Kawasaki Takeshi, Kim Kang, Onuki Akira
Department of Physics, Kyoto University, Kyoto 606-8502, Japan.
Department of Physics, Niigata University, Niigata 950-2181, Japan.
J Chem Phys. 2014 May 14;140(18):184502. doi: 10.1063/1.4873346.
We perform molecular dynamics simulation on a tetrahedral network glassformer using a model for viscous SiO2 by Coslovich and Pastore [J. Phys.: Condens. Matter 21, 285107 (2009)]. In this system, Si and O particles form a random network at low temperature T. We attach an ellipsoid to each particle to represent its time-averaged vibration tensor. We then examine the anisotropic vibrations of Si and O, where the ellipsoid orientations are correlated with the network. The ellipsoids exhibit marked vibrational heterogeneity. The configuration changes occur as breakage and reorganization of the network, where only one or two particles undergo large jumps at each rearrangement leading to diffusion. To the time-correlation functions, however, the particles surrounding these largely displaced ones yield significantly T-dependent contributions, resulting in a weak violation of the Stokes-Einstein relation. This crossover is mild in silica due to the small Si-O bond numbers per particle, while it is strong in fragile glassformers with large coordination numbers. On long timescales, jump events tend to occur in the same regions forming marked dynamic heterogeneity. We also calculate the diffusion constants and the viscosity. The diffusion obeys activation dynamics and may be studied by short-time analysis of irreversible jumps.
我们使用Coslovich和Pastore [《物理学报：凝聚态物质》21, 285107 (2009)]提出的粘性SiO₂模型，对四面体网络玻璃形成体进行分子动力学模拟。在这个系统中，硅（Si）和氧（O）粒子在低温T下形成随机网络。我们给每个粒子附上一个椭球体，以表示其时间平均振动张量。然后我们研究Si和O的各向异性振动，其中椭球体的取向与网络相关。椭球体表现出明显的振动不均匀性。构型变化以网络的断裂和重组形式发生，每次重组时只有一两个粒子经历大的跳跃从而导致扩散。然而，对于时间关联函数，围绕这些大幅位移粒子的粒子会产生显著的与温度T相关的贡献，导致对斯托克斯 - 爱因斯坦关系的轻微违背。由于每个粒子的Si - O键数较少，这种交叉在二氧化硅中较为轻微，而在具有高配位数的脆性玻璃形成体中则较为强烈。在长时间尺度上，跳跃事件倾向于在相同区域发生，形成明显的动态不均匀性。我们还计算了扩散常数和粘度。扩散服从活化动力学，可以通过对不可逆跳跃的短时间分析来研究。
