First- and zero-sound velocity and fermi liquid parameter F 2 s in liquid3He determined by a path length modulation technique

We have measured the velocity of first- and zero-sound in liquid³He at 12.6 MHz over the pressure range of 0.6 to 14.5 bar using a path length modulation technique that we have recently developed. From these measurements, the pressure dependent value of the Fermi liquid parameter F₂^s was calculated and found to be larger at low pressure than previously reported. These new values of F₂^s indicate that transverse zero-sound is a propagating mode at all pressures. The new values are important for the interpretation of the frequencies of order parameter collective modes in the superfluid phases. The new acoustic technique permits measurements in regimes of very high attenuation with a sensitivity in phase velocity of about 10 ppm achieved by a feedback arrangement. The sound velocity is thus measured continuously throughout the highly attenuating crossover ( 1) regime, even at the lowest pressures.