## 1956 [Energy Spectrum of the Excitations in Liquid Helium](https://knowen-production.s3.amazonaws.com/uploads/attachment/file/5578/feynman%2Bhelium.pdf) 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. 

- [A CLASSICAL MODEL OF A ROTON](https://www.pnas.org/content/pnas/39/5/443.full.pdf) BY C. L. PEKERIS


https://www.washington.edu/news/2014/02/03/solving-a-physics-mystery-those-solitons-are-really-vortex-rings/

## [Microscopic dynamics of superfluid 4 He: a comprehensive study by inelastic neutron scattering](https://hal.archives-ouvertes.fr/hal-01810737/document)

- [Marques - microscopic model](https://knowen-production.s3.amazonaws.com/uploads/attachment/file/5577/he-2%2Bmicro.pdf)


## https://en.wikipedia.org/wiki/Superfluid_helium-4

## https://en.wikipedia.org/wiki/Lambda_point

[UITRASONIC VELOCITY AND ABSORPTION IN LIQUID HELIUM](https://pdfs.semanticscholar.org/24b9/69e1be4583669739a354f4e313559d883c06.pdf)
By John R. Pellam and Charles . Squire