The superfluid transition is displayed by quantum liquids 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. See fermion condensate. A unified description of superconductivity and superfluidity is possible in terms of gauge symmetry breaking.
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.
Superfluidity was first discovered in 1937 in the helium-4 isotope by the Russian physicist Pyotr Leonidovich Kapitza (Nobel Prize in physics, 1978) and it is considered one of the most remarkable breakthroughs of low-temperature physics.
The phase change to the superfluid state in helium-4 is referred to as the lambda transition, because the shape of the specific heat curve vs. temperature resembles the Greek letter lambda(λ).
Superfluids are also used in high precision devices such as gyroscopes, which allow the measurement of some theoretically predicted gravitational effects, for example see Gravity Probe B article.
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