"Superconductivity in the cuprates could be caused by a saving of the Coulomb energy associated with long wavelengths and midinfrared frequencies," said Anthony J. Leggett, the John D. and Catherine T. MacArthur Professor of Physics at the U. of I. "This saving of Coulomb energy is a natural result of the formation of Cooper pairs, but is not included in the BCS theory, or most generalizations of it; it may be specially important in very 'two dimensional' materials like the cuprates."

The BCS theory -- developed in 1957 by John Bardeen, Leon Cooper and John Schrieffer (all three researchers were at the U. of I. at that time)-- explains superconductivity at temperatures close to absolute zero, but has difficulty accounting for the higher temperatures that were later achieved with the cuprates.

According to BCS theory, electrons can be attracted to one another through interactions with the crystal lattice. These electrons--called Cooper pairs-- can share the same quantum-wave function, which results in a lower energy state for the superconductor.

Cooper-pair production in the cuprates, however, is probably not dependent upon the crystal lattice, Leggett said. Instead, electrons may form Cooper pairs because of a net saving of Coulomb energy.

"My fundamental hypothesis is that the driving force leading to superconductivity in the cuprates is the saving of Coulomb energy in the regime of long wavelengths and midinfrared frequencies," Leggett said. "The main effect of Cooper-pair formation in this region is to reduce the force of repulsion between electrons, which results in a net saving of Coulomb energy."

Whether the Coulomb energy is being saved in the midinfrared region could be answered directly by differential electron-energy-loss spectroscopy measurements, Leggett said.

"For any given material, the electron-energy-loss spectroscopy cross-section is a direct measure of the Coulomb energy locked up in the region," he said. "The midinfrared scenario predicts a spectacularly large decrease in the electron-energy-loss spectroscopy cross-section in the midinfrared region when the material undergoes a transition from the normal state to the superconducting state."

Several research groups are preparing to make such measurements. The scenario also predicts a significant change in the optical spectra, but the theoretical situation here is more complicated.

"Correctly answering the question 'Where is the Coulomb energy saved?' would go a long way toward constraining possible theories of the mechanism behind cuprate superconductivity," said Leggett, who described his midinfrared scenario in the July issue of the Proceedings of the National Academy of Sciences.

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