Supersolid
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Image:Supersolid phase.png A supersolid is a spatially ordered superfluid.
When quantum fluids, like helium-4, are cooled below a certain characteristic temperature, they undergo the superfluid transition and enter a state of zero viscosity.
In helium, it has been proven to be possible to create a supersolid, which means a part of the solid shows zero-viscous fluid behavior. This phenomenon was experimentally observed in 2004 due to the work of physicists Moses Chan and Eun-Seong Kim at Pennsylvania State University. The significance of this experiment is that it heralds the observation of a new quantum phase of matter.
In this crystalline state, there are holes in the structure which have physical properties just as much as the atoms in the structure do. These holes are created because, at tenths of a kelvin, any energy in an atom is enough for it to leave the structure in the same way that it would normally in the change from solid to liquid or liquid to gas. These holes in the structure are mobile, even at 0 K.
Physicists think of these gaps as having energy and mass just as atoms do. The theory is not yet clear, though some set of physicists believe that these holes (vacancies) and defects are the main cause for superfluidity in solid helium. Their picture is that the vacancies can move through the solid in synchronicity as if one solid were moving through another. But this theory has so far not made any quantitative predictions to explain the observed superfluid fraction in solid helium.
This would happen because of the peculiar nature of quantum mechanics. Close to absolute zero, quantum theory says that groups of atoms can lose their individual identities and start behaving like a single, giant atom. Instead of dancing around in a gas or a liquid, these atoms can condense into a single quantum state and start moving in perfect lock-step. This happens because their quantum wave function, the region of space in which a particle is found, spreads out and grows larger than the distance between atoms.
Because the vacancies have physical properties, this applies to them too. The effect this has on a solid made up of bosons (no two identical fermions can share the same quantum state) is that once it has been cooled to below 0.4 K, and crystallized, the gaps in the structure could condense into one giant vacancy. This has the implication that just like a ghost passing through a wall, a supersolid made up of vacancies would pass through the rest of the crystal.
This effect has been observed by spinning crystallized helium at 0.2 K and then stopping it—some has been observed to continue moving through the rest of the crystal that had stopped moving.
In short, a supersolid is a vacancy in a solid that has physical properties, allowing it to pass through other solids.