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Hot times for buckyball superconductors.

As the pace of buckyball discoveries continues to accelerate, scientists report another major increase in the temperature at which compounds containing these soccerball-shaped molecules conduct electricity without resistance.

The 60-carbon buckyball is the most prominent member of a family of all-carbon molecules called fullerenes. By adding rubidium and thallium to a film of buckyballs, scientists at Allied-Signal, Inc., in Morristown, N.J., have now made a superconductor that works up to at least 42 kelvins. Just last month, Japanese scientists combined rubidium and cesium with buckyballs to create a compound that superconducts at 33 kelvins.

Physicist Zafar Iqbal of Allied-Signal described the latest increase last week at the University of Pennsylvania Workshop on Fullerites and Solid-State Derivatives. Another participant at the Philadelphia workshop, Paul W.C. Chu of the University of Houston, described growing large crystals of [C.sub.60] and reported that buckyballs exerted unexpected and baffling effects on known superconductors.

The Allied-Signal team created several samples of the thallium-rubidium-buckyball material, which remained superconducting to between 42.5 and 45 kelvins. They have yet to determine the exact ratios of these elements in the different samples, says Iqbal, but previous research suggests that a superconducting [C.sub.60] compound should contain three "dopant" atoms for every buckyball. This is the first report of a buckyball superconductor that incorporates elements other than alkali metals such as cesium and potassium, Iqbal and others note.

"It's a very encouraging result," says Robert C. Haddon, a chemist at AT&T Bell Laboratories in Murray Hill, N.J., who helped develop the first buckyball superconductor (SN: 4/20/91, p.244). "It broadens the scope of materials that have been shown to dope [C.sub.60]."

In the July 18 Nature, K. Tanigaki and colleagues at NEC Corp.'s Fundamental Research Laboratories in Tsukuba, Japan, described a new superconductor that contained two cesium atoms and one rubidium atom for each buckyball. Their material maintained its superconductivity up to 33 kelvins, suggesting that the bigger than metal atoms, the higher the superconducting temperatures of the buckyball film.

In the same issue of Nature, Charles M. Lieber, a Harvard University chemist, reported success in using alloys to make superconducting buckyball films that work up to 30 kelvins. This approach made it easier to combine cesium with buckyballs in the right proportions, he says. Unlike the the Allied-Signal scientists, Lieber and his co-workers added only one metal, cesium, into their buckyball lattice.

Chu, one of the pioneers in high-temperature ceramic superconductors, took a different tack in investigating [C.sub.60]. While trying to create a new material, he put a niobium superconductor into a chamber filled with buckyballs and heated the two. Chu expected only a small amount of carbon to diffuse into the superconductor, and he thought that impurity might have a slight effect on the material's superconductivity. Such an effect would indicate that the buckyballs had entered the niobium.

But the buckyballs completely eliminated the compound's superconductivity, he reported at the workshop. The results were even more astonishing when he put the buckyball-niobium compound into a magnetic field. The field reinstated the compound's superconducting properties, Chu says. When he heated the material, the superconductivity vanished again - as expected - but lowering the temperature did not restore the property, as it does for most superconducting materials.

Chu repeated the experiment with a tiny niobium ring, which actually transported current with no resistance. This confirmed that something extraordinary occurred throughout the sample when he added buckyballs to the ring, he says. Furthermore, when he exposed the sample to air, it acted as if the buckyballs were not present.

"This really defies all the rules of physics and all the rules of chemistry," Chu told Science News.

In his quest for a better understanding of buckyballs, Chu also spent three months trying to form a large single crystal of these carbon spheres. His 1.7-millimeter-long, nearly flawless specimen represents one of the biggest so far, he says.
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Title Annotation:buckminsterfullerene
Author:Pennisi, Elizabeth
Publication:Science News
Date:Aug 10, 1991
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