Superconductivity possible at 250 kelvins.

"If it's true, it would be fantastic," says Miles V. Klein, a physicist at the University of Illinois at Urbana-Champaign. "250 degrees kelvin is almost room temperature in Siberia."

Klein was referring to the report this week by a French team that it had attained superconductivity at 250 kelvins (-23 degrees C) in a thin film. Albeit a fleeting phenomenon in tiny samples, superconductivity at this temperature is the highest officially reported so far, nearly 100 kelvins above what other groups have published (SN: 10/2/93, p. 214).

Reporting in the Dec. 17 SCIENCE, Michel Lagues, a physicist at a Centre National de la Recherche Scientifique (CNRS) facility in Paris, and his colleagues describe a method for making a finicky, thin film of copper oxide. The film contains eight layers of copper and oxygen molecules sandwiched between other layers of bismuth, strontium, calcium, and oxygen. The scientists built the thin film painstakingly atom by atom, using a method called sequentially imposed layer epitaxy.

The material itself - a type of cuprate compound belonging to a well-known family of materials denoted BiSrCaCuO is not fundamentally new.

"The basic architecture of this material is well known," Lagues says. "What's new here is the way we grew these materials, layer by layer. It's not hard to make a sequence of layers - copper, calcium, copper, calcium - many times. But it's very difficult to get the structure just right, with the right conditions. So what we've done is made the material well enough to obtain superconductivity at this temperature."

Subjecting the material to standard tests for superconductivity, the researchers watched its electrical resistance drop by a factor of 100,000 as the sample cooled from 280 to 250 kelvins. At 235 kelvins, its resistance fell below detection levels. The sample also showed magnetic characteristics typical of superconductivity

Still, many researchers, while hopeful, remain cautious about these results. Other groups have described superconductivity at such temperatures, but colleagues discounted the findings when they could not document or reproduce them. Lagues, in contrast, says his group has reproduced their results, though other laboratories have not yet confirmed the report.

"This paper improves the chances of there really being superconductivity at 250 kelvins, but it's not conclusive," says Theodore H. Geballe, a physicist at

Stanford University "The amount of material is very small, the signals they've measured are small, and more work must be done to find out what this material really is." He stresses the uncertainty associated with tiny samples. "But their paper is a step forward," he adds. "It's more conclusive than previous work."

Thirumalai Venkatesan, at the University of Maryland's Center for Superconductivity Research, also remains skeptical. "Their data are interesting but not entirely convincing," he says. "The magnetization signals are so low that it's difficult to use them as evidence. Also, there are not enough data points in the temperature-versus-resistance curve. It's not clear to me that this is reproducible. Something other than superconductivity could explain these results."

While some researchers question the French report, Paul C.W. Chu, a physicist at the Texas Center for Superconductivity at the University of Houston, doesn't find the results outlandish. "Based on what I've heard, these results sound reasonable."

"Let me put it this way," Chu adds. "Any group can have credible results. And any credible group can have incredible results."