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  • Open Access

Rare Cage Escapes Drive Relaxation in Deeply Supercooled Liquids

Francesco Rusciano*, Raffaele Pastore, and Francesco Greco

Walter Kob†

  • *Contact author: francesco.rusciano@unina.it
  • †Contact author: walter.kob@umontpellier.fr
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Phys. Rev. X 16, 021061 – Published 26 June, 2026

DOI: https://doi.org/10.1103/7m7x-zxqv

Abstract

The relaxation dynamics of glass-forming liquids shows at a temperature Tc, somewhat above the glass-transition temperature Tg, a crossover, indicating the conjunction of two different dynamical regimes. For temperatures slightly above Tc, experiments and computer simulations have extensively probed this dynamics on the particle level and identified several universal scaling laws that are often compatible with the predictions of ideal mode coupling theory, while for temperatures below Tc, the nature of the relaxation dynamics, and hence the reason for the crossover, has so far remained elusive. Here we use large-scale computer simulations to investigate how in the low-temperature regime the relaxation mechanism differs from the one at higher temperatures. Our analysis reveals unexpected scaling laws that allow us to give a surprisingly simple description of the relaxation dynamics at very low T’s. Space-time correlation functions show that the cage-escape process involves rare but large particle displacements. These displacements give rise to distinctive subdiffusive power laws in the mean squared displacement and the intermediate scattering function, and also rationalize the origin of the universal excess wing found in the dynamic susceptibility. This insight advances our understanding on the dynamics of glass-forming systems at temperatures that are close to the experimental glass transition.

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Popular Summary

As glass-forming liquids are cooled, the viscosity dramatically increases by up to 15 orders of magnitude. Identifying the microscopic mechanism underlying this viscosity change has proven difficult, resulting in a multitude of competing theoretical approaches. Progress has been hampered by the difficulty to monitor in experiments the dynamics on the molecular level, and of performing numerical simulations at sufficiently low temperatures. Using large-scale computer simulations, we investigated the microscopic dynamics in this challenging temperature regime. We found that at low temperatures the nature of particle motion changes, crossing over from flowlike dynamics to simple hopping motion. Our insight into microscopic behavior should improve our understanding of glass-former dynamics.

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Phys. Rev. X 16, 021061– Published 26 June, 2026
  • Received 21 November 2025
  • Accepted 27 April 2026
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