Dynamics of coherent acoustic phonons in nanostructures

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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.