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Frozen helium has been turned into a solid that behaves like a superfluid.
Particles march along coherently in superfluids and supersolids.
Researchers in the United States think they have found a new state of matter - a supersolid.
If their findings are correct, this state is deeply bizarre. Although it is a crystalline solid, it can 'flow' like the
most slippery liquid imaginable - in fact, like a liquid with no viscosity.
Liquids that can do this have been known about for years. They are called superfluids, and because they have no viscosity
they behave in strange ways. Once stirred, a superfluid will keep swirling forever, whereas a vortex stirred into a
normal liquid will eventually exhaust itself and disappear. A superfluid will even creep up the side and out the top of a
The first two superfluids made by researchers were created using helium-4 and helium-3. Both need to be cooled close to
absolute zero (-273 °C) before their superfluid behaviour appears.
Eun-Seong Kim and Moses Chan of Pennsylvania State University now claim to have turned frozen helium-4 into a
supersolid1. They did this by filling the narrow channels of a porous form of glass (called Vycor) with helium, and
freezing it by cooling it down and squeezing it to more than 60 times atmospheric pressure. A disk of the helium-filled
glass was then set spinning. At about 0.175 °C above absolute zero, the disk suddenly started to rotate more easily -
precisely what would be expected if the helium became a supersolid.
The same thing happens when liquid helium as a superfluid is used in the Vycor pores, because the non-viscous liquid
spins more readily.
Kim and Chan say it is hard to explain their findings without invoking a supersolid state. But John Beamish of the
University of Alberta in Edmonton, Canada, says their claim "is sure to generate some controversy". Some researchers
might wonder, for example, whether some liquid helium might still be coating the Vycor pore walls and become a
superfluid, causing the disk to spin more freely. But Kim and Chan insist that this is unlikely.
Solid helium itself is a fairly weird substance. At very low temperatures, the behaviour of helium atoms is dictated by
quantum mechanics, and this prevents helium from freezing at all, no matter how cold it is, unless it is pressurized to
at least 25 atmospheres. The 'quantum solid' that results is very loose - unlike most frozen solids it can be squeezed
like rubber, and the regular crystal lattice of atoms is full of gaps, called vacancies, that move about.
When liquid helium becomes a superfluid, the laws of quantum mechanics make all the atoms move coherently, like a
regiment of soldiers. This is why flow in a superfluid, once started, cannot easily be stopped.
In supersolid helium, all of the vacancies in the crystal likewise start to move coherently, which means that waves can
progress through the lattice.
The onset of this coherent motion is called Bose-Einstein condensation. In superconductors, Bose-Einstein condensation of
electrons at low temperatures allows electrical current to pass with no resistance. Since 1995, Bose-Einstein
condensation of atoms has been seen in a variety of ultracold gases.