Phonons in ultrathin layered structures

Phonon subsystem of ultrathin films was analysed using Greens functions of the displacement-displacement type, including quantum size effect and effect of boundaries onto Hamiltonian parameters. Diagonal components of Greens functions were evaluated, which enabled determination of the dispersion law, local densities of phonon states and Debye frequencies for thin films. A comparison with crystal bulk has shown that properties of thin films are strongly influenced both by the sample dimensions and boundary conditions. We have stated the conditions under which metal conductors could be made to become better superconductors. (C) 1999 Elsevier Science Ltd. All rights reserved