GAP SOLITONS IN GAPLESS COUPLED CHAINS

We consider a system consisting of two coupled atomic chains in a subs trate potential; it is assumed that the atoms do not interact along on e of the chains. We demonstrate that intersection of the dispersion cu rves of the two uncoupled subsystems is a general mechanism producing gap envelope solitons in the presence of a weak coupling and weak nonl inearity. Moreover, considering the model in which one subsystem is de generate (due to the absence of the interaction inside the correspondi ng chain, its dispersion curve is a straight horizontal line), we find a new effect: while the width of the gap in the linearized system is equal to zero (or is very small), the nonlinearity produces an effecti ve gap in which the solitons exist. The corresponding soliton solution s are found in an exact analytical form.