VIBRATIONS LOCALIZED NEAR SURFACES AND INTERFACES IN NONTRADITIONAL CRYSTALS

This survey is devoted to studying some peculiar features of various s urface and interface vibrational modes in strongly anisotropic and oth er nontraditional crystals. The group of the nontraditional crystals i ncludes such anisotropic systems as layered and chain-type crystals, c omplex compounds of the ferroelastic and ferroelectric types in the vi cinity of structural phase transitions and many compounds with weak in teratomic bonds in some crystallographic directions or in certain plan es. The majority of such materials can be considered as quasi-low-dime nsional systems. It is shown that the presence of the weak interatomic bonds in the nontraditional crystals causes a considerable large effe ct of anharmonicity of the vibrations on properties of the longwavelen gth surface and interface modes. New types of deeply penetrating surfa ce waves are analyzed in anisotropic crystals and crystals near struct ural phase transitions, and in nonlinear crystals. The contribution of capillary effects to the peculiarities of the surface and interface w aves in the anisotropic crystals is estimated. The effect of the stron g 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 (t he inverse velocity surfaces) with the existence of the generalized su rface waves is discussed. The longwave dynamics of twins in the matrix of layered crystals, such as high T-c superconductors are also consid ered. In such systems, the dispersion relation for the interface elast ic waves have the form of omega similar to root kappa, characteristic of the spectrum of two-dimensional plasmons.