The dynamics of superfluid He4 at and above the Landau quasiparticle regime is investigated by high-precision inelastic neutron scattering measurements of the dynamic structure factor. A highly structured response is observed above the familiar phonon-maxon-roton spectrum, characterized by sharp thresholds for phonon-phonon, maxon-roton, and roton-roton coupling processes. The experimental dynamic structure factor is compared to the calculation of the same physical quantity by a dynamic many-body theory including three-phonon processes self-consistently. The theory is found to provide a quantitative description of the dynamics of the correlated bosons for energies up to about three times that of the Landau quasiparticles.
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We are grateful to X. Tonon for his help with the experiment, and to E. Farhi for his help with the program McStas. This work was supported, in part, by the Austrian Science Fund FWF grant I602, the French grant ANR-2010-INTB-403-01 and the European Community Research Infrastructures under the FP7 Capacities Specific Programme, MICROKELVIN Project No. 228464.
©2016 American Physical Society.