Superfluidity is a phase of matter characterised by the complete absence of viscosity. Thus superfluids, placed in a closed loop, can flow endlessly without friction. Superfluidity was discovered by Pyotr Leonidovich Kapitsa, John F. Allen, and Don Misener in 1937. The study of superfluidity is quantum hydrodynamics.
The superfluid transition is displayed by quantum liquidss below a characteristic transition temperature. Helium-4, the most abundant isotope of helium, becomes superfluid at temperatures below 2.17 K (−270.98 °C). The less abundant isotope helium-3 becomes superfluid at a much lower temperature of 2.6 mK, only a few thousandths of a kelvin above absolute zero.
Although the phenomenology of superfluidity in these two systems is very similar, the nature of the two superfluid transitions is very different. Helium-4 atoms are bosons, and their superfluidity can be understood in terms of the Bose statistics that they obey. Specifically, the superfluidity of helium-4 can be regarded as a consequence of Bose-Einstein condensation in an interacting system. On the other hand, helium-3 atoms are fermions, and the superfluid transition in this system is described by a generalisation of the BCS theory of superconductivity. In it, Cooper pairing takes place between atoms rather than electrons, and the attractive interaction between them is mediated by spin fluctuations rather than phonons. A unified description of superconductivity and superfluidity is possible in terms of gauge symmetry breaking.
One important application of superfluidity is in dilution refrigerators.
Recently in the field of chemistry, superfluid helium-4 has been successfully used in spectroscopic techniques, as a quantum solvent. Referred to as Superfluid Helium Droplet Spectroscopy (SHeDS), it is of great interest in studies of gas molecules, as a single molecule solvated in a superfluid medium allows a molecule to have effective rotational freedom - allowing it to behave exactly as it would in the gas phase.
