ELEMENTARY EXCITATIONS AND A COLLECTIVE MODE IN HCP HE-4

We have measured the velocity and attenuation of sound at 10, 30, and 50 MHz in solid4He at higher purity, lower density, and lower acoustic power than have been measured previously. At 10 MHz the velocity varies as T4, as would be expected for a very high quality crystal with remarkably low dislocation density. The temperature dependence of the attenuation reveals coupling to thermally activated excitations that are consistent with vacancies measured in other work. However, in our work the activation energy is ∼0.7 K so that their concentration is sufficiently high to require Bose statistics. At 30 and 50 MHz, an unexpected, additional resonant behavior was observed. We show that the resonance is consistent with the existence of a collective mode of the vacancies and with a finite population of ground state vacancies. © 1990 Plenum Publishing Corporation.