Dynamics of coherent acoustic phonons in nanostructures

皮秒超声学的最新发展使得在各种纳米尺度的金属和半导体结构中实验研究声子的动力学及其与电子的相互作用成为可能。然而，对这些结构中声子动力学的理论研究还很缺乏，是亟待开发的课题。在这种情况下，我们从理论上分析了与纳米结构中的声子相关的以下几个问题：(1)超晶格中热导率的反常降低。(2)金属超晶格中声学声子的衰减。(3)周期超晶格中声子的群速度。(4)表面声子在TeO_2中的各向异性传播。总结如下：(1)用分子动力学方法分析了半导体超晶格中的晶格热导率。在理想超晶格中，我们发现结果与以前基于群速度计算得到的结果基本相同。然而，考虑到界面原子构型的无序的新计算再现了实验测量的导热系数的大小和与双层厚度的关系。(2)我们预言了金属超晶格中的电子-声子相互作用导致声学声子的共振吸收。这是由于布里渊区折叠引起的电子色散关系的改变，以及由此导致的能够吸收声子的电子态密度的发散造成的。(3)研究了周期超晶格中声子群速度的大小，发现声子群速度在带隙中的增强。随着周期数的增加，这一点会被放大。我们还讨论了超晶格中声子群速度反常行为的物理根源。(4)关于这一主题，请参阅小册子，其中有更详细的报告和原始文件。

Recent development of picosecond ultrasonics makes it possible to study experimentally the dynamics of phonons and their interaction with electrons in various metallic and semiconducting structures of nano-dimensions. However, the theoretical studies on the dynamics of phonons in those structures are still lacking and are the subjects to be exploited urgently. Under such a circumstance, we analyzed theoretically the following topics related to the acoustic phonons in nano-structures : (1) Anomalous reduction of thermal conductivity in superlattices. (2) Attenuation of acoustic phonons in metallic superlattices. (3) Group velocities of phonons in periodic superlattices. (4) Anisotropic propagation of surface phonons in TeO_2. The summaries of these topica are : (1) Molecular dynamics calculations are conducted to analyze the lattice thermal conductivity in semiconductor superlattices. In a perfect superlattice we found the results essentially the same as those obtained previously based on the group-velocity calculation. However, the new calculation taking account of the disorder in the atomic configuration at interfaces reproduced experimentally measured thermal conductivities both in their magnitude and dependence on the bilayer thickness. (2) We predicted that the electron-phonon interaction in metallic superlattices leads to the resonant absorption of acoustic phonons. This is caused by the modification of electron dispersion relations due to the Brilouin zone folding and the resulting divergence of the density of states of electrons capable of absorbing phonons. (3) We studied the magnitude of phonon group velocities in periodic superlattices and found their enhancement in frequency gaps. This is magnified as the number of periods is increased. We also discussed the physical origin of this anomalous behavior of phonon group velocities in superlattices. (4) On this topic please look at the booklet with more detailed report and original papers.