in 1941. [L. D. Landau, J. Phys. USSR 5, 71 (1941), "The Theory Of Superfluidity In Helium II".] Landau received the Nobel Prize for this work in 1962.

In liquid helium-4, the superfluidity occurs at far higher temperatures than it does in helium-3. Each atom of helium-4 is a boson particle, by virtue of its integer spin. A helium-3 atom is a fermion particle; it can form bosons only by pairing with