Lü Y J, Wang W H
School of Physics, Beijing Institute of Technology, Beijing 100081, People's Republic of China.
Institute of Physics, Chinese Academy of Sciences, Beijing 100190, People's Republic of China.
Phys Rev E. 2016 Dec;94(6-1):062611. doi: 10.1103/PhysRevE.94.062611. Epub 2016 Dec 28.
The single-particle dynamics of the glass-forming Cu_{50}Zr_{50} alloy, from the supercooled liquid well above the glass-transition temperature, T_{g} to the glassy state, is studied by using the molecular dynamics simulations. When the liquid is cooled below 1.2T_{g}, the dynamics heterogeneity characterized by the cage-jump motion becomes increasingly pronounced. The analyses based on the continuous time random walk method indicate that the liquid falls out of equilibrium in the present simulation time scale when it is cooled into the regime below 1.02T_{g}. However, we find that the jump length and the jump rate do not display the non-equilibrium behaviors even in the glassy state below T_{g}, which allows us to study the intrinsic dynamic characteristics through T_{g}. The mean waiting time between two successive jumps has a rapid growth following the Vogel-Fulcher-Tammann law as the non-equilibrium regime is approached, in analogy with the temperature behaviors of transport properties for fragile supercooled liquids. In contrast, the jump rate maintains the Arrhenius decay and the jump length has even a weaker temperature dependence when the liquid is cooled into glassy state. We find that a pronounced enhancement of the spatial correlation of jumps occurs accompanied by the glass transition: the string-like cooperative jumps dominate the fast motion instead of the uncorrelated and individual jumps. Our work offers an insight into the equilibrium effect of the single-particle dynamics in glass transition.
通过分子动力学模拟研究了玻璃形成合金Cu₅₀Zr₅₀从远高于玻璃化转变温度Tg的过冷液体到玻璃态的单粒子动力学。当液体冷却到1.2Tg以下时，以笼跃运动为特征的动力学非均匀性变得越来越明显。基于连续时间随机游走方法的分析表明，在本模拟时间尺度下，当液体冷却到1.02Tg以下时，液体偏离平衡态。然而，我们发现即使在Tg以下的玻璃态，跳跃长度和跳跃速率也不表现出非平衡行为，这使我们能够通过Tg研究其内在动力学特性。随着接近非平衡态，两个连续跳跃之间的平均等待时间遵循Vogel-Fulcher-Tammann定律快速增长，这与易碎过冷液体的输运性质的温度行为类似。相反，当液体冷却到玻璃态时，跳跃速率保持Arrhenius衰减，跳跃长度对温度的依赖性甚至更小。我们发现，伴随着玻璃转变，跳跃的空间相关性显著增强：串状协同跳跃主导快速运动，而不是不相关的单个跳跃。我们的工作为玻璃转变中单粒子动力学的平衡效应提供了见解。
