STABLE NONLINEAR HEAVY-MASS IMPURITY MODES

We demonstrate that stable nonlinear localized modes can exist not onl y at a Light-mass isotopic impurity, but also at a heavy-mass impurity , due to an interplay between discreteness and nonlinearity. This is i n sharp contrast to the theory of linear lattice vibrations that an im purity mode is possible only at a light-mass defect. We determine the structure of the nonlinear impurity modes analytically using a four-pa rticle approximation, and confirm the stability by direct numerical si mulations.