Hypersonic Attenuation in the Vicinity of the Superfluid Transition of Liquid Helium
The attenuation of first sound has been measured in the vicinity of the lambda transition of liquid helium in a number of experiments and at frequencies between 16.8 kHz and 1 GHz. The experiments of Williams and Rudnick 1 have shown that for frequencies between 600 kHz and 3.17 MHz the attenuation maximum occurs at a temperature T p below T λ such that ω|T λ - T p |−1 =const, where ω is the angular frequency. The temperature relative to T λ at which the maximum occurs and the amplitude of the maximum are both proportional to the frequency in the range 16.8 kHz to 3.17 MHz. The data were interpreted by assuming that the attenuation was due to independent mechanisms in addition to the classical absorption due to thermal and viscous losses: (1) a relaxation process occurring only below the lambda point, described by Pokrovskii and Khalatnikov,2 in which the relevant relaxation time is given by ξ/C 2 where ξ is a coherence length and C 2 is the velocity of second sound [near T λ this time is proportional to (T λ — T)−1]; and (2) a critical attenuation which is nonsingular and symmetric about TA due to inherent fluctuations of the order parameter. Other experiments were performed at a frequency of 1 GHz3 over a wide temperature range. In the vicinity of the lambda transition they showed that the attenuation undergoes a very sharp maximum and that the peak is nonsingular and occurs 3–4 m°K below the lambda point. These measurements, however, were performed in an open bath and it was not possible to reach the submillidegree accuracy which is necessary to firmly establish confidence in the location of the peak.
KeywordsLiquid Helium Coherence Length Cadmium Sulfide Ultrasonic Attenuation Viscous Loss
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- 4.R.D. Williams, Doctoral Dissertation, Department of Physics, University of California, Los Angeles, (1970).Google Scholar