Observation of a High-Frequency Cutoff for Phonon Propagation in Liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$

A new high-frequency cutoff for phonon propagation which lies beyond the well-known low-frequency cutoff due to anomalous dispersion is observed in phonon propagation experiments in liquid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ using superconducting Sn and A1 tunnel-junction generators and detectors. The cutoff depends strongly on pressure and is interpreted as arising from a strong increase in phase space for phonon-roton scattering as normal dispersion in the phonon branch sets in.