Ultrasonic Absorption in Liquid Helium at Temperatures below 0.6°K

Abstract

An explanation is given for the sound absorption observed in liquid He ii in the phonon region below 0.6°K, arising from an interaction between acoustic phonons and thermal phonons. This interaction occurs between the acoustic wave and only those thermal phonons which propagate parallel to the wave. There is a "bunching" of thermal phonons within certain regions of the acoustic wave, analogous to the bunching of electrons in a traveling wave tube. The sound absorption calculated varies with temperature, frequency, and pressure in agreement with all available experimental data and is of the right order of magnitude. When the mean free path of the thermal phonons is reduced, for instance, by adding ${\mathrm{He}}^3$, the absorption is expected to decrease as found experimentally by Harding and Wilks.