HIGH-FREQUENCY ACOUSTIC MEASUREMENTS IN LIQUID-HE-3 NEAR THE TRANSITION-TEMPERATURE

We have made accurate measurements of the phase velocity in liquid He- 3 at 0.6 bar near T-c using a path length modulation technique. The fr equency used in this measurement (113 MHz) is much higher than any of the collective mode frequencies and well above the threshold for pair- breaking, omega much greater than Delta and omega > 2 Delta(0). We hav e observed a sharp but small linear increase in the phase velocity bel ow T-c and a small decrease from the expected behavior in the normal f luid region very close to the transition temperature.