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

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 $\ensuremath{\epsilon}(p)=cp[1\ensuremath{-}\ensuremath{\gamma}{p}^{2}\ensuremath{-}\ensuremath{\delta}{p}^{4}\ensuremath{\cdots}]$ with $\ensuremath{\gamma}$ negative, we are able to reconcile previous disagreements between theory and experiment.

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