Nonmonotonic Dynamical Correlations beneath the Surface of Glass-Forming Liquids
HL Peng and HS Liu and T Voigtmann, PHYSICAL REVIEW LETTERS, 129, 215501 (2022).
Collective motion over increasing length scales is a signature of the vitrification process of liquids. We demonstrate how distinct static and dynamic length scales govern the dynamics of vitrifying films. In contrast to a monotonically growing static correlation length, the dynamical correlation length that measures the extent of surface- dynamics acceleration into the bulk displays a striking nonmonotonic temperature evolution that is robust also against changes in detailed interatomic interaction. This nonmonotonic change defines a crossover temperature T-* that is distinct from the critical temperature T-c of mode-coupling theory. We connect this nonmonotonic change to a morphological change of cooperative rearrangement regions of fast particles, and to the point where the decoupling of fast-particle motion from the bulk relaxation is most sensitive to fluctuations. We propose a rigorous definition of this new crossover temperature T-* within a recent extension of mode-coupling theory, the stochastic beta-relaxation theory.
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