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Electron pairs in superconducting rings.

When a material becomes a superconductor, current-carrying electrons behave as if they were paired, even though electrons at close quarters repel each other. In conventional superconductors, this pairing results from the coordinated interaction between electrons and vibrations of the crystal lattice through which the electrons move.

However, such interactions are too weak to account for superconductivity in high-temperature copper-oxide superconductors. Although theorists are certain that pairing occurs, they have so far been unable to agree on what mechanism leads to the formation of electron pairs in these materials.

Now, researchers have obtained new experimental evidence that may help establish how high-temperature superconductivity works. John R. Kirtley, Chang C. Tsuei, and their coworkers at the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y., have found that a superconducting ring can exhibit magnetization half the size of the magnetic flux quantum.

Kirtley described the findings at an American Physical Society meeting held last week in Pittsburgh.

Kirtley and his colleagues worked with a set of four microscopic rings made from thin films of the high-temperature superconductor yttrium barium copper oxide (see image). Each ring contained a different number of grain boundaries, where superconducting films with different lattice orientations meet to create junctions.

When the rings are chilled to 4.2 kelvins, a tiny electrical current begins to circulate spontaneously within each one. This supercurrent generates a magnetic field. Using a scanning SQUID (superconducting quantum interference device) microscope, the researchers can detect and measure the resulting magnetization at each ring.

Kirtley and his team discovered that a three-junction ring- unlike those having no junction or two junctions - reveals magnetization in "packages" half the size of the magnetic flux quantum.

"If [yttrium barium copper oxide] were an ordinary superconductor, you would never see the effect they claim to see," says Frederick C. Wellstood of the University of Maryland in College Park, who has been working on a similar kind of experiment.

The IBM results may shed light on the particular type of pairing that occurs between two electrons in a high-temperature superconductor. Theorists have debated heatedly whether electron pairing is characterized by socalled s-wave or d-wave symmetry, The new findings - if they hold up - suggest the existence of d-wave pairing.

Such evidence doesn't settle the argument concerning the mechanism by which pairing occurs in high-temperature superconductors, but it may rule out certain possibilities. At the same time, the experimental results need to be checked. For example, magnetic impurities at the junctions may have affected the measurements.
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Title Annotation:high-temperature superconductivity
Author:Peterson, Ivars
Publication:Science News
Article Type:Brief Article
Date:Apr 2, 1994
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