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

TL;DR

This paper investigates the glass transition in a Fe50Co50 superlattice, revealing a decreasing correlation length during cooling and proposing a new perspective on strong and fragile liquids related to order-disorder transitions.

Contribution

It demonstrates that the glass transition can occur with decreasing correlation length, challenging the conventional view of increasing correlation length near Tg.

Findings

Glass transition occurs with decreasing correlation length.

Fe50Co50 superlattice exhibits strong glass-forming kinetics.

Correlation length behavior distinguishes strong and fragile liquids.

Abstract

The glass transition GT is usually thought of as a structural arrest that occurs during the cooling of a liquid, or sometimes a plastic crystal, trapping a metastable state of the system before it can recrystallize to stabler forms1. This phenomenon occurs in liquids of all classes, most recently in bulk metallic glassformers2. Much theoretical interest has been generated by the dynamical heterogeneity observed in cooling of fragile liquids3, 4, and many have suggested that the slow-down is caused by a related increasing correlation length 5-9. Here we report both kinetics and thermodynamics of arrest in a system that disorders while in its ground state, exhibits a large !Cp on arrest (!Cp = Cp,mobile - Cp,arrested), yet clearly is characterized by a correlation length that is decreasing as GT is approached from above. We show that GT kinetics in our system, the disordering superlattice Fe50Co50, satisfy the kinetic criterion for ideally 'strong' glassformers10, and since !Cp behavior through Tg also correlates10, we propose that very strong liquidsand very fragile liquids exist on opposite flanks of an order-disorder transition - one that is already known for model systems.