UNUSUAL CERAMICS COULD EXPAND POSSIBILITIES FOR SUPERCONDUCTORS
COLUMBUS, Ohio -- 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 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.
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.
"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
Scientists have speculated that cuprates could sustain s-wave superconductivity
at temperatures as high as 90° Kelvin
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.