Vibrating Quartz Crystal Studies of Wall-Film Superfluidity in Liquid 3He/4He Mixtures Near the Tricritical Concentration

We are using a thin quartz crystal immersed in the liquid, vibrating in shear modes at 24.8 and 94.0 MHz, to investigate the onset and growth of superfluidity in a thin film at the crystal surface in liquid 3He/4He mixtures having 3He mole fractions x near the tricritical value x=0.675. A parallel-plate capacitor in the cell monitors the phase-separation transition. Preliminary measurements of frequency shifts at x=0. 644 and x=0.705 show a relatively abrupt onset of wall-film superfluidity in the bulk normal region of the phase diagram at temperatures in good agreement with earlier studies of wall-film superfluidity by other methods.