Nuclear spin-spin relaxation of isotopic impurities in solid hydrogen

We report systematic studies of the transverse nuclear spin-spin relaxation times T₂ of HD impurities in solid H₂. The experimental results indicate three different temperature regimes: (1) an exponential temperature dependence at high temperatures, (2) a minimum in T₂ for 7 KT11 K, and (3) an approximately temperature-independent region for 1.5 KT7 K. The latter is attributed to the quantum diffusion of the impurities at low temperatures. At very low temperatures (0.10 KT1.5 K), T₂ increases approximately linearly with temperature. This is attributed to the formation of bound ortho-H₂ pairs. The behavior in the intermediate-temperature regime is analyzed in terms of vacancy-impurity scattering with quantum tunneling of vacancies.