Comment on anomalous dispersion and scattering rates for multiphonon spontaneous decay in He II

Abstract

We report on new measurements of the spontaneous decay threshold energy E_c for high-frequency phonon propagation in He II at saturated vapor pressure at T=0.1 K. Superconducting tin tunnel generators and aluminum tunnel detectors were used in this study. The measurements show that the mean free path becomes much larger than the propagation length of 1.1 mm for a value of E_c =9.8±0.15 K. This agrees with the value originally reported (E_c =9.5±0.4 K) by Dynes and Narayanamurti using aluminum tunnel generators, but is shown to correspond to the point where the phase velocity equals the sound velocity, when the phonons become stable, as first proposed by Pitayevski and Levinson. Evidence for n-phonon decay at energies lower than E_c is presented for n≥2 with a short mean free path (<0.3 mm) at the two-phonon decay energy. The measured values of the dispersion parameters are shown to agree closely with the spline fit to neutron data due to donnelly, donnelly, and Hills.