Role of structural relaxations and vibrational excitations in the high-frequency dynamics of liquids and glasses
Role of structural relaxations and vibrational excitations in the high-frequency dynamics of liquids and glasses
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DOI：
10.1103/physreve.74.031205
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发表时间：
2006-09-01
影响因子：
2.4
通讯作者：
Chong, Song-Ho
中科院分区：
文献类型：
--
作者：
Chong, Song-Ho
基于理想液体-玻璃相变的模耦合理论，我们对液体和玻璃中的高频集体动力学进行了微观尺度和太赫兹频段的理论研究。我们集中在最近从非弹性x射线散射和计算机模拟研究动态结构因素和纵向和横向电流谱所研究的问题：高频声速的反常色散和称为玻色子峰的低频激发的性质。将讨论声音模式如何干扰系统中存在的其他低位模式。因此，我们从结构松弛过程和被称为反常振荡峰(AOP)的振动激发的贡献的角度，系统地解释了从高温液体到玻璃态深处的系统中的反常声速色散。讨论了当声速弥散由振动动力学的贡献主导时，观察到负色散的可能性--声速随波数增加而减小。我们还指出，在同一共振频率下，在玻璃态的纵向和横向电流谱中都能观察到的低频激发是AOP的表现。由于横向电流谱中AOP的存在，预测横向声速也表现出反常的频散。对于Lennard-Jones系统的模型，证明了理论的这些结果。
We present theoretical investigation on the high-frequency collective dynamics in liquids and glasses at microscopic length scales and in the terahertz frequency region based on the mode-coupling theory for ideal liquid-glass transition. We focus on recently investigated issues from inelastic-x-ray-scattering and computer-simulation studies for dynamic structure factors and longitudinal and transversal current spectra: the anomalous dispersion of the high-frequency sound velocity and the nature of the low-frequency excitation called the boson peak. It will be discussed how the sound mode interferes with other low-lying modes present in the system. Thereby, we provide a systematic explanation of the anomalous sound-velocity dispersion in systems-ranging from high temperature liquid down to deep inside the glass state-in terms of the contributions from the structural-relaxation processes and from vibrational excitations called the anomalous-oscillation peak (AOP). A possibility of observing negative dispersion-the decrease of the sound velocity upon increase of the wave number-is argued when the sound-velocity dispersion is dominated by the contribution from the vibrational dynamics. We also show that the low-frequency excitation, observable in both of the glass-state longitudinal and transversal current spectra at the same resonance frequency, is the manifestation of the AOP. As a consequence of the presence of the AOP in the transversal current spectra, it is predicted that the transversal sound velocity also exhibits the anomalous dispersion. These results of the theory are demonstrated for a model of the Lennard-Jones system.