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