When liquid helium-4 is cooled to a temperature close to absolute zero, it acquires an unusual set of properties known as superfluidity, and the material is said to be in a superfluid state. The superfluid flows without friction, and its viscosity is zero. Recently, researchers have developed several applications for superfluids. For instance, they have been used as specialized solvents (quantum solvents) in spectroscopy, as agents to trap and dramatically reduce the speed of light, and as materials needed in high-precision gyroscopes.
Superfluidity was discovered by Pyotr Leonidovich Kapitsa, John F. Allen, and Don Misener in 1937. It is a major facet in the study of quantum hydrodynamics.
Below its boiling point of 4.21 K and above a temperature of 2.1768 K (called the "lambda point" for helium), the helium-4 isotope behaves as a normal, colorless liquid and is called helium I. When cooled below the lambda point, part of it enters a state called helium II, which is a superfluid. Upon further cooling, increasing amounts of the helium are converted to the superfluid state.
Thus the behavior of helium below its lambda point is explained in terms of a mixture of a normal component, with properties characteristic of a normal liquid, and a superfluid component. The superfluid component flows without friction. It has zero viscosity, zero entropy, and "infinite" thermal conductivity. It is thus impossible to set up a temperature gradient in a superfluid, just as it is impossible to set up a voltage difference in a superconductor.
Superfluid helium also exhibits a "creeping" effect—some of it creeps up the sides of the container in which it is placed, rising against the force of gravity and forming a film (called a "Rollin film"). If the vessel is not sealed, it evaporates and escapes from the opening.
One of the most spectacular results of these properties is known as the thermomechanical effect or "fountain effect." If a capillary tube is placed in a bath of superfluid helium and then heated, even by shining a light on it, the superfluid helium will flow up through the tube and out the top.
The thermal conductivity of helium II is greater than that of any other known substance, a million times that of helium I and several hundred times that of copper. This is because heat conduction occurs by an exceptional quantum-mechanical mechanism. When heat is introduced, it moves through helium II in the form of waves, at 20 meters per second at 1.8 K, in a phenomenon called second sound.
A more fundamental property than the disappearance of viscosity becomes apparent if the superfluid is placed in a rotating container. If the container is rotated below a certain velocity (called the first critical velocity), the liquid remains perfectly stationary. Once the first critical velocity is reached, the superfluid quickly begins spinning at what is called the "critical speed." The speed is quantized—that is, it can spin only at certain speeds.
The isotope helium-3 also has a superfluid phase, but only at much lower temperatures. As a result, less is known about such properties of helium-3.
Although the phenomenologies of the superfluid states of helium-4 and helium-3 are very similar, the microscopic details of the transitions are very different. Helium-4 atoms are bosons, and their superfluidity 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 generalization of the "BCS theory" of superconductivity.
Recently, some physicists have been able to create a Fermionic condensate from pairs of ultra-cold fermionic atoms. Under certain conditions, fermion pairs form diatomic molecules and undergo Bose–Einstein condensation. At the other limit, the fermions (most notably superconducting electrons) form Cooper pairs that also exhibit superfluidity. This recent work with ultra-cold atomic gases has allowed scientists to study the region in between these two extremes, known as the BEC-BCS crossover.
Additionally, physicists at Penn State University might have discovered supersolids, in 2004. When helium-4 is cooled below about 200 mK under high pressures, a fraction (about one percent) of the solid appears to become a superfluid.
All links retrieved January 7, 2020.
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