Longitudinal optical-like excitations in binary liquid mixtures
Longitudinal optical-like excitations in binary liquid mixtures
复制标题
二元液体混合物中的纵向类光学激发
DOI：
10.1088/0953-8984/14/25/102
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发表时间：
2002
期刊：
影响因子：
--
通讯作者：
I. Mryglod
中科院分区：
文献类型：
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作者：
T. Bryk;I. Mryglod
研究了非离子液体二元混合物质量浓度波动广义朗之万方程的类扩散解和传播解。导出了纵向类光激励存在的条件。从开发的方法获得的结果进行比较，对致密的混合物KrAr和He0.65Ne0.35的模拟结果。讨论了类光激发对谱函数贡献的振幅。二元液体中的集体动力学长期以来一直是一个争论的问题，特别是关于高频传播激发的存在以及在实验强度谱函数中观察其表现的可能性。虽然对于库仑液体[1]，传播电荷波的存在（通过与固体中的光学声子类比，它们被称为光学类激发）已经得到了很好的证实，但人们不知道如何解释非离子混合物中的高频激发。在最近的文献[2，3]中，我们用广义集体模（GCM）方法[4]研究了Lennard-Jones和金属混合物的横向动力学，发现非离子二元液体中存在横向类光激发。结果表明，质量集中传播模式的阻尼对于色散关系的估计是非常重要的。在长波极限下，横向类光激发的阻尼由互扩散和静态结构因子Sxx（k）的值决定，其中k = 0是二元混合物离分层有多近的标志。在这封信中，我们报告的纵向高频动力学的二元混合物的GCM研究。主要目的是证明广义Langevin方程存在类光学解。使用分析三变量模型来处理质量浓度波动，我们将表明，光学类（质量浓度）传播激发必须存在于非离子液体混合物中，但在某些情况下可能会被抑制的特殊条件。两个二元系统MD模拟的数值例子
Diffusive-like and propagating solutions of the generalized Langevin equation for mass–concentration fluctuations for liquid non-ionic binary mixtures are studied. A condition of existence for longitudinal optical-like excitations is derived. The results obtained from the approach developed are compared against simulation results for the dense mixtures KrAr and He0.65Ne0.35. Amplitudes of contributions from optical-like excitations to the spectral functions are discussed. Collective dynamics in binary liquids has been for a long time a matter of debate, especially as regards the existence of high-frequency propagating excitations and the possibility of observing their manifestation in experimental intensity spectral functions. While for coulombic liquids [1] the existence of propagating charge waves (by analogy with optical phonons in solids they have been called optical-like excitations) is well established, it is not known how one should interpret high-frequency excitations in non-ionic mixtures. In recent papers [2, 3] we studied the transverse dynamics of Lennard-Jones and metallic mixtures within the generalized collective modes (GCM) approach [4] and found that transverse optical-like excitations can exis te ve ni nnon-ionic binary liquids. It was shown that the damping of mass–concentration propagating modes is very important for estimatio no ft heir dispersion relations. In the longwavelength limit the damping of transverse optical-like excitations is determined by the value of the mutual diffusion and the static structure factor Sxx(k) ,w hich atk = 0i s am easure of how close the binary mixture is to demixing. In this letter we report a GCM study of longitudinal high-frequency dynamics in binary mixtures. The main aim is to show that there exist opticallike solutions for the generalized Langevin equation. Using an analytical three-variable model to treat mass–concentration fluctuations, we will show that optical-like (mass–concentration) propagating excitations must exist in non-ionic liquid mixtures, but may be suppressed in some cases by special conditions. Numerical examples from MD simulations of two binary