Researchers have turned frozen helium into a solid, which behaves like a superfluid
Philip Ball, Nature (translation: Dikla Oren)
Researchers from the USA think that they have discovered a new state of matter - a super-solid. If their findings are correct, the state of this material is extremely strange. Despite being a crystalline solid, it can "flow" like the most slippery liquid imaginable - in fact, it flows like a liquid with no viscosity at all.
It is known that there are liquids, which can flow without viscosity, for years. They are called superfluids, and because they lack viscosity, their behavior is strange. Once stirred, superfluids will continue to swirl forever, while a swirl in a normal liquid will eventually subside and disappear. A superfluid can also climb out of a container in which it is located.
Two of the first liquids, created by researchers, were made from helium-4 and helium-3. Both must be cooled to a temperature close to absolute zero (-273 degrees Celsius) for their superfluidity properties to appear.
Un-Sung Kim and Moses Chan of the University of Pennsylvania now claim to have turned frozen helium-4 into a supersolid. They did so by filling tiny channels in porous glass (called "vicor") with helium and freezing it by cooling it and compressing it to more than 60 atmospheres. Then let the glass disc filled with helium spin. At a temperature of about 0.175 degrees Celsius above absolute zero, the disk suddenly began to rotate more easily - exactly what should have happened, if the helium had become a superfluid.
The same thing happens, even when you put super-liquid helium in a vikor glass, because the viscosity-free liquid circulates too easily.
Kim and Chan say that it is difficult to provide an explanation for their findings without assuming that this is a state of supersolidity. However, John Beamish from the University of Alberta in Edmonton, Canada, says that their claim "will certainly arouse controversy". Some researchers wonder, for example, if it's possible that some superfluid helium wraps the walls of the tiny channels in glass and causes the disc to spin more freely. Kim and Chan, however, insist that the likelihood of this is low.
Solid helium itself is a rather strange substance. At such low temperatures, the behavior of helium atoms is dictated by quantum mechanics. This prevents the helium from freezing at all, regardless of how cold it is, unless it is compressed to a pressure of at least 25 atmospheres. The resulting "quantum solid" is quite soft - unlike most frozen solids it can be crushed like rubber, and the lattice of the atomic crystal is full of spaces, called voids, that move through them.
When liquid helium turns into a superfluid, the laws of quantum mechanics cause all the atoms to move coherently, like a battalion of soldiers. For this reason stopping the flow of a superfluid once it has started is not an easy task.
In supersolid helium the voids in the crystal also begin to move coherently, meaning that the waves can travel through the lattice.
The onset of coherent motion is called Bose-Einstein condensation. In superconductors, the Bose Einstein condensation of electrons at low temperatures allows the electric current to move without resistance. Since 1995 Bose-Einstein condensation of atoms has been seen in a variety of extremely cold gases.
One response
It's just a shame that the energy invested in keeping the superfluid in its state is greater than the energy that can be produced from the never-ending rotation!!!