TRICRITICAL POINT AND SUPERFLUID TRANSITION IN HE-3-HE-4 MIXTURES IN SILICA AEROGEL

We have ultrasonically studied He-3-He-4 mixtures in 87% porosity aero gel. We find only a single transition for He-3-rich mixtures, in contr ast to the detached phase separation curve seen in 98% porosity aeroge l. However, it is a continuous transition, much like the superfluid tr ansition seen in He-4 films on aerogels, rather than a first-order pha se separation as seen in bulk mixtures. A kink occurs in the lambda li ne of the phase diagram at a He-3 concentration (corrected for the He- 4 inert layer) of X-c = 0.51 and is accompanied by a change in the cou pling to the superfluid component, indicating two distinct superfluid morphologies.