Unusual Ceramics Could Expand Possibilities For Superconductors

Researchers here have learned that these ceramic materials, called cuprates (pronounced KOOP-rates), switch between two different kinds of superconductivity under certain circumstances.

The finding could settle a growing controversy among scientists and point the way to buckyball-like superconductivity in ceramics.

Scientists have been arguing for years whether cuprates exhibit one type of superconductivity, called d-wave, or another type, called s-wave, explained Thomas Lemberger, professor of physics.

The difference depends on how the electrons are arranged within the material, he said. Materials with s-wave behavior are more desirable, because they should have better technical properties at high temperatures. Unfortunately, most of the high-temperature cuprate compounds seem to exhibit d-wave behavior. S-wave superconductivity at high temperatures is still a possibility and is a goal of current research, Lemberger said.

For instance, buckyballs -- soccer-ball-shaped carbon molecules discovered at Bell Labs in 1991 -- exhibit s-wave superconductivity at 40° Kelvin (-388°F, -233°C), a very high temperature for superconductors. To achieve this, the Bell Labs scientists mixed, or "doped," the buckyballs with potassium.

Now Lemberger and his colleagues have found they can change the behavior of a certain class of cuprates from d-wave to s-wave if they dope it with sufficient amounts of the element cerium -- a common ingredient in glassware.

"It seems that the mechanisms for both kinds of behavior are always present in these materials," Lemberger said. "So if you do something to suppress one behavior, a cuprate will automatically switch to the other."

They report their results in two papers in a recent issue of the journal Physical Review Letters. Lemberger, doctoral student John Skinta and postdoctoral researcher Mun-Seog Kim collaborated with Tine Greibe and Michio Naito, both materials scientists at NTT Basic Research Laboratories in Japan.

Since their discovery in 1986, cuprates have puzzled scientists. Ceramics are normally insulators, but when doped with atoms of elements like lanthanum or cerium, cuprates suddenly become excellent conductors.

"That's what's so amazing about these materials," Lemberger said. "A cuprate could start out as a very good insulator; you could subject it to thousands of volts and it won't conduct electricity at all. But change the composition just a little, and you've turned it into a superconductor. With the tiniest wisp of voltage, you'll get huge currents flowing."

Normal doping involves adding small quantities of a secondary material in order to boost the number of mobile electrons in a sample. Over-doping, as the Ohio State physicists and their colleagues did, is roughly equivalent to over-stuffing the material with electrons -- as many electrons as the cuprate would hold while still maintaining its unique crystal structure.

They created thin films of cuprates with different amounts of cerium, and studied how the electrons arranged themselves within the material. They did this by measuring how deeply a magnetic field could penetrate each film.

As the researchers pushed the cerium content of the cuprates to the limit, the magnetic field measurements suggested that the electrons had changed their formation from d-wave to s-wave.

Scientists have speculated that cuprates could sustain s-wave superconductivity at temperatures as high as 90° Kelvin(-298°F, -183°C). That would make the materials useful conductors for commercial electronics. If metal conductors were replaced with superconducting ceramics, devices would be more efficient, and new types of devices would become possible. And 90° Kelvin, while very cold, is still easier and less expensive to achieve than 10° Kelvin (-442°F, -263°C), the operating temperature of conventional metallic superconductors.

Lemberger said the scientific controversy surrounding the nature of superconductivity in cuprates will come to a head this summer, as researchers gather in Taiwan to debate which of the two "personalities," d-wave or s-wave, is the true state of the material.

"Our work bridges the gap between the two camps," Lemberger said. "We propose that it's just a matter of composition."

"The question now is, how high can we push s-wave superconductivity?" he added.

The National Science Foundation funded this work.