Excitation lines and the breakdown of Stokes-Einstein relations in supercooled liquids

Abstract

By applying the concept of dynamical facilitation and analyzing the excitation lines that result from this facilitation, we investigate the origin of decoupling of transport coefficients in supercooled liquids. We illustrate our approach with two classes of models. One depicts diffusion in a strong glass former, and the other in a fragile glass former. At low temperatures, both models exhibit violation of the Stokes-Einstein relation, $D\ensuremath{\sim}{\ensuremath{\tau}}^{\ensuremath{-}1}$, where $D$ is the self-diffusion constant and $\ensuremath{\tau}$ is the structural relaxation time. In the strong case, the violation is sensitive to dimensionality $d$, going as $D\ensuremath{\sim}{\ensuremath{\tau}}^{\ensuremath{-}2∕3}$ for $d=1$ and as $D\ensuremath{\sim}{\ensuremath{\tau}}^{\ensuremath{-}0.95}$ for $d=3$. In the fragile case, however, we argue that dimensionality dependence is weak, and show that for $d=1$, $D\ensuremath{\sim}{\ensuremath{\tau}}^{\ensuremath{-}0.73}$. This scaling for the fragile case compares favorably with the results of a recent experimental study for a three-dimensional fragile glass former.