Probing superconductor electron pairs.

Copper oxide superconductors maintain their ability to conduct electricity, without resistance, to unusually high temperatures. Theorists generally agree that current-carrying electrons in these materials behave as if they were paired, even though electrons repel each other. So far, however, they have failed to pinpoint the mechanism that leads to the pairing responsible for superconductivity.

One way to study the behavior of electrons in these materials is by bombarding the surface of a superconductor with high-energy photons. These photons knock electrons out of the material, and researchers can measure the energies of the ejected particles. Zhi-Xun Shen of Stanford University and his collaborators have now used this technique, known as photoemission spectroscopy, to take a detailed look at six hightemperature superconductors. They describe their findings in the Jan. 20 SCIENCE.

The researchers determined the binding force between superconducting electron pairs by measuring differences in the energy and direction of electrons emitted from a material in its normal and superconducting states. They found that this binding force was a little stronger in one direction -- relative to the material's lattice of atoms -- than another.

This result supports the idea that electron pairing in hightemperature superconductors is characterized by so-called dwave symmetry (SN: 4/2/94, p.213). And this suggests that some kind of magnetic effect may be responsible for pairing the electrons in these superconductors.

But the issue remains far from settled. "While the evidence for exotic pairing... appears quite strong, uniform agreement on this point has yet to be reached," Daniel L. Cox of Ohio State University in Columbus and M. Brian Maple of the University of California, San Diego, comment in the February PHYSICS TODAY.