Distribution of Diffusion Constants and Stokes-Einstein Violation in supercooled liquids

摘要

It is widely believed that the breakdown of the Stokes-Einstein (SE) relationbetween the translational diffusivity and the shear viscosity in supercooledliquids is due to the development of dynamic heterogeneity i.e. the presence ofboth slow and fast moving particles in the system. In this study we\emph{directly} calculate the distribution of the diffusivity for a modelsystem for different temperatures in the supercooled regime. We find that withdecreasing temperature, the distribution evolves from Gaussian to bimodalindicating that on the time scale of the typical relaxation time, mobile (fluidlike) and less mobile (solid like) particles in the system can be\emph{unambiguously} identified. We also show that less mobile particles obeythe Stokes-Einstein relation even in the supercooled regime and it is themobile particles which show strong violation of the Stokes-Einstein relation inagreement with the previous studies on different model glass forming systems.Motivated by some of the recent studies where an ideal glass transition isproposed by randomly pinning some fraction of particles, we then studied the SEbreakdown as a function of random pinning concentration in our model system. Weshowed that degree of SE breakdown increases quite dramatically with increasingpinning concentration, thereby providing a new way to unravel the puzzles of SEviolation in supercooled liquids in greater details.