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. 
V.B.Bobrov, S.A.Trigger, D.I. Litinski
cond-mat.other cond-mat.stat-mech