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 called quantum hydrodynamics.
The superfluid transition is displayed by quantum liquids below a characteristic transition temperature. The phase change to the superfluid state is referred to as the lambda transition, because of the shape of the specific heat curve vs. temperature resembles the greek letter lambda(Λ). 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.
Superfluids, such as supercooled helium-4, exhibit many unusual properties. A superfluid acts as if it is a mixture between a normal component, with all the properties associated with normal fluid, and a superfluid component. The superfluid component has zero viscosity, zero entropy, and infinite thermal conductivity. (It is thus impossible to set up a temperature gradient in a superfluid, much as it is impossible to set up a voltage difference in a superconductor.) One of the most spectacular results of these properties is known as the thermomechanical or fountain effect. If a capillary tube is placed in a bath of superfluid helium, if the tube is heated (even by shining a light on it), the superfluid helium will flow up through the tube and out the top (this is a result of the Clausius-Clapeyron relation). A second unusual effect is that superfluid helium can form a layer, a single atom thick, up the sides of any container it is placed in.
A more fundamental property than the disappearance of viscosity becomes visible if superfluid is placed in a rotating container. Instead of rotating uniformly with the container, the rotating state consists of quantized vorticity.
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.
Also superfluids 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.
- Hagen Kleinert, Gauge Fields in Condensed Matter, Vol. I, "SUPERFLOW AND VORTEX LINES", pp. 1--742, World Scientific (Singapore, 1989); Paperback ISBN 9971502100 (also available online here)
- Superfluid phases of helium