Helium (i.e., 4He) was first liquefied by H. K. Onnes in 1908. Its normal boiling point is 4 K, and it is now known that helium will never solidify, even at 0 K, at ambient pressure. Liquid helium undergoes a phase change at 2.2 K, the so-called lambda transition, where it has maximum density and heat capacity. Above the lambda transition, it is called liquid helium I. Below 2.2 K, it is known as liquid helium II and becomes superfluid. At slow flow velocities, helium II acts as if it is a mixture of a superfluid and normal fluid. Superfluid liquid helium exhibits the so-called fountain effect, and can be made to circulate indefinitely around a loop.
3He becomes superfluid at much lower temperatures but, unlike superfluid 4He, behaves as a quantum liquid and has a phase transition which can be described by BCS theory. Superfluid 3He was first produced by D. Osheroff in May 1972 at a temperature of 2.6 mK (although this was recognized as a phase transition, it was not immediately recognized transition to a superfluid phase), for which Osheroff subsequently shared the 1996 Nobel Prize in physics with David Lee and Robert Richardson.
Alpha Particle, Fountain Effect, Pomeranchuk Cooling
References
Dobbs, E. R. Solid Helium Three. Oxford, England: Oxford University Press, 1994.
Donnelly, R. J. "The Discovery of Superfluidity." Phys. Today 48, 30-36, 1995.
Glyde, H. R. Excitations in Liquid and Solid Helium. New York: Oxford University Press, 1995.
Rose-Innes, A. C. Low Temperature Laboratory Techniques: The Use of Liquid Helium in the Laboratory, 2nd ed. London: English Universities Press, 1973.
Weisstein, E. W. "Books about Helium." http://www.ericweisstein.com/encyclopedias/books/Helium.html.
Wilks, J. The Properties of Liquid and Solid Helium. Clarendon, 1966.
