  In supercooled liquids, vitrification generally suppresses crystallization.
Yet some glasses can still crystallize despite the arrest of diffusive motion.
This ill-understood process may limit the stability of glasses, but its
microscopic mechanism is not yet known. Here we present extensive computer
simulations addressing the crystallization of monodisperse hard-sphere glasses
at constant volume (as in a colloid experiment). Multiple crystalline patches
appear without particles having to diffuse more than one diameter. As these
patches grow, the mobility in neighbouring areas is enhanced, creating dynamic
heterogeneity with positive feedback. The future crystallization pattern cannot
be predicted from the coordinates alone: crystallization proceeds by a sequence
of stochastic micro-nucleation events, correlated in space by emergent dynamic
heterogeneity.
