NMR investigation of the low-temperature dynamics of solid 4He doped with 3He impurities.

The lattice dynamics of solid 4He has been explored using pulsed NMR methods to study the motion of 3He impurities in the temperature range (0.05-0.20 K) where experiments have revealed anomalies attributed to superflow or unexpected viscoelastic properties of the solid 4He lattice. We report the results of measurements of the nuclear spin-lattice and spin-spin relaxation times that measure the fluctuation spectrum at high and low frequencies, respectively, of the 3He motion that results from quantum tunneling in the 4He matrix. The measurements were made for 3He concentrations 16 < x3 < 2000 ppm. For 3He concentrations X3 = 16 and 24 ppm, large changes are observed for both the spin-lattice relaxation time T1 and the spin-spin relaxation time T2 at temperatures close to those for which the anomalies are observed in measurements of torsional oscillator responses and the shear modulus. These changes in the NMR relaxation rates were not observed for higher 3He concentrations. [ABSTRACT FROM AUTHOR]