4 K. Enough for to make "low temperature superconductors" like regular metals superconducting, e.g. the superconducting temperature of aluminum if 1.2 K.
Contrast with liquid nitrogen, which is much cheaper but only goes to 77K.
Surprisingly, it can also become a superfluid even though each atom is a fermion! This is because of Cooper pair formation, just like in superconductors, but the transition happens at lower temperatures than superfluid helium-4, which is a boson.
aps.org/publications/apsnews/202110/history.cfm: October 1972: Publication of Discovery of Superfluid Helium-3 contains comments on the seminal paper and a graph which we must steal.
Also sometimes called helium II, in contrast to helium I, which is the non-superfluid liquid helium phase.
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Helium is the chemical element with atomic number 2 and symbol . It is a noble gas. It has a closed-shell electronic structure and is exceptionally unreactive. At ordinary pressure it remains liquid to very low temperatures without freezing; helium-4 also provides the classic example of a superfluid at sufficiently low temperature.[ref]
Helium is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that is the second lightest and second most abundant element in the observable universe, after hydrogen. Its atomic number is 2, and its chemical symbol is He. Helium is classified as a noble gas, which means it has a very low reactivity due to its filled electron shell.