Frequency and Temperature Dependence of Sound Velocity in Liquid Helium-4: A Test of the Phonon Dispersion Relation

Abstract

Changes $\frac{\ensuremath{\Delta}C}{C}$ in sound velocity $C$ in liquid helium-4 over the frequency range 1 to 15 MHz were studied as a function of temperature for $0.45>~T>~0.1$ K. The experimental results are in qualitative agreement with predictions of positive phonon dispersion for small momentum values.