The Role of Spatially Heterogeneous Dynamics in Ultrastable Glasses*

Unlike conventional glasses, ultrastable glassy films transform into liquids upon heating via a propagating equilibration front, resembling the heterogeneous melting of crystals. We simulate this heterogeneous transformation using ultrastable configurations prepared with the swap Monte Carlo algorithm, and find transformation rates in excellent agreement with experiments.  Unlike experiments, the simulations can resolve the liquid–glass interface both in space and in time as well as the underlying particle motion responsible for its propagation. We show that the dynamic heterogeneity of the bulk liquid is passed on to the front that propagates heterogeneously in space and intermittently in time. This observation naturally explains why the averaged front velocity has the temperature dependence of the equilibrium diffusion coefficient of the liquid. We suggest that an experimental characterization of the interface geometry during the heterogeneous devitrification of ultrastable glassy films could provide direct experimental access to the long-sought characteristic length scale of dynamic heterogeneity in bulk supercooled liquids.