Superconductivity glimpsed near 300 K.

Superconductivity Glimpsed Near 300 K

Room-temperature superconductivity is a dream of condensed-matter physicists that seems on the verge of coming true. New experimental results point tantalizingly to its existence, including a short appearance of superconductivity at a temperature of 292 kelvins, or 66[deg.]F. The experiment was conducted by Alex Zettl, Angelica Stacy and Marvin Cohen of the Lawrence Berkeley Laboratory and the University of California at Berkeley.

The problem with the many reports and rumors of superconductivity at or near room temperature is that other experiments, or even the same experimenters, have not been able to confirm or repeat the reported results. Nevertheless, one of the physicists involved in the search for high-temperature superconductivity, Paul Grant of the IBM Almaden Research Center in San Jose, Calif., speaks of what he calls the "church" of high-temperature superconductivity. "We have to believe there's something out there," he says.

Room-temperature superconductivity would mean resistanceless flow of electricity at temperatures requiring no special refrigeration, and that means no generation of waste heat and no power loss. This would be a great advantage to closely confined circuitry, such as computers, and it might even bring worthwhile savings in long-distance transmission.

Zettl and his collaborators reported in Berkeley at last week's Workshop on Novel Mechanisms of Superconductivity that they took an yttrium-barium-copper-oxide and cooled it down from 300 K. Between 292 and 280 K it lost resistance. The resistanceless quality seemed stable, Zettl says, lasting two or three hours. The next day they tried to repeat the experiment, heating the same sample above 300 K and cooling it back down, but in the second cooling the high-temperature resistance loss did not occur.

The precise chemical composition can vary within these samples. Zettl thinks that inside the sample there was a filament or "link" of a specific composition that went superconducting at 292 K, but that the thermal stress of reheating and recooling broke it. Therefore he was not able to apply the second standard test for superconductivity, the Meissner effect, in which a superconductor resists penetration by a magnetic field imposed from outside.

Paul C. W. Chu of the University of Houston and his colleagues from the University of Houston an the University of Alabama at Huntsville were able to test for the Meissner effect in a sample that lost all resistance at 225 K (-54[deg.]F), but only 1 percent of the sample showed the Meissner effect. Therefore Chu is not making any out-and-out claims. In his view, repeatable superconductivity has not been confirmed above 100 K.

By replacing some of the oxygen in these compounds with flourine, a group at Energy Conversion Devices, Inc., in Troy, Mich., led by Stanford R. Ovshinsky, produced a compound in which they found bulk superconductivity at 155 K (-180[deg.]F) and a filamentary Meissner effect at 260 K (8.6[deg.]F). At the Berkeley meeting, Alex Braginski of Westinghouse Research Laboratories in Pittsburgh reported a "resistance anomaly" but not total resistance loss after substitution of two flourines for oxygen. He calls the anomaly "a partial agreement with Ovshinsky."