What is supercooled liquid helium

Superfluid helium is what you get when you cool helium to near absolute zero under normal atmospheric pressure (1 atm). From absolute zero up, the phases of helium at normal atmospheric pressure are superfluid, liquid, gas; higher pressure than atmospheric is required to produce solid helium. Superfluid is a state of matter that can only be understood using quantum mechanics. It is similar to a liquid, but different in certain ways: in particular, zero viscosity and infinite thermal conductivity. It is related to a Bose-Einstein Condensate. The term superfluid helium usually refers to superfluid ^4Helium, helium-4 with a nucleus containing two protons and two neutrons and overall quantum mechanical spin zero. Superfluid ^4He exists below about 4 degrees Kelvin (at 1 atm). The isotope ^3He, helium with a nucleus with only one neutron, exists as a superfluid at lower temperatures, below one kelvin, and different mathematics is required to describe it; the differences follow from the fact that the ^3He atom has quantum mechanical spin 1/2.