Electron superconductors and more.

Electron superconductors and more

The recent discovery of a new family of high-temperature superconductors in which electrons carry the superconducting current (SN: 3/4/89, p.143) has prompted a flurry of research activity. Some of this work aims at finding materials that lose their resistance to the flow of electrical current at temperatures higher than the 24 kelvins (-416[deg.]F) achieved in the initially discovered compounds. Now, Allen M. Hermann of the University of Arkansas in Fayetteville reports a formulation that may boost the transition temperature for an electron superconductor to 85 kelvins.

The new compound consists of thallium, barium, cerium, copper and oxygen. "It's quite reproducible and easy to make," Hermann says. However, his samples contain a mixture of components, or phases, in which the proportion of each element may vary. Because the superconducting phase hasn't been isolated and identified yet, Hermann can't tell whether the observed electron current is in the superconducting part of the material or in some other, nonsuperconducting phase. "The results are very preliminary but intriguing," he says.

To study the role oxygen plays in high-temperature superconductors, James E. Schirber of the Sandia National Laboratories in Albuquerque, N.M., and his collaborators have gone back to lanthanum copper oxide, a relatively simple compound that becomes a superconductor with the substitution of barium for some of the lanthanum. Schirber and his colleagues use high pressure to force oxygen into a heated sample of lanthanum copper oxide. They find that raising the amount of oxygen in the sample by less than 1 percent turns the material into a superconductor with a transition temperature of 40 kelvins. They want to get a better sense of where the oxygen goes and what it does to the copper lanthanum oxide's crystal structure.

"This is a very important material to try to understand," Schirber says. "It's a very simple copper-oxide system. We feel we can get a handle on what the oxygen is doing, how it turns the antiferromagnetic insulator into a 40-degree superconductor with a very, very small amount of oxygen."