Figure 2: Helium in phase
Helium-4 is the more common of two stable isotopes
of helium, the other being helium-3. At everyday pressures and
temperatures helium exists as a gas that is commonly used to float
balloons due to its lightness. When cooled close to absolute zero,
helium-4 becomes a liquid and at a temperature of 2.17 K turns into a
superfluid that flows without viscosity (due to the fundamental rules of
quantum mechanics, helium-3 atoms, being fermions rather than
bosons like helium-4, become superfluid at the much lower
temperature of 0.0025 K once they have paired up to form bosons).
Unlike any other element, helium does not solidify at absolute zero, but
only does so under pressure. Predicted several decades ago, recent
experiments show that pressurized helium-4 can enter a new state of
matter called a supersolid below a temperature of about 200 mK.