标题： 结构弛豫和振动激发在液体和玻璃高频动力学中的作用 显示英文标题

标题： Role of structural relaxations and vibrational excitations in the high-frequency dynamics of liquids and glasses

摘要： 我们基于理想液-玻璃转变的模式耦合理论，对微观尺度和太赫兹频率区域液体和玻璃中的高频集体动力学进行了理论研究。 我们关注通过非弹性X射线散射和计算机模拟研究最近探讨的问题，即动态结构因子以及纵向和横向电流谱：高频声速的异常色散以及被称为玻色峰的低频激发的本质。 将讨论声学模如何与系统中存在的其他低能模相互作用。 从而，我们系统地解释了从高温液体到玻璃态深处的系统中异常声速色散，这涉及结构弛豫过程的贡献以及称为异常振荡峰（AOP）的振动激发的贡献。 当声速色散主要由振动动力学贡献时，论证了观察负色散——即波数增加时声速的{\em 减少}——的可能性。 我们还表明，在玻璃态的纵向和横向电流谱中，在相同共振频率下可观测到的低频激发是AOP的表现。 由于横向电流谱中存在AOP，预测横向声速也表现出异常色散。 这些理论结果在Lennard-Jones系统的模型中得到验证。 显示英文摘要

摘要： We present theoretical investigation on the high-frequency collective dynamics in liquids and glasses at microscopic length scales and 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 {\em 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.