Only liquids and gases can be superfluids.
A superfluid is a phase of matter capable of flowing indefinitely without loss of energy. This property of certain isotopes was discovered by Pyotr Leonidovich Kapitsa, John F. Allen and Don Misener in 1937. It was achieved at very low temperatures with at least two helium isotopes, a rubidium isotope and a lithium isotope.
Only liquids and gases can be superfluids. For example, the freezing point of helium is 1 K ( Kelvin ) and 25 atmospheres of pressure, the lowest of any element, but the substance begins to exhibit superfluid properties at about 2 K. The phase transition occurs when all the constituent atoms of a sample begin to occupy the same quantum state. This happens when atoms are placed very close together and cooled so much that their quantum wave functions begin to overlap and the atoms lose their individual identities, behaving more like a single superatom than a cluster of atoms.
One limiting factor in which materials can exhibit superfluidity and which cannot is that the material must be very, very cold (less than 4 K) and remain fluid at that cold temperature. Materials that become solid at low temperatures cannot assume this phase. When cooled to very low temperatures, a set of superfluid-ready bosons, atoms with an even number of nucleons, form 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 to a few Kelvin, this is not enough to cause this transition.
Since only bosons can readily become a Bose-Einstein condensate, fermions must first pair up to become a superfluid. This process is similar to the Cooper electron pairing that occurs in superconductors. When two atoms with odd numbers of nucleons pair up, they collectively have an even number of nucleons and begin to behave like bosons, condensing together into a superfluid state. This is called a fermion condensate, and it arises only at the mK (milliKelvin) temperature level, not a few Kelvins. The main difference between pairing atoms in a superfluid and pairing electrons in a superconductor is that atomic pairing is mediated by quantum spin fluctuations rather than phonon exchange (vibrating energy).
Superfluids have some impressive and unique properties that set them apart from other forms of matter. Because they have no internal viscosity, a vortex formed within one persists forever. A superfluid has zero thermodynamic entropy and infinite thermal conductivity, which means that no temperature differential can exist between two superfluids or two parts thereof. They can also climb up and out of a container in an atom-thick layer if the container is not sealed. A conventional molecule embedded in a superfluid can move with complete rotational freedom, behaving like a gas. Other interesting properties may be discovered in the future.
Most so-called superfluids are not pure, but are actually a mixture of a fluid component and a superfluid component. The potential applications of superfluids are not as exciting and far-reaching as those of superconductors, but dilution coolers and spectroscopy are two areas where they have found use. Perhaps the most interesting application today is purely educational, showing how quantum effects can become macroscopic in scale under certain extreme conditions.