Zhang Hao, Wang Xinyi, Yu Hai-Bin, Douglas Jack F
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 1H9, Canada.
Wuhan National High Magnetic Field Center, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China.
J Chem Phys. 2021 Feb 28;154(8):084505. doi: 10.1063/5.0039162.
We investigate the fast β- and Johari-Goldstein (JG) β-relaxation processes, along with the elastic scattering response of glass-forming (GF) liquids and the boson peak, in a simulated Al-Sm GF material exhibiting a fragile-strong (FS) transition. These dynamical processes are universal in "ordinary" GF fluids and collectively describe their "fast dynamics," and we find these relaxation processes also arise in a GF liquid exhibiting a FS transition. String-like particle motion, having both an irreversible and a reversible nature (stringlets) component, occurs in the fast-dynamics regime, corresponding to a ps timescale. String-like collective motion associated with localized unstable modes facilitates irreversible and intermittent particle "jumping" events at long times associated with the JG β-relaxation process, while stringlets associated with localized stable modes and corresponding perfectly reversible atomic motion give rise to the boson peak. To further clarify the origin of the boson peak, we calculate the density of states for both the stringlet particles and the "normal" particles and find that the stringlet particles give rise to a boson peak, while the normal atoms do not. The growth of stringlets upon heating ultimately also leads to the "softening" of these excitations, and the boson peak frequency and shear modulus drop in concert with this softening. The growth of string-like collective motion upon heating in the fast-dynamics regime is further shown to be responsible for the growth in the intensity of the fast relaxation process. Relaxation in cooled liquids clearly involves a hierarchy of relaxation processes acting on rather different timescales and spatial scales.
我们研究了快速β弛豫和乔哈里-戈尔茨坦（JG）β弛豫过程，以及玻璃形成（GF）液体的弹性散射响应和玻色子峰，研究对象是一种呈现脆-强（FS）转变的模拟Al-Sm GF材料。这些动力学过程在“普通”GF流体中具有普遍性，并共同描述了它们的“快速动力学”，我们发现这些弛豫过程也出现在呈现FS转变的GF液体中。在快速动力学区域，出现了具有不可逆和可逆性质（弦线）成分的类弦状粒子运动，其对应的时间尺度为皮秒。与局域不稳定模式相关的类弦状集体运动促进了与JGβ弛豫过程相关的长时间内不可逆且间歇性的粒子“跳跃”事件，而与局域稳定模式及相应的完全可逆原子运动相关的弦线则产生了玻色子峰。为了进一步阐明玻色子峰的起源，我们计算了弦线粒子和“正常”粒子的态密度，发现弦线粒子产生了玻色子峰，而正常原子则没有。加热时弦线的增长最终也导致了这些激发的“软化”，玻色子峰频率和剪切模量随着这种软化而协同下降。在快速动力学区域加热时类弦状集体运动的增长进一步表明是快速弛豫过程强度增长的原因。冷却液体中的弛豫显然涉及作用在相当不同时间尺度和空间尺度上的一系列弛豫过程。
