More surprises from new superconductors.
Although the search for new superconductors has focused largely on ceramic compounds containing copper and oxygen, one of the earliest ceramic superconductors, discovered in 1975, consisted of barium, lead, bismuth and oxygen. To gain a better understanding of the factors influencing superconductivity, researchers are now taking a closer look at the bismuth family. They find that very similar compounds can have strikingly different temperatures at which they lose their resistance to electrical current.
The original member of the family, barium lead bismuth oxide, becomes a superconductor below 12 kelvins. By using potassium instead of lead, researchers at AT&T Bell Laboratories in Murray Hill, N.J., can push the material's transition temperature to 30 kelvins--the highest transition temperature for a ceramic material not containing copper (SN: 5/14/88, p.309). But substituting the element antimony for bismuth in the original compound lowers the transition temperature to only 3.5 kelvins, even though the antimony- and bismuth-based compounds appear quite similar. This surprising result, reported in the May 25 NATURE, highlights the need to discover exactly why bismuth -- rather than its close chemical relative, antimony--is the magic ingredient necessary for superconductivity at elevated temperatures.
Another puzzle concerns the behavior of recently discovered superconductors in which electrons rather than "holes" (the absence of electrons) carry the current (SN: 3/4/89, p.143). Almost all hole-doped copper-oxide compounds respond to increasing pressure by raising their superconducting transition temperatures, sometimes considerably. But a team of Japanese researchers, mainly from the University of Tokyo, reports in the May 25 NATURE that pressure appears to have almost no effect on the electron-doped superconductor neodymium cerium copper oxide.
According to the researchers, this difference in behavior may relate to the way copper atoms bind to oxygen atoms in the crystal lattice. In hole-doped superconductors, each copper atom usually sits at the base of a pyramid of oxygen atoms, with four oxygen atoms in the same plane as the copper atom and one oxygen atom above the plane. Electron-doped superconductors have copper-oxygen arrangements in which the upper oxygen atoms are missing. The new experiments suggest that pressure effects appear to arise from changes in the distance between copper atoms and nonplanar oxygen atoms.
Both recent discoveries illustrate how much scientists have to learn about the way ceramic superconductors work. "Every time we look, we find something new," says Robert J. Cava of Bell Labs. Theorists so far have provided little guidance on where to look for new superconductors or why known superconductors behave as they do. Adds Cava, "After two years of research, we still don't know why."
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|Title Annotation:||Physical Sciences|
|Date:||Jun 10, 1989|
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