Nuclear magnetic resonance studies of isotopic impurity tunneling in solid hcp4He

Nuclear magnetic relaxation studies of isotopically impure solid⁴He have been made in the temperature-independent region of the relaxation spectrum, below 0.7 K. The Torrey theory for nuclear relaxation has been applied to measurements ofT ₁ andT ₂ to calculate the characteristic fluctuation time of the³He atoms due to the zero-point motion of³He and⁴He atoms. The fluctuation rates have been determined as a function of molar volume and Larmor frequency in samples where the mole fractions of³He in the gaseous mixtures used to form the solids were 0.02, 0.01, 0.005, and 0.002. The volume dependence of the fluctuation rate has been found to be far greater than that of the exchange rate of a³He-³He pair.T ₁ has been found to vary as ₀ ² , and bothT ₁ andT ₂ increased as the concentration decreased, in agreement with the Torrey theory. Preliminary investigations of the solid isotopic phase separation have also been carried out.Research supported in part by the National Science Foundation through Grant No. GP-29682 and the Advanced Research Projects Agency through the Materials Science Center at Cornell University, MSC Report # 1675.