In 1986, certain compounds containing copper-oxide layers were
discovered to be superconducting at relatively high temperatures.
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
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
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
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.
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