Ultrasonic Propagation Near the Critical Point in Helium

Abstract

A logarithmic singularity in the specific heat at constant volume C v at the critical point has recently been reported to occur in argon1 and oxygen.2 Such a result is in direct contradiction to standard theory,3,4 according to which C v must remain finite at the critical point. It can readily be shown5 that these results imply the existence of a corresponding singularity in the adiabatic compressibility κ s , accompanied by a zero in the sound velocity u and large absorption. Sound velocity measurements might thus serve to confirm the reported behavior of C v ; in addition, they have several other advantages. The available resolution (≈0.01%) is much greater than in specific-heat measurements, and each observation is made at a fixed temperature instead of over a temperature interval. If measurements are made along a suitable line in the phase diagram, e.g., an isotherm or an isobar, it is possible to remain in a homogeneous phase at all times, whereas measurements of C v along an isopycnal are necessarily carried out in the mixed phase below T c . In the latter circumstances small departures of ρ from the critical value may lead to large contributions from the latent heat close to T c . We have accordingly undertaken a study of the sound velocity at 1 Mcps in the critical region of helium (T c = 5.1994°K, P c = 1718 mm of mercury). Similar measurements have previously been made in xenon,6 but with insufficient resolution to detect the presence of a singularity in the compressibility.