Dark-bright solitons with positive mass in Manakov cases with repulsive interactions.

Dark-bright solitons of the integrable Manakov model with repulsive interactions were widely believed to possess negative mass due to the total densities admit dips. We report the dark-bright soliton can also admit positive effective mass for the Manakov case, by finding two branches of the phases and widths underlying the vector soliton solutions. The positive-mass soliton can evolve stably against noise and weak deviations from the Manakov conditions, and even a line-dark-bright soliton can exist stably in two-dimensional cases with suppressed snake instability, allowing further experimental investigations. When the soliton is driven by a constant force, it diffuses (for negative-mass ones) or splits (for positive-mass ones) as its velocity approaches the maximum moving speed, in contrast to the oscillating ones in nonintegrable cases. The diffusion can be understood by the excitation probabilities of the ground state (bright soliton) to the first excited state in the effective potential well. These results provide important supplements for the vector solitons in integrable cases, and would motivate more discussions on the dispersion relations of soliton in nonlinear coupled systems.