Soliton molecules and breather positon solutions for the coupled modified nonlinear Schrödinger equation

Abstract

The coupled modified nonlinear Schrödinger equation, which can be regarded as a combination of nonlinear Schrödinger and derivative nonlinear Schrödinger equations, is investigated by Darboux transformation (DT) method. Based on the vector modified derivative nonlinear Schrödinger equation spectral problem, the related Lax pair and DT in compact determinant form are all successfully constructed to ensure integrability of the coupled modified nonlinear Schrödinger equation. According to DT method and the limiting technique, two main types of solutions that soliton molecules (SMs) and breather positon (B-P) solutions are systematically discussed. Beginning form zero plane wave backgrounds, the general $M-$SM-$(N-M)$-soliton solutions ($0\le M\le N$), including $M$ SMs and $N-M$ solitons, are subtly derived by the received DT. In particular, two degenerate cases can be reduced from the above general solutions, i.e., $N$-SM solutions ($M=N$) and $N$-soliton solutions ($M=0$). From the nonzero plane wave backgrounds, the higher-order B-P solutions are constructed via both DT and the limiting technique. It is interestingly shown that the central region of B-P solutions exhibit the patterns of rogue waves, and thus they are suggested to explain the generating mechanism of rogue waves. Finally, the corresponding dynamics of these received solutions are discussed in detail.