Editing Superfluid helium-4 (section)

=== Hard-sphere models ===
The models are based on the simplified form of the inter-particle potential between helium-4 atoms in the superfluid phase. Namely, the potential is assumed to be of the hard-sphere type.<ref>{{cite journal|author=T. D. Lee|author2= K. Huang|author3=C. N. Yang|name-list-style=amp|journal= Phys. Rev. |volume=106|pages= 1135–1145 |year=1957|doi=10.1103/PhysRev.106.1135|title=Eigenvalues and Eigenfunctions of a Bose System of Hard Spheres and Its Low-Temperature Properties|issue=6|bibcode = 1957PhRv..106.1135L }}</ref><ref>{{cite journal|author=L. Liu|author2=L. S. Liu|author3=K. W. Wong|name-list-style=amp|journal= Phys. Rev. |volume=135|pages= A1166–A1172 |year=1964|doi=10.1103/PhysRev.135.A1166|title=Hard-Sphere Approach to the Excitation Spectrum in Liquid Helium II|issue=5A|bibcode = 1964PhRv..135.1166L }}</ref><ref>{{cite journal|author=A. P. Ivashin|author2=Y. M. Poluektov|name-list-style=amp|journal= Cent. Eur. J. Phys. |volume= 9|pages= 857–864 |year=2011|doi=10.2478/s11534-010-0124-7|title=Short-wave excitations in non-local Gross-Pitaevskii model|issue=3|bibcode = 2011CEJPh...9..857I |arxiv=1004.0442|s2cid=118633189}}</ref>
In these models the famous Landau (roton) spectrum of excitations is qualitatively reproduced.