Editing Superfluid helium-4 (section)

=== Vortex ring model ===
Landau thought that vorticity entered superfluid helium-4 by vortex sheets, but such sheets have since been shown to be unstable.
[[Lars Onsager]] and, later independently, Feynman showed that vorticity enters by quantized vortex lines. They also developed the idea of [[quantum vortex]] rings.
[[Arie Bijl]] in the 1940s,<ref>{{Cite journal
| last = Bijl
| first = A
|author2=de Boer, J
|author3=Michels, A
 | title = Properties of liquid helium II
| journal = Physica
| volume = 8
| issue = 7
| pages = 655–675
|year = 1941
| doi = 10.1016/S0031-8914(41)90422-6
| bibcode=1941Phy.....8..655B}}</ref>
and [[Richard Feynman]] around 1955,<ref>{{Cite book
| editor = Braun, L. M.
| title = Selected papers of Richard Feynman with commentary
| publisher = World Scientific
| series = World Scientific Series in 20th century Physics
| volume = 27
| date = 2000
| isbn = 978-9810241315 }}
[https://books.google.com/books?id=qnwkqcVixucC&q=feynman Section IV (pages 313 to 414)] deals with liquid helium.</ref> developed microscopic theories for the roton, which was shortly observed with inelastic neutron experiments by Palevsky. Later on, Feynman admitted that his model gives only qualitative agreement with experiment.<ref>{{cite journal|author=R. P. Feynman|journal= Phys. Rev.|volume= 94|page= 262 |year=1954|doi=10.1103/PhysRev.94.262|title=Atomic Theory of the Two-Fluid Model of Liquid Helium|issue=2|bibcode = 1954PhRv...94..262F |url=http://authors.library.caltech.edu/3539/1/FEYpr54a.pdf}}</ref><ref>{{cite journal|author=R. P. Feynman|author2=M. Cohen|name-list-style=amp|journal= Phys. Rev. |volume=102|pages= 1189–1204 |year=1956|doi=10.1103/PhysRev.102.1189|title=Energy Spectrum of the Excitations in Liquid Helium|issue=5|bibcode = 1956PhRv..102.1189F |url=https://authors.library.caltech.edu/3549/1/FEYpr56.pdf}}</ref>