The acoustic impedance of liquid helium-three

Abstract

Measurements of the transverse acoustic resistance of liquid helium-3 have been made at pressures between 0.3 bar and 28.0 bar in the temperature range 0.015 Kelvin to 1.0 Kelvin using a helium-3/helium-4 dilution refrigerator. The method involved observation of the decay of a series of echos generated in a piezoelectric rod immersed in the liquid, one end of the rod being excited by a resonant cavity. The frequency of this cavity was 242 MHz in the greater proportion of this work but similar measurements were also made at 1048 MHz. The measurements at the lower frequency have confirmed the existence of the collective oscillation known as transverse zero sound in liquid helium-3 at higher pressures, as predicted by the Landau Theory of a Fermi liquid. This has enabled an estimate to be made of the magnitude of the symmetrical Landau parameter, F₂S. The result obtained was in agreement with those of other experimenters who have used a variety of methods to determine F₂S.

A similar experimental technique has been used to study the longitudinal acoustic resistance of liquid helium-3 in order to investigate the anomalously high value of F₂S indicated by previous measurements of this phenomenon. These measurements, both transverse and longitudinal, have mainly been carried out on pure helium-3 (ie. better than 99.9997 per cent) but some data have been obtained for the transverse acoustic resistance of helium-3 in which a small amount of helium-4 (about 1%) was present. These results enabled the determination of the acoustic resistance of an assembly of non-interacting Fermions.