NMR, OPTICAL, AND PLASTIC-FLOW EXPERIMENTS IN BCC HE-3-HE-4 MIXTURE CRYSTALS - IN PURSUIT OF A VACANCY FLUID

We describe experiments on the properties of bcc He-3 -He-4 solid mixt ures on the melting curve between 0.5K and 1.9K. In this paper we focu s on effects related to the presence of thermal vacancies. First, we u sed NMR to image the He-3 distribution within the solid in equilibrium with the superfluid as well as its T-1 and T-2. The most surprising r esult was that above about 1K, vacancy related motion. of He-3 atoms i n the solid becomes faster than in the liquid. To check the macroscopi c aspects of this motion, we used the vibrating wire technique to look at plastic flow of the bcc solid phase, by moving the wire through th e crystal. The temperature dependence of the plastic flow velocity ind icates that the vacancy population in the bcc solid behaves like a vis cous fluid. The extent to which the vacancy population, causes the sol id to have liquid like properties is best demonstrated through optical observations of the distillation of He-3 atoms out of the crystal, wh ich tak es place via formation bubbles within the solid, which then pe rcolate into the liquid, creating a vivid impression of boiling.