Superfluidity

(cryogenics) The frictionless flow of liquid helium at temperatures very close to absolute zero through holes as small as 10^-7 centimeter in diameter, and for particle velocities below a few centimeters per second.

The frictionless flow of liquid helium at low temperature; also, the flow of electric current without resistance in certain solids at low temperature (superconductivity).

Both helium isotopes have a superfluid transition, but the detailed properties of their superfluid states differ considerably because they obey different statistics. ⁴He, with an intrinsic spin of 0, is subject to Bose-Einstein statistics, and ³He, with a spin of ½, to Fermi-Dirac statistics. There are two distinct superfluid states in ³He called A and B.

The term “superfluidity” usually implies He II or the A and B phases of ³He, but the basic similarity between these and the “fluid” consisting of pairs of electrons in superconductors is sufficiently strong to designate the latter as a charged superfluid. Besides flow without resistance, superfluid helium and superconducting electrons display quantized circulating flow patterns in the form of microscopic vortices. See also Bose-Einstein statistics; Liquid helium; Quantized vortices; Second sound; Superconductivity.