Liquid helium-4 was found by Peter Kapitza, John F. Allen, and Don Misener to exhibit this property when it is cooled below a characteristic transition temperature called the lambda point. Also superfluidity is observed when superconductors are cooled below a critical temperature; however, before the recent observation of supersolid-like behavior in solid helium-4, superfluidity was considered to only be a property of the fluid state, e.g. superconducting electron fluids, gases with Bose-Einstein condensates, or unconventional liquids such as helium-4 or helium-3 at sufficiently low temperatures.
Superfluidity in helium arose from the normal liquid by a second-order phase transition ("lambda transition"). In a dilute gas of Bose particles it comes about by a phase transition that belongs to the universality class of the spherical model. In thin liquid helium films it arises from the normal liquid by a Kosterlitz-Thouless transition. In the case of helium-4, it has been conjectured since 1970 that it might be possible to create a supersolid.
In most theories of this state, it is supposed that vacancies, empty sites normally occupied by particles in an ideal crystal, exist even at absolute zero. These vacancies are caused by zero-point energy, which also causes them to move from site to site as waves. Because vacancies are wellness bosons, if such clouds of vacancies can exist at very low temperature then a Bose-Einstein condensation of vacancies could occur at temperatures less than a few tenths of a kelvin. A coherent flow of vacancies is equivalent to a “superflow” (frictionless flow) of particles in the opposite direction. Despite the presence of the gas of vacancies the ordered structure of a crystal is maintained, although with less than one particle on each lattice site on average.
Mentioned by Sheldon.
For further details and history, view Supersolids in Wikipedia.
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