Researchers Report First Proof of Theory of High-Temperature Superconductivity

BUFFALO, N.Y. -- Ever since high-temperature superconductors were discovered in the 1980s, most scientists struggling to pin down an explanation for the phenomenon have put their confidence in one of two theories.

One, known as the s-wave theory, is based on explanations used to describe low-temperature superconductivity. The other, known as the d-wave theory, has recently attracted proponents based on some intriguing, though not definitive, results published during the past four years.

Now, in a paper published in the May 29 issue of Nature, scientists at IBM, the University at Buffalo and Université Paris-sud report the first irrefutable proof that d-wave theory is entirely responsible for the high-temperature superconductivity seen in a thallium thin film fabricated at UB.

Theorists see the advance as essential to developing a general theory of high-temperature superconductivity, the field's holy grail, which would allow scientists to manipulate and therefore, greatly enhance, the phenomenon for practical applications.

For years, the s-wave theory has been considered the most likely candidate for explaining the phenomenon, with some scientists conceding that other theories may also come into play, but only in conjunction with s-wave.

This latest finding is a boon to proponents of the d-wave theory, who, until now, have lacked decisive experimental evidence to support their position.

"This is the first time that an experiment has shown that s-wave behavior in electrons is not critical to high-temperature superconductivity," said Jui H. Wang, Ph.D., Einstein Professor of Chemistry at UB and co-author on the paper with Zhifeng Ren, Ph.D., research assistant professor at UB. "It is the only evidence that a high-temperature superconductor is demonstrating 100 percent d-wave behavior."

The theories are based on wave functions, the probability of finding a particle -- in this case, the electron pair -- at a certain location in space.

They describe how electrons in a superconducting material overcome their natural repulsion to one another and then move together in pairs without resistance in either an s-shaped or d-shaped pattern through a crystal lattice.

The new finding builds on work the IBM and UB investigators published in January 1996 in Science that showed that this Tl2Ba2CuO6+d thin film was probably the first example of a superconductor that demonstrated d-wave behavior.

But in that design, Wang and Ren explained, it was not possible to determine whether or not the behavior the electron pairs exhibited was exclusively d-wave behavior or was perhaps some combination of d-wave and s-wave.

The latest findings are convincing in part because of the simplicity of the superconductor the UB team developed and the new substrate design developed by the IBM team.

The UB scientists developed, fabricated and characterized the thallium thin film. They then sent it to IBM where researchers using the scanning magnetometer, the instrument they had invented to detect the symmetry of the wave of superconducting electron pairs, decisively detected d-wave behavior in the thin film. X-ray diffraction studies on the material were conducted at the Université Paris-sud.

The UB researchers are the only ones to have made a thin film from this thallium superconductor, which the lab now fabricates for about a dozen labs in the U.S., Canada and Europe.

"Because of its structural simplicity, it's the film of choice for making decisive experimental tests of theoretical predictions," said Ren.

According to Ren, the authors are confident because they know their findings are not dependent on boundary complications, which can make interpreting results tricky.

Such complications result from designs where a superconducting thin film is deposited onto a substrate composed of several different domains, which could influence the behavior of electrons traveling through these boundaries from domain to domain.

"The conclusion here is not dependent on these boundary complications," said Ren. "The symmetry of the film corresponds exactly to that of the substrate."

The UB portion of the work was funded by the New York State Institute on Superconductivity at UB and the Department of Energy's Argonne and Oak Ridge National Laboratories.