Superfluidity is the frictionless flow and other exotic behavior observed in liquid helium at temperatures near absolute zero. The unusual behavior arises from quantum mechanical effects.
What are these "quantum mechanical effects"?
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Superfluidity phase transition occurs when all the constituent atoms of a sample begin to occupy the same quantum state. This happens when the atoms are placed very close together and cooled down so much that their quantum wave functions begin to overlap and the atoms lose their individual identities, behaving more like a single super-atom than an agglomeration of atoms.
When (helium-4) cooled to very low temperatures, a superfluid-ready set of bosons, atoms with an even number of nucleons, forms into a Bose-Einstein condensate, a superfluid phase of matter. When fermions, atoms with an odd number of nucleons such as the helium-3 isotope, are cooled down to a few Kelvin, this is not sufficient to cause this transition.