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An analysis of experimental data shows that, in addition to phonon--rotonexcitations in superfluid helium, there necessarily exist at least one branchof elementary excitations whose energy spectrum strongly depends ontemperature. On this basis, the Landau superfluidity criterion is generalizedfor several branches of elementary excitations, taking into account that thecritical velocity should vanish during the phase transition of liquid heliumfrom the superfluid state to the normal state. We suppose that a new spectrumof excitations with a gap, depending on interparticle interaction, correspondsto the transition of helium to superfluid state. This gap exists only insuperfluid state and disappears at the transition temperature to normal state.The phonon-roton branch of excitations has no crucial influence onsuperfluidity. Rotons, as well as phonons, are not the specific excitations forthe superfluid helium, but exist as universal excitations in liquid state, andform the continuous branch of excitations in different liquids. This point ofview is confirmed experimentally and numerically.

Universality of rotons in liquids, generalized superfluidity criterion and helons in helium II