FIRST-SOUND AND ZERO-SOUND VELOCITY AND FERMI-LIQUID PARAMETER F-2(S)IN LIQUID-HE-3 DETERMINED BY A PATH-LENGTH MODULATION TECHNIQUE

We have measured the velocity of first- and zero-sound in liquid He-3 at 12.6 MHz over the pressure range of 0.6 to 14.5 bar using a path le ngth modulation technique that we have recently developed. From these measurements, the pressure dependent value of the Fermi liquid paramet er F-2(3) was calculated and found to be larger at low pressure than p reviously reported. These new values of F-2(3) indicate that transvers e zero-sound is a propagating mode at all pressures. The new values ar e important for the interpretation of the frequencies of order paramet er collective modes in the superfluid phases. The new acoustic techniq ue permits measurements in regimes of very high attenuation with a sen sitivity in phase velocity of about 10 ppm achieved by a feedback arra ngement. The sound velocity is thus measured continuously throughout t he highly attenuating crossover (omega tau approximate to 1) regime, e ven at the lowest pressures.