Investigation of the excitation spectrum of liquid 4He by inelastic neutron scattering technique

It is found that the one-particle excitation spectrum of liquid 4He has a complex multicomponent structure. Two components of the peak of slow neutron scattering are observed in the phonon part of the normal phase of liquid helium: the narrow component (s) and the wide component (w). The maxon-roton spectral region contains only the wide component w. In the superfluid phase of liquid helium, one more narrow scattering component (n) is added to the maxon-roton part of the spectrum, the intensity of this component increasing sharply upon a decrease in temperature. On the other hand, the intensity of the wide component drops rapidly. Therefore, the structure of the excitation spectrum undergoes significant changes upon a transition from the normal to the superfluid phase, as well as in both phases upon a transition from phonon to maxons. The obtained results are in qualitative agreement with the model of excitation spectrum of liquid 4He, which has been proposed in recent years by Griffin, Glyde, and Stirling. However, some of the details of the observed pattern cannot be explained by using prevailing concepts.

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