Glass transition with decreasing correlation length during cooling of Fe50Co50 superlattice and strong liquids

The glass transition is usually understood as a structural arrest that occurs during the cooling of liquids, trapping the system before it can crystallize. It occurs for all liquid classes, including metals. Theoretical interest has focused on the dynamical heterogeneity encountered during supercooling of 'fragile' liquids. Many suggest that the slow-down is caused by increasing dynamical correlation lengths. Here we report kinetics and thermodynamics of arrest in a system that disorders while in its ground state, exhibits a large heat capacity change (Delta C-p = C-p,C-mobile - C-p,C-arrested) on arrest, yet clearly is characterized by a static correlation length that decreases when approaching the transition temperature T-g from above. We show that our system, the Fe50Co50 superlattice, kinetically mimics an ideal 'strong' liquid with a critical point. Introducing liquid critical-point simulations, we can then argue that strong liquids differ from fragile liquids by occupying opposite flanks of an underlying order-disorder transition, which can be continuous, critical or weakly first order.