Star-Studded Superconductors

A star appears on a high-temperature superconductor when an electron pair hits a zinc atom and breaks apart.

MINNEAPOLIS--Scientists studying high-temperature superconductors are seeing stars. A painstaking scanning technique reveals delicate star-shaped patterns on the materials that may help physicists understand how superconductors whisk electricity along without any resistance at temperatures as high as 120 Kelvin. The study was presented here 20 March at the annual meeting of the American Physical Society.

Since their discovery in 1986, high-temperature superconductors have defied full explanation. Researchers know that pairs of electrons dance effortlessly along planes of copper and oxygen atoms within larger crystal structures made of elements such as bismuth, strontium, and calcium. But they still don't understand how the electrons in a pair attract each other, in part because they have not been able to see the quantum mechanical wave of pairs pass through the material. J. C. Séamus Davis of the University of California, Berkeley, and Shuheng Pan of Boston University suspected that the wave would reveal itself when electron pairs scattered off impurities in the material and broke apart.

To detect the scattering, the researchers used a scanning tunneling microscope, an exquisitely sensitive device that measures the flow of electrons between the material and a tiny fingerlike probe hovering less than a billionth of a meter above it. By scanning the region around individual zinc atoms, sometimes for months at a time, Davis and colleagues mapped regions of high current, which signaled the surf zone where the quantum wave crashed against the impurity and the electron pairs split apart. The key to the experiment were carefully prepared crystals in which researchers Hiroshi Eisaki and Shin-Ichi Uchida of the University of Tokyo managed to replace individual copper atoms with zinc atoms. "Without these samples, the experiments would have been utterly impossible," Davis says.

The star patterns show that electron pairs break apart more easily when traveling diagonally across the copper-oxygen plains. This in turn tells researchers that the quantum wave looks a bit like a four-leaf clover with its petals lying along the rows of atoms--a fact suspected, but never before proved.

More insights may come from systematically replacing atoms elsewhere in the crystal and by using other impurities such as nickel, says Alexander Balatsky, a theoretical physicist at Los Alamos National Laboratory in New Mexico who predicted the presence of the star patterns. Such experiments may even reveal why the electrons embrace one another in the first place. "By breaking things," Balatsky says, "we learn how they work from the inside out."

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