NEW-TYPE OF GAP SOLITON IN A COUPLED KORTEWEG-DE VRIES WAVE SYSTEM

We show that, in a narrow gap in the spectrum of two linearly coupled Korteweg-de Vries equations with opposite signs of the dispersion coefficient, a two-parameter family of solitons of a novel type may exist. These are envelope solitons with decaying oscillating tails, which are radically different from the gap solitons previously known in nonlinear optics. In particular, they may become singular at some value of the velocity, and degenerate into algebraic solitons in another special case. It is demonstrated that gap solitons of the same type may also exist in a nonlinear optical system consisting of focusing and defocusing tunnel-coupled planar lightguides.