By measuring the pressure dependence of the velocity of sound, we have determined both the pressure dependence of the density and the Grüneisen constant u of liquid He4. Measurements were made below 0.1 K and in the vicinity of 0.5 K. Our determinations of the pressure dependence of the density agree quite well with that determined by Boghosian and Meyer, who used a capacitance bridge. Since the latter results rely on the validity of the Clausius-Mossotti relation and a pressure-independent electric polarizability, the present work can be interpreted as supporting both of these assumptions. We found that u(ρ0)â‰¡(ρc)dcdρ=2.84 under the vapor pressure at 0.1 K. Using this value of u to calculate the attenuation of sound according to a three-phonon mechanism, we obtain an attenuation of less than half the measured value. Thus, the present theory of sound attenuation must be incomplete.
ASJC Scopus subject areas
- Atomic and Molecular Physics, and Optics