Graduate School of Arts and Sciences, University of Tokyo, Komaba, Tokyo 153-8902, Japan.
Research Center for Complex Systems Biology, Universal Biology Institute, University of Tokyo, Komaba, Tokyo 153-8902, Japan.
Proc Natl Acad Sci U S A. 2023 Apr 4;120(14):e2215153120. doi: 10.1073/pnas.2215153120. Epub 2023 Mar 29.
Supercooled liquids undergo complicated structural relaxation processes, which have been a long-standing problem in both experimental and theoretical aspects of condensed matter physics. In particular, past experiments widely observed for many types of molecular liquids that relaxation dynamics separated into two distinct processes at low temperatures. One of the possible interpretations is that this separation originates from the two-scale hierarchical topography of the potential energy landscape; however, it has never been verified. Molecular dynamics simulations are a promising approach to tackle this issue, but we must overcome laborious difficulties. First, we must handle a model of molecular liquids that is computationally demanding compared to simple spherical models, which have been intensively studied but show only a slower process: relaxation. Second, we must reach a sufficiently low-temperature regime where the two processes become well-separated. Here, we handle an asymmetric dimer system that exhibits a faster process: Johari-Goldstein relaxation. Then, we employ the parallel tempering method to access the low-temperature regime. These laborious efforts enable us to investigate the potential energy landscape in detail and unveil the first direct evidence of the topographic hierarchy that induces the relaxation. We also successfully characterize the microscopic motions of particles during each relaxation process. Finally, we study the correlation between low-frequency modes and two relaxation processes. Our results establish a fundamental and comprehensive understanding of experimentally observed relaxation dynamics in supercooled liquids.
过冷液体经历复杂的结构弛豫过程，这在凝聚态物理的实验和理论方面一直是一个长期存在的问题。特别是，过去的实验广泛观察到许多类型的分子液体在低温下弛豫动力学分为两个不同的过程。一种可能的解释是，这种分离源于势能景观的两尺度层次结构；然而，这从未得到验证。分子动力学模拟是解决这个问题的一种有前途的方法，但我们必须克服艰巨的困难。首先，我们必须处理与简单的球形模型相比计算要求更高的分子液体模型，这些模型已经被深入研究，但只显示出较慢的过程：弛豫。其次，我们必须达到足够低的温度范围，使两个过程能够很好地分离。在这里，我们处理一个表现出更快过程的不对称二聚体系统：Johari-Goldstein 弛豫。然后，我们采用并行温度法来进入低温范围。这些艰苦的努力使我们能够详细研究势能景观，并揭示导致弛豫的地形层次结构的第一个直接证据。我们还成功地描述了每个弛豫过程中粒子的微观运动。最后，我们研究了低频模式与两个弛豫过程之间的相关性。我们的结果为超冷液体中实验观察到的弛豫动力学建立了一个基本而全面的理解。
