Phonon Dispersion and the Propagation of Sound in Liquid Helium-4 below 0.6°K

Abstract

Recent experiments on the attenuation and velocity of sound in liquid ${\mathrm{He}}^4$ at low temperatures are discussed in terms of the excitation model of liquid ${\mathrm{He}}^4$. By assuming for the long-wavelength excitations the dispersion relation $\epsilon \mathrm{}\left(p\right)=cp[1-\gamma p^2-\delta p^4\cdots ]$ with $\gamma$ negative, we are able to reconcile previous disagreements between theory and experiment.