What you see isn't always what you get.

What you see isn't always what you get

If you have to judge a book by its cover, make sure you look at the correct cover. That's the metaphoric message from a collaborative study by investigators at four national laboratories who took a new look at surface electrons of the so-called "1-2-3" high-temperature ceramic superconductors.

By now, scientists have gotten over the initial shock of finding the celebrity ceramics behave like superconducting metals. "They're supposed to be rocks," observes Aloysius J. Arko, a physicist at Los Alamos (N.M.) National Laboratory and a principal investigator on the project. Recent observations that the electronic structure of these materials appears nonmetallic have added to the head-scratching. Instead of having many conduction electrons as in a metal, the ceramics appears to have very few -- a meaty paradox for superconductor theorists to demystify.

But Arko and more than a dozen colleagues say the paradox may be more apparent than real. They argue that their refined observations of the ceramics' electronic structure once again portray the material as metals. In earlier studies, scientists assumed the surface properties of a superconductor crystal represent its bulk properties. Also, many scientists tacitly assumed the surfaces wouldn't change in the time it takes to determine an electronic structure. Arko and company see it differently. they discovered that surfaces of some superconductor crystals rapidly transform (by losing oxygem) from a metallic superconducting form into a nonmetallic one, especially at the relatively high temperature at which most prior studies had been done (77 kelvins and up). This transformation has led to wrong inferences about the superconductor's bulk properties, Arko says.

The scientists did their electronic structure measurements at colder temperatures (20 kelvins) and only on freshly exposed crystal surfaces. Other researchers looking at different classes of the new superconductors now are finding similar metal-like electronic structures, Arko says. "We are quite sure they are metals," he adds. "But we still don't understand why they are superconductors."