Ultrasonic Attenuation in LiquidHe4under Pressure

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Abstract
In an attempt to help resolve previously observed discrepancies between the theory for ultrasonic attenuation and the data at the vapor pressure, we have extended our measurements to higher pressures. We report data on the temperature dependence of the attenuation from 0.1 to 1.0 K at frequencies of 15, 45, 105, and 256 MHz for pressures between 0 and 24.7 atm. At low pressures or temperatures, the data resemble the theoretical predictions, but the measured attenuation is about twice that predicted by most theories. At higher pressures and for temperatures above about 0.3 K, the attenuation shows a sharp reduction below the value extrapolated from low temperatures according to previous theories. It has recently been suggested that this unexpected behavior is due to a restriction on the number of thermal phonons that are able to play a part in the attenuation process at higher temperatures and pressures.