Sound Absorption in Liquid Helium Below 0.6 K

Abstract

The temperature dependence of ultrasonic attenuation in superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ below 0.6 K is studied. It is shown that vertex corrections in the phonon self-energy improve the agreement with experiment, especially near the transition between the collisionless and collision-dominated regions. The introduction of one average lifetime for the relevant thermal phonons is critically examined.