Crossover from classical to quantum spin diffusion in a nondegenerate gas

We study the crossover from classical to quantum diffusion by observing the equilibration of longitudinal spin domains in a trapped 87Rb sample just above quantum degeneracy. By controlling the degree of spin coherence in the domain wall, we can dramatically alter the relaxation dynamics of the system. Coherence in the domain wall leads to transverse-spin-mediated longitudinal spin diffusion that is slower than classical predictions, as well as altering the domains' oscillation frequency. We also investigate an instability in the longitudinal spin dynamics as the longitudinal and transverse spin components couple, and a conversion of longitudinal spin to transverse spin is observed, leading to longer lived coherent spin oscillations.