Helium-4 and bosons

1956 Energy Spectrum of the Excitations in Liquid Helium Richard Feynman and Michael Cohen

The roton state represented by the function (21) can be described roughly classically as a vortex ring of such small. radius that only one atom can pass through the center. Outside the ring there is a slow drift of atoms returning for another passage through the ring. There are at least three ways that the classical picture is modified.