Superconductivity: a hard frost.

Superconductivity: A hard frost

As superconductivity appears athigher and higher temperatures, the prospect of a resistanceless electrical technology glimmers more brightly. As of last week the limit has risen to 240 kelvins, claimed by a group at Wayne State University in Detroit.

At this rate there may be an advantageto living in the frost belt: This temperature is the equivalent of -33|C or -27|F. Indications of superconductivity at temperatures near this had previously been seen by the group working with Paul Chu at the University of Houston and the group working with Marvin Cohen at the University of California at Berkeley. At the recent meeting of the American Physical Society, Cohen had spoken of sudden drops in resistance. This could be a precursor of superconductivity, but as Lowell E. Wegner of Wayne State points out, it could also be what happens when an insulator becomes an ordinary conductor --a normal phenomenon that does not lead to superconductivity.

To verify superconductivity, physicistswant either a demonstration of zero resistance in a sample of the material or a showing of some other phenomenon that requires the presence of a superconductor. The Detroit group, which includes Wegner, J.T. Chen, Eleftherios M. Logothetis, Charles J. McEwan and Winston Win, used a kind of inverse-Josephson effect.

In the Josephson effect, a direct voltagein a particular kind of superconducting junction will cause a stepwise alternating current to flow, and under certain conditions this current can generate a radio wave. In the inverse, the experimenters bathe the sample in radio waves and look for the alternating current and the direct voltage. The Josephson effect requires a superconductor, Wegner says.

What the Wayne State researchersthink they have is a material in which a part becomes superconducting at 240 K, and another part at 90 K. Cohen told SCIENCE NEWS that this is what his group thought they had, too: a "filament' inside the sample that went superconducting at 234 K, but then broke so that the indication disappeared. "We always had the 90 K phase,' Choen says, "and we see indications at 150 K and 180 K also.'

These materials are compounds of copper,oxygen and rare-earth elements. They are granular, and the exact proportion of the different elements can vary from place to place. Chemists refer to these variations as different "phases.' Wegner says his group is now trying to find the exact composition of the phase that goes superconducting at 240 K, in the hope of being able to prepare a pure sample of it.