Facilitation governs the fate of nuclei in the equilibration of glasses

For glasses relaxing towards a low-temperature supercooled liquid, it has been claimed that equilibration proceeds via a crystallisation-like process of nucleation and domain growth of a dense disordered state. Similarly, a recent theoretical explanation for the melt fronts seen in the melting of ultrastable glasses is couched in terms of proximity to a first-order thermodynamic transition. In this talk, we present in-silico experiments that challenge these ideas. We artificially seed a nucleus and characterise its evolution during the equilibration of a heated or cooled glass towards a supercooled liquid. We find that upon cooling, a pre-equilibrated nucleus in fact shrinks. Upon heating, melt fronts are only obtained for extremely strongly annealed glasses, with front growth proceeding much more slowly than the ballistic growth seen in simple models of crystal melting. We argue that both of these features are consistent with a picture in which facilitation plays the dominant role in the equilibration dynamics of glasses.