Helium has two isotopes: helium-4, a boson; and helium-3, a fermion. Superfluidity is a form of Bose-Einstein condensation, which means it is only accessible to bosons. Yet, in 1971, Osheroff, Richardson and Lee observed superfluidity in helium-3, at only 0.002 K from absolute zero! They won the Nobel Prize in 1996 thanks to this discovery.
Helium-3 presents two types of superfluidity depending on its pressure and its temperature. In a superconductor, electrons (which are fermions) form Cooper pairs in order to form bosons, which enables condensation. For helium-3, the situation is similar: the helium-3 atoms (which are fermions) pair up and form bosons. However, in that case, it is not the atom vibrations that are responsible for the formation of pairs, but rather the fact that the atom magnetizations become parallel to one another.
The total magnetization of a pair of helium-3 atoms taking part in superfluidity is in that case not equal to zero, contrary to the magnetization of a Cooper pair in the BCS theory. Physicists call this type of superfluidity “triplet superfluidity”, and it is very different from helium-4 superfluidity, especially as far as “gap” symmetries are concerned. Antony Legget, one of the physicists who understood this helium-3 superfluidity, won the Nobel Prize for his works in 2003. This situation can be found in some exotic superconductors where Cooper pairs also form “triplet pairs”, with a magnetization not equal to zero.
