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Kinetic Heterogeneities at Dynamical Crossovers

We perform molecular dynamics simulations of a model glass-forming liquid to measure the size of kinetic heterogeneities, using a dynamic susceptibility $χ_{\rm ss}(a, t)$ that quantifies the number of particles whose dynamics are correlated on the length scale $a$ and time scale $t$. By measuring $χ_{\rm ss}(a, t)$ as a function of both $a$ and $t$, we locate local maxima $χ^\star$ at distances $a^\star$ and times $t^\star$. Near the dynamical glass transition, we find two types of maxima, both correlated with crossovers in the dynamical behavior: a smaller maximum corresponding to the crossover from ballistic to sub-diffusive motion, and a larger maximum corresponding to the crossover from sub-diffusive to diffusive motion. Our results indicate that kinetic heterogeneities are not necessarily signatures of an impending glass or jamming transition.