Ultrasonic Attenuation in Liquid Hellium above 1°K at 1 GHz

Abstract

The attenuation of first sound in liquid helium in the temperature range from about 1°–4°K was measured with a two crystal interferometer with a variable path length. Because of the short acoustic wavelength of about 2000 Å and the large attenuation, some details of the interferometer system are unusual and will be discussed. The ultrasonic attenuation measurements in this temperature range can be divided into several regions. There is a classical region above the superfluid transition with ordinary viscous and thermal attenuation processes. There is a region near the lambda transition in which our measurements reveal an anomalously high attenuation peak located at, or very nearly at, the lambda point. This peak is not due to the Landau-Khalatnikov mechanism, which produces a large peak in this region at lower frequencies, but is postulated to be due to critical point fluctuations. There is a region below the lambda transition in which roton and phonon interactions control the attenuation process. Our measurements show the local maximum which separates the hydrodynamic régime from the collisionless régime and agree with the theory of Khalatnikov and Chernikova.