Universality of phonon-roton spectrum in liquids and superfluidity of He II
Based on numerous experimental data on inelastic neutron and X-ray scattering in liquids, we assert that the phonon-roton spectrum of elementary excitations, predicted by Landau for superfluid helium, is a universal property of the liquid state. We show that the existence of the roton minimum in the spectrum of collective excitations is caused by the short-range order in liquids. Using the superfluidity criterion, we assume that one more branch of collective excitations should exist in He II, whose energy spectrum differs from the phonon-roton spectrum.
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