STUDY OF HIGH-FREQUENCY VIBRATIONAL-MODES FOR NONLINEAR DIATOMIC CHAINS

The model of a one-dimensional diatomic chain with two different types of nonlinearity (quartic nearest-neighbor interparticle interaction p otential and substrate potential, respectively) is studied. The nonlin ear partial differential-difference equations in the two parallel case s for the model are transformed into nonlinear ordinary differential e quations by the approach cf decoupling ansatz plus continuum approxima tion. Through the qualitative analysis tile nonlinear vibrational mode s with various frequencies above the cut-off frequency of tile linear spectrum are obtained for the systems. The difference of the modes for the two types of nonlinearity almost disappears as the mass of heavy atoms is three times greater than that of light ones. This similarity of interparticle interaction nonlinearity to substrate nonlinearity fo r the nonlinear diatomic chain is in agreement with the origin of tile nonlinear substrate potential.