Elucidating the origin of decoupling in glass-forming liquids through high-temperature activation free energies

It is well established that many glass-forming liquids exhibit a deviation from the Stokes-Einstein relation, which is commonly quantified by a power-law relation (so-called fractional Stokes-Einstein relation) between the mass diffusion coefficient [Formula: see text] and the momentum diffusion coefficient, corresponding to the shear viscosity [Formula: see text]. This ubiquitous phenomenon is often attributed to dynamic heterogeneity upon cooling, characterized by the progressive growth of the average size and lifetime of dynamic clusters of particles having excessively high or low mobility. However, a predictive theoretical understanding of the non-universal power-law decoupling exponent [Formula: see text] has been elusive. Here, for a wide variety of simulated glass-forming systems, we determine the power-law relation between the structural relaxation time [Formula: see text] and the peak time [Formula: see text] of the non-Gaussian parameter, that is, [Formula: see text], analogous to the power-law relation between [Formula: see text] and [Formula: see text] in the widely studied Kob-Andersen glass-forming liquid. We confirm the prediction from the string model of glass formation that [Formula: see text] is determined by the ratio of the high-temperature activation free energies for [Formula: see text] and [Formula: see text] in the temperature regime where dynamic heterogeneity is minimal. This finding suggests that dynamic heterogeneity is not the cause of decoupling, but rather a symptom. Our study emphasizes the role of the high-temperature liquid dynamics in understanding the fundamental mechanisms of glass formation.