BALTIMORE, MARYLAND--Two years ago, a team of physicists created a stir when they reported that solid helium could flow without resistance, like a liquid utterly devoid of viscosity. Now, three different groups have confirmed the bizarre observation. But mounting evidence also suggests the "supersolid" flow occurs only in crystals riddled with defects. In fact, one group reports that the effect vanishes if the crystal is gently heated and cooled to smooth out the flaws.
The first signs of supersolidity were spotted by Eunseong Kim and Moses Chan of Pennsylvania State University in University Park in a tiny can filled with pressurized solid helium (ScienceNOW, 14 January 2004). When they set the can twisting atop a thin shaft and cooled it to below 0.2 kelvin, the frequency of the twisting suddenly shot up. That indicated that about 1% of the helium had let go of the can and was standing still as the rest of the helium crystal continued to gyrate. The baffling conclusion: The solid helium flowed freely through itself. But theorists disagree on how the flow occurs, or whether it's even possible in a perfectly ordered crystal. And experimenters hadn't reproduced the spectacular results--until now.
New experiments also reveal the helium letting go, Keiya Shirahama of Keio University in Yokohama, Japan, told the meeting of the American Physical Society here yesterday. "We have confirmed the Kim and Chan observation," Shirahama says. Minoru Kubota and colleagues at the University of Tokyo, who were not at the meeting, have obtained similar results, as have Ann Sophie Rittner and John Reppy of Cornell University in Ithaca, New York. But Rittner and Reppy have gone a step further, showing that the flow vanishes if the crystal is heated to just below its melting temperature and then slowly cooled, a process called annealing that generally eliminates defects. "The more perfect the crystal, the less supersolid there is," Rittner says.
That's discouraging news to those who had hoped supersolidity was an inherent feature of crystalline helium. Some researchers had argued that the flow might arise when many helium atoms crowd into a single quantum wave in a phenomenon called Bose-Einstein condensation. That weird quantum effect enables liquid helium to flow without resistance, but some theorists had argued it was impossible in a crystalline solid. The results of the new experiments make Bose-Einstein condensation in the solid "very unlikely" says David Ceperley, a theorist at the University of Illinois in Urbana-Champaign.
Reppy disagrees. Instead of atoms, defects in the crystal may undergo Bose-Einstein condensation to create the flow, he says. Meanwhile, Chan says that he and his team have annealed their helium crystals many times and never seen the flow go away.