Vibrations localized near surfaces and interfaces in nontraditional crystals

Abstract

This survey is devoted to studying some peculiar features of various surface and interface vibrational modes in strongly anisotropic and other nontraditional crystals. The group of the nontraditional crystals includes such anisotropic systems as layered and chain-type crystals, complex compounds of the ferroelastic and ferroelectric types in the vicinity of structural phase transitions and many compounds with weak interatomic bonds in some crystallographic directions or in certain planes. The majority of such materials can be considered as quasi-low-dimensional systems. It is shown that the presence of the weak interatomic bonds in the nontraditional crystals causes a considerable large effect of anharmonicity of the vibrations on properties of the longwavelength surface and interface modes. New types of deeply penetrating surface waves are analyzed in anisotropic crystals and crystals near structural phase transitions, and in nonlinear crystals. The contribution of capillary effects to the peculiarities of the surface and interface waves in the anisotropic crystals is estimated. The effect of the strong elastic anisotropy on the properties of the ordinary and generalized Rayleigh waves and the shear surface waves is described analitically. A possible connection of the geometry of the isofrequency surfaces (the inverse velocity surfaces) with the existence of the generalized surface waves is discussed. The longwave dynamics of twins in the matrix of layered crystals, such as high T c superconductors are also considered. In such systems, the dispersion relation for the interface elastic waves have the form of ω ~ √k, characteristic of the spectrum of two-dimensional plasmons.