates heterogeneous diffusion in the active regions. However, the diffusion process becomes homogeneous on time scales longer than the life time of the heterogeneity structure (similar to 3 tau(alpha)). [S0031-9007(98)07758-8].