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Superfluidity is a state of matter characterised by the complete absence of viscosity. Thus superfluids, placed in a closed loop, can flow endlessly without friction.

The superfluid transition is displayed by quantum liquids[?] below a characteristic transition temperature. The most abundant isotope of Helium, 4He, becomes superfluid at temperatures below 2.17K (-270.98°C). The less abundant isotope, 3He, becomes superfluid at a much lower temperature: 2.6mK (only a few thousands of a degree above the absolute zero, that is -273.15°C).

Although the phenomenology of superfluidity in these two systems is very similar, the nature of the two superfluid transitions is very different. 4He atoms are bosons, and their superfluidity can be understood in terms of the Bose statistics[?] that they obey. Specifically, the superfluidity of 4He can be regarded as the generalisation of Bose-Einstein condensation (which takes place only in a non-interacting gas) to interacting systems. On the other hand, 3He 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[?].

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