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Superfluids are materials that can flow indefinitely without losing energy, discovered in 1937 with at least two isotopes of helium, one of rubidium, and one of lithium. Only liquids and gases can be superfluids, and they must be very cold. Bosons can easily become a Bose-Einstein condensate, while fermions must first pair with each other to become a superfluid. Superfluids have unique properties, including zero thermodynamic entropy and infinite thermal conductivity. They have limited applications, but are useful in dilution refrigerators and spectroscopy.
A superfluid is a phase of matter capable of flowing indefinitely without loss of energy. This property of some isotopes was discovered by Pyotr Leonidovich Kapitsa, John F. Allen and Don Misener in 1937. It was obtained at very low temperatures with at least two isotopes of helium, one isotope of rubidium and one isotope of lithium.
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 start to overlap and the atoms lose their individual identities, behaving more like a single super-atom than a conglomeration of atoms.
A limiting factor on which materials can exhibit superfluidity and which they cannot is that the material must be very very cold (less than 4K) and remain fluid at this 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—forms in a Bose-Einstein condensate, a superfluid phase of matter. When fermions, atoms with an odd number of nucleons such as the isotope helium-3, are cooled to a few Kelvins, this is not enough to cause this transition.
Since only bosons can easily become a Bose-Einstein condensate, fermions must first pair with each other to become a superfluid. This process is similar to the Cooper pairing of electrons that occurs in superconductors. When two atoms with odd numbers of nucleons pair with each other, they collectively possess an even number of nucleons and start behaving like bosons, condensing together into a superfluid state. This is called a Fermionic Condensate and only emerges at the mK (milliKelvin) temperature level rather than a few Kelvin. The key difference between the coupling of atoms in a superfluid and the coupling of electrons in a superconductor is that the atom coupling is mediated by quantum spin fluctuations rather than phonon exchange (vibratory energy).
Superfluids have some impressive and unique properties that set them apart from other forms of matter. Since they have no internal viscosity, a vortex formed inside 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 any two parts thereof. They can also climb up and out of a container in an atom-thick layer if the container isn’t sealed. A conventional molecule incorporated in a superfluid can move with full 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 aren’t as exciting and wide-ranging as those of superconductors, but dilution refrigerators 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 gross under certain extreme conditions.
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