Interaction-Driven Spin Rotations in a Two-component BEC Reflecting from a Barrier

Reflection from a barrier perturbed by weak Raman coupling beams generates spin waves in a BEC of ⁸⁷Rb. Due to the coincidence of scattering lengths in ⁸⁷Rb, a BEC in a mixture of two hyperfine states behaves as a phase-coherent yet distinguishable two-component fluid. Reflection from the barrier creates a counter-propagating matter wave with spin partly transverse to the spin of the forward-going wave, initiating interaction-driven rotations. The observed spin rotations are well-described by mean-field simulations with equal inter and intra-spin interaction strengths, demonstrating that the spin dynamics do not arise from nonequilibrium dynamics caused by spin-dependent interactions or immiscibility of the two components. Rather, the driving mechanism for spin rotations is the different interaction energy experienced by parallel versus anti-parallel spins in different spatial modes, much in the same way the identical spin rotation effect is known to generate spin waves in non-condensed gases. We observe one oscillation of a spin wave for low Rabi frequencies and study the transition where spin rotations become independent of the external coupling and instead are dominated by the interaction-driven effects.