VACANCY INDUCED ATOMIC MOTION IN HE-3-HE-4 MIXTURE CRYSTALS ON THE MELTING CURVE STUDIED BY NMR IMAGING

We report the results of an NMR imaging study of He-3-He-4 mixture cry stals in coexistence with the liquid. Data were obtained for 7 differe nt He-3 concentrations between 0.7% and 12.12%, in the temperature ran ge between 0.45 K and 1.3 K, along the melting curve. We imaged the sp atial distribution of the density He-3, and that of T-1 and T-2 relaxa tion times in the solid and in the liquid along the vertical direction of the experimental cell, perpendicular to the solid-liquid interface . We find significant differences between the dynamics of He-3 atoms i n hcp and bcc solid phases. These can be attributed to the different p roperties of thermally activated vacancies in the two phases. Specific ally, in the bcc phase above 1 K, vacancy induced motion of He-3 atoms in the solid is faster than the motion in the liquid. We interpret th is finding in terms of a He-3-vacancy bound state. Finally, we examine our results within the broader context of finding a consistent descri ption of the properties of vacancies in solid He.