Dark and antidark soliton pairs in nonlocal media

Abstract We study soliton formation in a one-dimensional two-component defocusing nonlinear Schrödinger system with nonlocal interactions. The model features arbitrary intra- and inter-component nonlinearities, and a general symmetric nonlocal response kernel, allowing for a broad range of physical applications in nonlinear optics and Bose–Einstein condensates. Focusing on the miscible regime, where the uniform background is modulationally stable, we perform a multiscale expansion and derive an effective Korteweg–de Vries equation governing small-amplitude, long-wavelength excitations. This analytical framework reveals two distinct families of vector soliton pairs. The first, fast solitons, propagate at higher velocities and exhibit same-polarity structures in both components (dark–dark or antidark–antidark), depending on the strength of nonlocality. The second, slow solitons, represent a novel class of mixed-polarity solutions in which one component is dark and the other antidark. The polarity of each component is shown to depend on the relative background densities and the system’s nonlinear coefficients. Direct numerical simulations confirm the stability and accuracy of the asymptotic predictions, and reveal that—if the characteristic velocities are upshifted—solitons of different polarities emerge in the same component, thus suggesting that spatial nonlocal nonlinearity can induce symmetry-breaking internal structure within multi-component solitons.