Strongly localized anharmonic modes in perfect and imperfect crystals

Abstract

Localized modes of large amplitudes in nonlinear lattices are considered. The applied method allows the reduction of nonlinear problem to a linear inverse problem of phonons scattering on a local potential. The method is efficient in the case of strongly localized modes. Analytical description of such modes in monatomic chain is given. Results of numerical calculations of anharmonic local vibrations of light ions in pure and impure alkali halide crystal are presented. It is found that, in the case of amplitudes ~0.5A, the vibrations depend very strongly on the crystallographic directions.