New superconductor clue.

New superconductor clue

High-temperature superconductors continue to tantalize researchers and attract funding. Yet despite mountains of experimental data and mind-years of theory, no one understands how these layered compounds work. A new computer study, focusing on the ordering of oxygen atoms in specific layers of yttrium-barium-copper-oxide (YBaCuO) superconductors, may supply a clue.

The molecular formula for YBaCuO superconductors includes six to seven oxygen atoms. The total amount of those atoms and their arrangement around copper atoms somehow determines the material's transition temperature -- the point at which it conducts electricity without resistance.

A team led by Henning Friis Poulsen at the Riso National Laboratory in Roskilde, Denmark, now reports computer calculations that seem to provide a quantitative picture of the link between oxygen and transition temperature.

The proportion of two copper-oxygen arrangements, which the Danish researchers correlate with two observed transition temperatures in the YBaCuO materials, shifts with changes in the amount of oxygen in the compounds. For compounds hosting both arrangements, possibly segregated into molecular neighborhoods too small to observe with conventional techniques, the transition temperature is a "weighted average of the two types of ordered oxygen domains," the researchers write in the Feb. 14 NATURE. The weights derive directly from the population of oxygen atoms in each kind of neighborhood.

In an accompanying commentary, James D. Jorgensen of Argonne (III.) National Laboratory notes that the team's calculations show "remarkable agreement" with experiments and provide "an important insight into the microscopic mechanisms that influence superconductivity in this material."