Riding the d-wave

Researchers believed that in conventional superconductors electricity flowed with no resistance while being carried by pairs of electrons in which each pair formed a sphere-like configuration called an s-wave.

As a result, an energy gap separating paired and unpaired electrons was the same in all directions. Despite substantial skepticism from the superconductivity research community, Balatsky and his collaborators suspected that the conducting electrons in superconductor materials might be paired differently,perhaps in a star- like configuration known as a d-wave, and that the energy gap would vary with direction.

Balatsky theorized that replacing copper atoms with impurity atoms, zinc or nickel atoms, for example, would validate his hypothesis. He predicted that the impurities would interfere with the mechanism that creates the superconductivity and would create, in the area of the impurity, a localized state that would help reveal the nature of the superconductivity.

Yet to prove this theory, researchers would need to use a scanning tunneling microscope to image, on the atomic scale, the localized state created by the presence of the impurities. At the time Balatsky and his colleagues proposed the theory, the capabilities to do the work did not exist. The world would have to wait to see the hypothesis proven.

Proof came recently when a University of California, Berkeley, team lead by Seamus Davis created the impure state predicted by Balatsky and company and then used a scanning tunneling microscope to capture the images necessary to confirm the prediction. The resulting scanning tunneling microscope image, which records the probability that electrons will tunnel across the energy gap, shows the predicted cross image.

The success story, however, does not end there. According to Balatsky, adding impurities to determine the nature of the superconductivity is very much like destroying a toy to see how it works. He theorizes that the use of defects to deliberately destroy the correlated state could be quite useful as a means to learn more about material states. The use of impurity as an analysis of the correlated state might be used in analyzing carbon nanotubes, strontium ruthenate superconductors and other similar materials.

Research in the area of high-temperature superconductivity has implica- tions in many critical areas of the economy. One of the most valuable applications is in the possibility of creating power transmission lines in which the flow of current through wires with virtually no resistance would dramatically lower the natural power losses caused by resistance.

Given the current power shortages in densely populated urban areas like California the potential for this kind of highly efficient power transmis- sion is enormous.

To picture the energy gap of the high-temperature superconductor, one could imagine four notches carved into the rim of a cup nearly filled with water.

Water added to the cup would leak out through this cross-like shape. Similarly, electrons in the impurity state leak out where the energy gap is zero.