NMR studies of vacancy motion in solid hydrogen

The temperature dependence of the nuclear-spin-lattice relaxation time of ortho-H₂ and HD impurities in solid parahydrogen has been studied from the melting point to 1.5 K. Three distinct temperature regimes are observed: (1) an exponential temperature dependence for 11 KT13.5 K, (2) a minimum for the relaxation time for 7 KT11 K, and (3) a weak, approximately linear dependence for 1.5 KT7 K. The overall behavior can be understood in terms of the cross relaxation between two weakly coupled energy reservoirs corresponding to the ortho-H₂ and HD impurities, respectively. The temperature dependence near the minimum is different from that at high temperatures and is interpreted as evidence for quantum tunneling of vacancies in the intermediate-temperature regime.