superfluid helium-3
/ SOO-per-FLOO-id HEE-lee-um THREE /
Helium comes in two flavors. The common kind, helium-4, has an even, balanced count of inner particles; its rarer sibling helium-3 is missing one neutron, and that single tiny difference completely changes how it behaves when it gets cold. Helium-3 will not become a superfluid at a few degrees the way helium-4 does — it holds out until temperatures more than a thousand times colder.
The reason is that helium-3 atoms are fermions, the antisocial particles that refuse to share a quantum state, so they cannot simply pile in together. Instead they must first find partners and join up in twos. Only once two helium-3 atoms bind loosely into a pair does that pair act like a boson and the liquid become a superfluid. The pairing is delicate and directional, so helium-3 has several distinct superfluid phases with rich internal structure.
Superfluid helium-3, discovered in the early 1970s, matters because its paired-atom mechanism is a near-twin of how electrons pair up in a superconductor, making it a uniquely controllable laboratory for those ideas. A common misconception is that helium-3 and helium-4 are basically the same liquid; the extra step of pairing makes helium-3's superfluidity far more intricate, and it only appears at temperatures within a few thousandths of a degree of absolute zero.
Helium-4 turns superfluid near 2 kelvin; helium-3 must be chilled to roughly 0.002 kelvin — about a thousand times colder — because its atoms must first pair up before any frictionless flow can begin.
Pairing is costly: helium-3 stays normal until almost a thousand times colder than helium-4.
Helium-3 pairs are the direct fluid analogue of the Cooper pairs of electrons that make a metal superconduct, which is why studying helium-3 teaches physicists so much about superconductivity.