Supersolid

A supersolid is a spatially ordered material with superfluid properties. Superfluidity is a special quantum state of matter in which a substance flows with zero viscosity.

Liquid helium-4 was discovered by Pyotr Kapitza, John F. Allen, and Don Misener to exhibit property of superfluidity when it is cooled below a characteristic transition temperature called the lambda point. The superfluid motion of pairs of electrons (Cooper pairs) within a cooled metallic lattice is also the mechanism behind superconductivity. However, before the recent observation of superfluid-like behavior in solid helium-4, superfluidity was considered to be a property exclusive to the fluid state, e.g. superconducting electron and neutron fluids, gases with Bose–Einstein condensates, or unconventional liquids such as helium-4 or helium-3 at sufficiently low temperature.

Superfluidity in helium arises 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 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.

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