Superfluidity
Helium is the 2nd lightest and 2nd most abundant element, right behind hydrogen. The two could not be more different. Helium’s stability is the opposite of hydrogen’s volatility.
Helium is a gas except under extreme conditions. Helium has the lowest boiling and melting points. Helium becomes a liquid at 4.2 K. Below 4 K, helium fiercely boils.
Below 2.172 K, the boiling stops; helium becomes a superfluid. A superfluid exhibits zero viscosity and zero entropy. Superfluid helium flows without friction; through tiny holes as small as a molecule, up and out of a tube, over the edge of a cup.
More particularly, helium-4, the ubiquitous isotope comprising 2 neutrons with 2 protons, becomes a superfluid more readily than helium-3, which is a rare isotope, with only 1 neutron per helium atom. Helium-3 only becomes a superfluid when chilled to 2 millikelvins.
The reason is that helium-4 atoms are bosons, whereas helium-3 atoms are fermions. Hence helium-4 corresponds to the characteristics of Bose-Einstein condensation, whereas helium-3 becomes a fermionic condensate.
A fermionic condensate interacts by Cooper pairing between atoms, as contrasted to the electron Cooper pairs that facilitate superconductivity.
Helium-4 is bosonic via the subatomic components (protons, neutrons, and electrons) canceling each other’s complementary spins, resulting in zero spin for the helium-4 atom as a whole. This facilitates the stability of helium-4.
Only neon is less reactive than helium, and it needs a cluster of buddies to be so. Neon is most commonly molecular as 20Ne, whereas helium is monoatomic (1He).
Helium is not the only element capable of superfluidity. Rubidium, a highly reactive silvery-white metal, becomes a superfluid at 500 nanokelvins (500 10–9 Kelvin).
