Physicists from Pennsylvania State University believe they have created a new state of matter—a supersolid. If their observations are accurate, this state is profoundly unusual. While a supersolid has all of the properties of a crystalline solid, it behaves like a liquid with zero viscosity, flowing without resistance.

Liquids without viscosity have been observed for years. Called "superfluids," these fluids exhibit bizarre behavior because they're extremely slippery. Stir them once and they will spin forever, in contrast to normal liquids that can only sustain a vortex temporarily. A superfluid will even slither up the side of a container and slide out of the top.

Researchers created the first two superfluids using helium-4 and helium-3. Both have to be cooled to near absolute zero (-273°C) before they enter the superfluid phase.

Researchers Eun-Seong Kim and Moses Chan of Penn State now report that they have transformed frozen helium-4 into a supersolid—or a solid that acts like a superfluid. They accomplished this by compressing helium-4 gas into a tiny glass disk with atomic-sized pores. They then put the disk in a capsule and applied a pressure of more than 60 atmospheres. They rotated the capsule and cooled it to close to absolute zero.

When the temperature hit about 0.175°C above absolute zero, the disk suddenly started spinning faster—exactly what is supposed to happen when helium turns into a supersolid. Kim and Chan say that this increased oscillation rate signifies that the helium-4 had "decoupled" from the disk's pores and had reached a supersolid state. "In this supersolid, the individual helium-4 atoms are continually flowing—without any friction—but because all the particles are in an identical quantum state, it remains a solid," Chan explains to PhysicsWeb.

The researchers observed the same effect when they filled the disk's pores with liquid helium as a superfluid as the non-viscous liquid also spins more easily.

In both supersolids and superfluids, particles move in a coherent fashion, like a military brigade. This is because their atoms have all fallen into the same quantum ground state to create a Bose-Einstein condensate. Researchers had yet to observe this effect in solids until Kim and Chan's experiment. Bose-Einstein condensation marks the start of the coherent motion of atoms. Since 1995, this phenomenon has been observed in an assortment of ultracold gases.

To doublecheck their findings, the researchers also conducted the experiment with helium-3 atoms, which are fermions. Unlike bosonic atoms such as helium-4, fermions cannot form a Bose-Einstein condensate. Sure enough, the researchers observed no variation in the oscillation rate of the capsule. "If our results are confirmed, this means that researchers have now been able to observe Bose-Einstein condensation in gases, liquids and solid phases," write Kim and Chan.

The duo says it's difficult to account for the results of their experiment without using the supersolid explanation. However, John Beamish of the University of Alberta in Canada says that their assertion "is sure to generate some controversy." Some researchers could contend, for example, that some liquid helium might still be covering the pore walls and turn into a superfluid, allowing the disk to rotate more readily. But Kim and Chan claim this is improbable.

The researchers published their findings in Nature.

Sources:

Glimpse of a New Type of Matter
Philip Ball
Nature, January 15, 2004
www.nature.com/nsu/040112/040112-7.html

Supersolid is Seen in the Lab
Belle Dume
PhysicsWeb, January 14, 2004
physicsweb.org/article/news/8/1/6