Fractional-charge vortex dipoles in spinor Bose-Einstein condensates

We theoretically and numerically investigate the generation of fractional-charge vortex dipoles in spinor condensates with non-zero magnetization. We find that in the antiferromagnetic phase of spin-1 and spin-2 and the cyclic phase of spin-2 condensate with non-zero magnetization coupling of the density (phonon) and a spin-excitation mode results in two critical speeds for vortex-antivortex pair creation in the condensate. As a result, a Gaussian obstacle potential moving across the antiferromagnetic spin-1 or spin-2 and cyclic spin-2 spinor condensates with non-zero magnetization can lead to the creation of fractional-charge vortex dipoles. On the other hand, for zero magnetization, the two modes get decoupled, which is illustrated by a single critical speed for vortex-antivortex pair creation in the condensate due to the phonon excitation mode.