Dispersion and Attenuation of Superthermal "Monochromatic" Phonons in He II

The propagation of phonons in He II at 0.1 K, in the frequency range (2 to 9)×${10}^{10}$ Hz, has been studied using superconducting Al-film generators. Such superthermal waves ($\hslash \omega \gg \mathrm{kT}$) are shown to be long-lived propagating excitations in the liquid and to have negligible dispersion (group velocity varies by less than 0.5%). These measurements reveal that the current theories of phonon lifetimes in He II are inadequate.