Superfluid

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[?].

External link

• http://www.aip.org/physnews/graphics/html/helium3.htm