Anomalous roton dispersion in He II

Analysis of experimental data on the viscosity of He II at saturation vapor pressure reveals that three-particle phonon–roton processes make a considerable contribution (about 25%) to the kinetics of superfluid helium. An investigation of the temperature dependence of the phonon viscous mean free path associated with these processes made it possible to establish the law of roton dispersion in the region of maximum roton velocity. As a result we found that in part of the spectrum the roton velocity exceeds the phonon velocity. This type of anomaly, as shown by Pitaevskii,1 results in the spontaneous emission of a phonon by rotons. A relation is shown to exist between the anomalies in the phonon and roton parts of the spectrum.

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