Superfluidity

Superfluidity, state of matter characterized by the complete absence of viscosity, or resistance to flow. The term superfluidity is applied primarily to phenomena observed in liquid helium at very low temperatures, but the term is also sometimes used to refer to the frictionless flow of electrons in certain metals and alloys at very low temperatures. See Absolute Zero; Cryogenics; Helium; Superconductivity.

The phenomenon of superfluidity was discovered in 1937 by the Russian physicist Peter Kapitza and independently in 1938 by the British physicist John Frank Allen and coworkers. They observed that liquid helium (⁴He), when cooled below 2.17 K (-270.98° C, or -455.76° F), could flow with no difficulty through extremely small holes, which liquid helium above that temperature cannot do. They also noticed that on the walls of its container superfluid helium formed a thin film (approximately 100 atoms thick) that flowed against gravity up and over the rim of the container. The temperature of 2.17 K is called the lambda (λ) point because the graph of the specific heat of liquid helium exhibits at that temperature a lambda-shaped maximum.

At normal pressure, helium liquefies at a temperature of 4.2 K. Between this temperature and the lambda point, helium behaves as a normal liquid, and is called helium I. Helium II refers to the liquid state of helium below the lambda point. Besides superfluidity, other unusual phenomena are observed in helium II. Its thermal conductivity is high, some 3 million times higher than that of helium I. Superfluid helium (helium II) spontaneously flows from a cool region to a region of higher temperature; helium I however, flows in the opposite direction. When a flow of superfluid helium is induced, moreover, temperature differences appear spontaneously in the liquid.

Helium II is believed to consist of a mixture of superfluid atoms and normal atoms. The proportion of superfluid atoms increases when the temperature approaches absolute zero. The superfluid atoms are in their lowest energy state (the ground state) and therefore carry no thermal energy. The absence of friction can be explained by the fact that these atoms do not interact with other atoms by momentum interchange, since a certain energy is required to get these superfluid atoms out of the ground state. The absence of thermal energy in the superfluid atoms also explains the unusual thermal properties. The high thermal conductivity of helium II is the result of the flow of normal helium atoms, carrying thermal energy to the lower temperature zone, and the frictionless flow of superfluid atoms to the warmer zone, where they are transformed to normal atoms. The stable isotope helium-3 also exhibits superfluid characteristics, but only at temperatures that are lower than 0.00093 K.