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Buckyball superconductors get warmer.

As the recent sweltering weather broke records, those 60-carbon molecules called buckyballs set a different kind of temperature standard.

In late May, two research groups announced they had increased by 60 percent the working temperature of an organic superconductor by combining buckyballs -- members of a new class of carbon molecules called fullerenes -- with the metal rubidium. And this week, four teams shed light on how this fullerence, normally an insulator, can switch to a conductor. Now, the race is on to make buckyballs that superconductor at even higher temperatures.

Matthew J. Rosseinsky and his colleagues at AT&T Bell Laboratories in Murray Hill, N.J., made a thin film of buckyballs-rubidium and found it conducted electricity with no resistance at 28 kelvins. This broke a record set at the same lab in April, when researchers made a buckyball-potassium superconductor that worked at 18 kelvins. (SN: 4/20/91, p.244). While these materials do not work at as high a temperature as ceramic superconductors, they do function at temperatures much higher than expected, Rosseinsky's group reports in the May 27 PHYSICAL REVIEW LETTERS.

In the May 24 SCIENCE, Karoly Holczer and co-workers at the University of California, Los Angeles, describe making fully superconducting samples of potassium-doped buckyballs and producing rubidium-based superconductors that work at 30 kelvins.

Superconductivity seems to arise in these materials because metal atoms so readily share an electron with a neighboring buckyball, two research groups propose in the June 7 SCIENCE. Gunther K. Wertheim's team at Bell Labs suggests that the electrons set up pathways across the thin film, which can then conduct electricity. Paul J. Benning and two colleagues at the University of Minnesota in Minneapolis, working with chemists at Rice University in Houston, draw a similar conclusion based on the energies of the electrons released when the researchers bombared fullerence solids with light. When there are three metal atoms for each buckyball, this path contains just the right number of electrons for the material to become a superconductor, they report.

Other scientists have found that they can stuff in up to six potassium or cesium atoms per buckyball. X-ray crystallography indicates that the buckyballs shift slightly to accommodate these metals, reports a team headed by John E. Fischer at the University of Pennsylvania in Philadelphia. Normally, buckyballs arrange themselves as if they were corners on a cube, with another buckyball sitting in the center of each face of the cube. When metal is added, the buckyballs on the faces move over to let the new atoms occupy the central positions, the researchers report in the June 6 NATURE.

In the same issue, William A. Goddard III and two students from Caltech in Pasadena calculate the lowest-energy--and consequently, most stable--arrangement of buckyballs in a solid. Their conclusions about how these molecules stack fit with previous results.

Taken together, the recent studies indicate that adding relatively large metal atoms, such as cesium, should result in higher superconducting temperatures. So far, no one has reported getting the cesium-fullerence combination to work, although many are trying.
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Title Annotation:60-carbon molecules used to increase the working temperature of organic superconductor
Author:Pennisi, Elizabeth
Publication:Science News
Date:Jun 8, 1991
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