Probing spin superfluidity in a spinor Bose-Einstein condensate with a moving magnetic obstacle

We study the critical energy dissipation in a spin-1 antiferromagnetic Bose-Einstein condensate by an oscillating magnetic obstacle, which is formed by focusing a near-resonant laser beam. When the magnetic obstacle is weak not to saturate local spin polarization, a sudden onset of spin-wave excitations is observed as the oscillation frequency increases, demonstrating the spin superfluid behavior of the system. For a strong obstacle that causes density perturbations via local spin saturation, half-quantum vortices (HQVs) are created by the moving magnetic obstacle, which shows that the critical dissipative dynamics evolves from spin-wave emission to HQV shedding. Critical HQV shedding is further investigated using a pulsed linear motion of the obstacle, and we identify two critical velocities to create HQVs with different core magnetization.