High-temperature superconductors

CERAMIC MATERIALS with `split personalities' could lead to new high-temperature superconductors, according to physicists at Ohio State University and their colleagues.

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 behaviour 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 behaviour. 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 degrees Kelvin (-388 degree Farenheit, -233 degree Centigrade), a very high temperature for superconductors. To achieve this, Bell Labs scientists mixed, or `doped,' buckyballs with potassium.

Now Lemberger and colleagues have found they can change the behaviour 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.

They report their results in two papers in a recent issue of the journal Physical Review Letters. 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 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 K (-2980 F, -1830 C). That would make materials useful conductors for commercial electronics.

If metal conductors were replaced with superconducting ceramics, devices would be 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 K (-4420 F, -2630 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 when the researchers will gather together in Taiwan to debate which of the two `personalities,' either the d-wave or the s-wave, is the true state of the material.

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