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Superconductivity: two teams, one view.

Superconductivity: Two teams, one view

Progress in science depends on independent confirmation of research results. But rarely does a finding receive simultaneous verification, as it did in recent work with high-temperature superconducting films. Using scanning tunneling microscopy (STM), U.S. and European research teams have produced strikingly similar images of the surfaces of ceramic thin films. These images show that the films have a rough topography landscaped with spiraling terraces.

Christoph Gerber and four co-workers at the IBM Zurich Research Laboratory in Switzerland report in the March 28 NATURE that yttrium-barium-copper-oxide ceramic films contain several billion of these spirals, called screw dislocations, per square inch. And in the March 29 SCIENCE, Marilyn Hawley and her colleagues at the Los Alamos (N.M.) National Laboratory report nearly identical results.

The consistency of the new images provides "a good affirmation of scientific research," says physicist Theodore H. Geballe, a superconductor specialist at Stanford University. "And it shows that nature is consistent from one side of the Atlantic to the other."

The detailed resolution of STM, which produces three-dimensional, atomicscale views of surfaces, helped the two teams resolve that these films start out as islands of material and do not build layer by layer as some had though, says Ian D. Raistrick, a materials scientist with the Los Alamos group. The islands grow by spiraling up and out until the spirals merge with adjacent islands.

The findings partly explain why these films make such good superconductors, and they point to ways to improve this property.

Geballe cautions, however, that the work "is very enlightening for one class of films, but it doesn't cover all films." Both teams used films made by a process called sputtering, one of several approaches to producing thin films.

Although superconductors conduct electricity with almost no resistance, this ability disappears when they try to conduct large currents. The problem is that conduction generates a magnetic field that interferes with electron flow if the current -- and consequently the magnetic flux -- is strong enough.

Superconducting thin films can handle much more current than can bulkier forms such as wires. Experts theorize that microscopic defects enhance the thin film's superconducting properties because the flux gets stuck, or "pinned," in the defects and can no longer impede conduction. "In these materials, one would like to have as many pinning sites as possible," physicist Darrell G. Schlom of the IBM team told SCIENCE NEWS.

The spirals provide pinning sites, the IBM scientists say. Increasing the number of spiral defects might increase the amount of current a superconductor can carry, they suggest.

Defects that form where the grains meet may actually be more important pinning sites than the spiral cores, the Los Alamos group proposes, because these imperfections are much more numerous. Introducing such defects might improve the superconductivity of wires and other bulk forms, Raistrick says.

Either way, says Schlom, the new findings imply that "if we control the defects, maybe we can control the properties [of the superconductor]."
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Title Annotation:IBM Zurich Research Laboratory and Los Alamos National Laboratory
Author:Pennisi, Elizabeth
Publication:Science News
Date:Apr 6, 1991
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