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Exact solutions of multicomponent nonlinear Schrödinger equations under general plane-wave boundary conditions

We construct exact soliton solutions of integrable multicomponent nonlinear Schrödinger (NLS) equations under general nonvanishing boundary conditions. Different components of the vector (or matrix) dependent variable can approach plane waves with different wavenumbers and frequencies at spatial infinity. We apply Bäcklund-Darboux transformations to the cubic NLS equations with a self-focusing nonlinearity, a self-defocusing nonlinearity or a mixed focusing-defocusing nonlinearity. Both bright-soliton solutions and dark-soliton solutions are obtained, depending on the signs of the nonlinear terms and the type of Bäcklund-Darboux transformation. The multicomponent solitons generally possess internal degrees of freedom and provide highly nontrivial generalizations of the scalar NLS solitons. The main step in the construction of the multicomponent solitons is to compute the matrix exponential of a constant non-diagonal matrix arising from the Lax pair. With a suitable re-parametrization of the non-diagonal matrix, the matrix exponential can be computed explicitly in closed form for the most interesting cases such as the two-component vector NLS equation. In particular, we do not resort to Cardano's formula in diagonalizing a $3 \times 3$ matrix, so our expressions for the multicomponent solitons are in some sense more explicit and useful than those obtained in [Q-H. Park and H. J. Shin, Phys. Rev. E 61 (2000) 3093].