Media Contact: Mario Aguilera, (619) 534-7572, mcaguilera@ucsd.edu

UCSD PHYSICS TEAM PINPOINTS NOVEL ENERGY SCALE
ASSOCIATED WITH SUPERCONDUCTING MATERIALS

A team of physicists led by the University of California, San Diego has taken a major step forward in the evolving story of superconductors, the materials that lose resistance to electricity.

Superconductivity was initially achieved earlier this century by Dutch physicist Haike Kamerlingh Onnes, who discovered the phenomenon by cooling metallic mercury to minus 452 degrees Fahrenheit. Superconductivity has been actively pursued by scientists due to the alluring ability of superconductive materials to conduct electrical currents without resistance, in contrast to currently-used metallic wires, and hence its ability to conserve energy and money.

"High-temperature" superconductors, discovered 12 years ago, were hailed as a more viable technology because they lose resistance at temperatures well above the levels of ordinary superconducting metals, such as lead and aluminum. High temperature superconductors are complex intermetallic compounds based on the oxide of copper, cuprates.

Yet many questions remained as to the energy associated with the phenomenon of superconductivity. Because electrons in ordinary metals interact weakly, conventional theories of ordinary superconducting said that superconductivity is a low-energy phenomenon. Could the same be said of the high temperature materials?

Using state-of-the-art spectroscopic instrumentation developed at UCSD, researchers Dimitri Basov, Robert Dynes and their colleagues analyzed the properties of cuprates. As reported in the Jan. 1 issue of the journal Science, the research team documented an anomalously broad energy scale associated with cuprates as they made the transition to superconduction.

In fact, Basov and his colleagues showed that cuprate high-temperature superconductors display an energy scale higher on the order of one or two magnitudes compared with ordinary superconducting metals. It’s almost as if the energy potential of a car was suddenly compared with the energy potential of a jet airplane.

"We found that the energy scale involved in the superconducting transition is much, much broader than the one observed in conventional superconductors. This is a new result that changes the way we think about high-temperature cuprates," said Basov, an assistant professor of physics at UCSD. "Many theoretical pictures had argued that the mechanisms explaining the superconductivity of aluminum or lead could be extended to these new oxide cuprates. But this experiment shows that the energy scale is qualitatively different in oxide superconductors."

Thus the cuprates have ushered in a new series of questions, including: Can the physics explaining ordinary metal superconduction be conformed to apply to the high-temperature cuprates? Or is a whole new set of concepts required to deal with this new energy range?

"One of the things I think this specific paper illustrates is that our conventional way of thinking about metals doesn’t work," said Dynes, UCSD chancellor, professor of physics and co-author of the study. "So this clearly tells us that we have to think of new ways to describe all these properties. We can’t just use extensions of concepts we’ve developed over the past 50 years."

To arrive at their results, the UCSD team used optical spectroscopy instruments that probe beyond the normal visible spectrum. The method gave the group insight into the fundamental properties of the cuprates, including characteristics that fall into the infrared range.

Basov hopes that by pinpointing the mechanisms responsible for high-temperature superconductivity, researchers may be able to develop new materials for specific purposes, including satellite communications and other areas.

"We are finding that there is a bouquet of effects in these cuprate materials," said Basov. "Sometimes their properties are unclear because there are several complicated things going on at once. So we have to study each flower separately and see if we can apply concepts in other materials that can be regarded as model systems."

Beside Basov and Dynes, other researchers in the study include Solomon Woods, Andy Katz, and E. Jason Singley from UCSD; M. Xu from the University of Chicago; David Hinks from the Argonne National Laboratory; and Christopher Homes and Myron Strongin from Brookhaven National Laboratory.

Funding for the study was supported by the Department of Energy, the Sloan Foundation, the Research Corp., National Science Foundation and Air Force Office of Scientific Research.