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 (ΔCp=Cp,mobileCp,arrested) on arrest, yet clearly is characterized by a static correlation length that decreases when approaching the transition temperature Tg 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.

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Figure 1: The glass transition in the Fe50Co50 order–disorder transition.
Figure 2: DSC upscans at fixed 20 K min−1 rates, following cooling at different slower rates between 1.5 and 20 K min−1.
Figure 3: Comparison of equilibrium heat capacities, derived from the scans of enthalpy recovery (shown in the Supplementary Information), with data obtained from fixed scan rate runs and also the theoretical functions from the Bragg–Williams model and the Kirkwood approximation.
Figure 4: Heat capacities of BeF2 and SiO2 per g-atom through Tg compared with those for Fe50Co50 and for confined water.


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We appreciate support received from the Deutsche Forschungsgemeinschaft (DFG). C.A.A. acknowledges helpful discussions with M. D. Ediger and JY-Z. Yue.

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C.A.A. and R.B. conceived the project, I.G. and S.W. planned and carried out the experimental work, S.W. and I.G. analyzed the data, and C.A.A. wrote the paper with important literature and diagrammatic input from S.W. and advice from I.G. and R.B.

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Correspondence to C. Austen Angell.

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Wei, S., Gallino, I., Busch, R. et al. Glass transition with decreasing correlation length during cooling of Fe50Co50 superlattice and strong liquids. Nature Phys 7, 178–182 (2011).

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