Quasi-two-dimensional behavior of phonon subsystems and the superconductivity mechanism in perovskitelike compounds

The results of group-theory analysis, lattice dynamical calculations, and experimental data on the vibrational spectra of perovskitelike superconductors have been considered together. The layer description of phonon subsystems in these materials has been shown to be valid. The quasi-two-dimensional nature of the phonon subsystems has been established. The genesis of the bulk modes has been studied. It has been shown that optical vibrations in these crystals can be divided into 3(NL-1) interlayer and 3(NA-NL) intralayer modes where NL and NA are the numbers of layers and atoms per primitive unit cell, respectively. The interlayer bulk vibrations originate from acoustic layer modes whereas the intralayer modes are induced by optical vibrations of isolated layers. A sandwich conception of the high-Tc superconductivity mechanism is discussed based on the different roles of different layers.