Layer upon layer to higher temperatures.

Layer upon layer to higher temperatures

With relatively little theoretical guidance, researchers have tested hundreds of combinations of chemical elements in their search for superconductivity at higher and higher temperatures. So far, all of the new superconductors contain copper and oxygen atoms arranged in layers sandwiched between layers of larger, heavier metal atoms.

The behavior of the newest class of superconductors -- those containing the elements thallium or bismuth -- seems to suggest a trend linking the temperature at which a material loses all electrical resistance with the number of copper oxide planes pressed between successive layers of thallium or bismuth atoms (SN: 4/2/88, p.213). The latest experimental results show that the trend holds for up to four consecutive copper oxide layers but not for five.

In the Sept. 2 SCIENCE, a group of researchers from Northeastern University in Boston describes the structure of a thallium-based compound containing four consecutive copper-oxygen layers within the material's basic structural unit, or unit cell. That material's superconducting transition temperature is 122 kelvins, or -240[deg.]F.

The new compound, made up of thallium, barium, calcium, copper and oxygen, belongs to a family of materials in which each member contains a different number of consecutive copper oxide planes. As the number of planes increases from two to three to four, the transition temperatures go from 90 to 110 to 122 kelvins. The diagram above shows the arrangement of atoms in a unit cell of the 122-kelvin material.

A team of Japanese researchers at the Electrotechnical Laboratory in Ibaraki also has synthesized the same family of superconductors, obtaining almost identical transition temperatures. However, its preliminary results, as reported in the Aug. 11 NATURE, suggest the transition temperature for a compound having five consecutive copper oxide planes is probably less than 120 kelvins.

Even before the Japanese results became known, some theorists, who had predicted the transition temperature should rise as the number of copper oxide layers increased, were hedging their bets. They now suggest that the spacing between the copper oxide layers may be just as important as the number of layers in determining a material's transition temperature. The closeness of that spacing helps determine how strongly electrons bind together, or couple, to produce superconductivity. If the spacing between copper oxide layers goes up as the number of layers increases, then the transition temperature would at some point start to fall.

Confusing the situation further, scientists at the Materials Research Laboratories in Hsinchu, Taiwan, report traces of superconductivity in a thallium-based compound at temperatures as high as 162 kelvins. That compound also appears to have four consecutive copper oxide layers in its unit cell. The Taiwanese discovery is described in the Aug. 15 HIGH-T.sub.c UPDATE, a newsletter monitoring recent superconductivity research results. However, no other group has yet succeeded in reproducing the Taiwanese findings.