ULTRASONIC ATTENUATION IN LIQUID HELIUM AT 1 GHz.
CALIFORNIA UNIV LOS ANGELES DEPT OF PHYSICS
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The ultrasonic attenuation in liquid helium at 1 GHz was meaasured from about 1.1 to 4.2K by a method which gives a direct measurement of the absolute attenuation. The results are described in terms of three regions a classical normal fluid region above 2.3K, a critical region near the lambda transition, and a superfluid region below 2.0K. In the normal fluid region, the attenuation is due to classical shear viscosity and thermal conductivity, and is described by the Stokes-Kirchhoff equation. The results give good agreement when compared with the theoretical Stokes-Kirchhoff attenuation curve. In the critical region, the results are shown to differ in a fundamental way from those found at lower frequencies. The Landau-Khalatnikov order parameter relaxation mechanism is shown to contribute insignificantly to the attenuation in this region. The measurements show a very large non-singular peak of about 2000 cm-1 centered at or very nearly at the lambda transition. In the superfluid region, the attenuation measurements show the local maximum at 1.5K which separates the hydrodynamic from the collisionless regime. The results are shown to be in agreement with the elementary excitation relaxation theory of Khalatnikov and Chernikova. Author