NMR of Dilute $o$-${\mathrm{H}}_2$ in Solid $p$-${\mathrm{H}}_2$ at Temperatures below 1 K

We report some unusual observations on the NMR spectrum and the clustering rate of $o$-${\mathrm{H}}_2$ at low ortho concentration in solid ${\mathrm{H}}_2$ at temperatures between 25 mK and 2 K. Evidence points towards motional narrowing of the HD impurities, the proton signal of which is observed for $T<\mathrm{}0.15\mathrm{}$ K, and a hopping of $o$-${\mathrm{H}}_2$ from one pair configuration into another. Furthermore, there is an unexpected increase of the clustering rate of the other $o$-${\mathrm{H}}_2$ molecules as the temperature decreases below ∼0.1 K.