Spin diffusion of correlated two-spin states in a dielectric crystal

Reciprocal space measurements of spin diffusion in a single crystal of calcium fluoride (CaF$_2$) have been extended to dipolar ordered states. The experimental results for the component of the spin diffusion parallel with the external field are $D_{D}^{||}=29 \pm 3 \times 10^{-12}$ cm$^{2}$/s for the [001] direction and $D_{D}^{||}=33 \pm 4 \times 10^{-12}$ cm$^{2}$/s for the [111] direction. The diffusion rates for dipolar order are significantly faster than those for Zeeman order and are considerably faster than predicted by simple theoretical models. It is suggested that constructive interference in the transport of the two spin state is responsible for this enhancement. As expected the anisotropy in the diffusion rates is observed to be significantly less for dipolar order compared to the Zeeman case.