New heights in superconductivity.

New heights in superconductivity 

For years, but with very little success,researchers have been trying to find materials that become superconductors at higher temperatures. Until recently, the best they could do were certain metal alloys that abruptly lose their electrical resistance at temperatures below 24|K, barely above absolute zero and well below the boiling point of liquid nitrogen.

That situation has changed suddenlywith the surprising discovery of a new class of materials that show onset of superconductivity at temperatures greater than 40|K. Raising the transition temperature means that less costly refrigeration techniques can be used to cool these materials enough to turn them into superconductors, thereby increasing the number of potential applications.

The new approach was suggested lastyear when researchers at the IBM research center in Zurich, Switzerland, found that lanthanum copper oxide, with barium randomly replacing some of the lanthanum atoms in the compound, becomes partially superconducting at 30|K. Now, researchers at the University of Houston and at AT&T Bell Laboratories in Murray Hill, N.J., in separate efforts, have tinkered with the material in various ways to push the superconducting transition temperature to an unprecedented 40|K.

Whereas most known superconductorsare metal alloys such as mixtures of niobium and tin, the new materials are conducting oxides, which in some ways are more like ceramics than metals. "This is quite surprising to us,' says Robert C. Dynes, physics and chemistry research director at Murray Hill. "For reasons that are not yet at all obvious to us, conducting oxides have properties that result in quite enhanced superconducting characteristics,' he says. "You have to throw away preconceived notions that metals are superconductors, and you have to start again.'

Houston's Paul C.W. Chu obtained increasedtransition temperatures by putting the oxide under high pressure to reduce the distance between atoms in the compound. More recently, he has achieved the same result by substituting strontium for barium. Researchers at Bell Labs have modified the conducting oxide's composition to produce a sharp transition to superconductivity that is less than 2| wide. Their material is completely superconducting at 36|K. Reports from Houston and Bell Labs are to be published in PHYSICAL REVIEW LETTERS, probably later this month.

Researchers have lots of room tosearch for ways of further increasing the superconducting transition temperature. "You can change the composition; you can change the elements; you can apply pressure,' says Chu. "With all this flexibility, I don't see a limit in the near future.'

Thin films of the new material have a"potentially excellent future' in detectors and electronic devices, says Stanford University's Theodore H. Geballe. "One can always find difficulties in working with this material,' he says, "but compared with the problems that were overcome in working with [niobium-tin alloys], for example, these are no greater, and the stakes are higher.' Japanese researchers already have a great deal of experience working with another superconducting oxide, barium bismuth lead oxide.

"I think it's very exciting and open-endedright now,' says Geballe. "Who knows how far it will go?'