Phonon and Roton Propagation in He II under Pressure

We have studied the transition from second sound to ballistic phonon flow in He II up to pressures of 24 bar. At saturated vapor pressure the phonon-phonon scattering rate $\tau _{\mathrm{pp}}^{}{}_{}{}^{-1\mathrm{}}\cong 1.7\times {10}^6{T}^3$ ${\sec }^{-1\mathrm{}}$ for $T<\mathrm{}0.6\mathrm{}$ K. At 24 bar, $\tau _{\mathrm{pp}}^{}{}_{}{}^{-1\mathrm{}}<8\mathrm{}\times {10}^4$ ${\sec }^{-1\mathrm{}}$ for $T<\mathrm{}0.7\mathrm{}$ K. The phonon-roton scattering rate $\tau _{\mathrm{pr}}^{}{}_{}{}^{-1\mathrm{}}$ is ∼ ${10}^6$ ${\sec }^{-1\mathrm{}}$ at 0.75 K and almost pressure independent. At 24 bar between 0.4 and 0.7 K, in addition to ballistic phonons, we observe a second pulse traveling at 16 m/sec, which we believe is the propagation of interacting rotons.