QUASISOLITONS ON A DIATOMIC CHAIN AT ROOM-TEMPERATURE

We have studied analytically and numerically a nonlinear diatomic latt ice with a cubic nearest-neighbor interaction potential. Our system is a one-dimensional chain of pairs of atoms interacting through a ''har d'' interaction, each pair being bound to the neighboring pairs by a ' 'soft'' interaction. This is a simple model for hydrogen-bonded molecu lar chains, like the spines in an alpha helix. We have used a multiple -scale reductive perturbative technique to transform the equations of motion, and derived a nonlinear Schrodinger equation describing the ti me evolution of localized solitonic excitations. We have also derived analytically, following the method introduced by Zakharov and Shabat, the thresholds for the creation of solitons when the chain is initiall y excited by a square wave, which is a model of a generic localized ex citation. We have performed afterwards several molecular-dynamics simu lations at zero temperature. We have found that localized solitonlike excitations can propagate along the chain without being significantly altered; if the initial excitation has a square-wave shape, it evolves into a solitonlike excitation also traveling along the chain. However , if the initial excitation is excessively broad it tends to disperse in a way similar to a linear system; on the other hand, if the excitat ion is too narrow it may become pinned at the initial position. Finall y, we have repeated our simulations in presence of thermal disorder co rresponding to temperatures ranging up to 300 K. We have found that th e thermal vibrations not only do not destroy the solitonlike excitatio ns, but do not even alter in any significant way their propagation alo ng the molecular chain.