SCIENCE & ENGINEERING NEWS
San Diego, CALIF. — Add a layer of aluminum oxide to a crystal made of neatly stacked soccer-ball shaped carbon molecules known as buckyballs, and the result is a superconductor that effortlessly carries electricity at relatively warm temperatures.
Writing in the current issue of the journal Nature, Kenneth Chang reports that researchers from Lucent Technologies Inc.’s Bell Labs in Murray Hill, N.J., report that the buckyball-aluminum oxide combination remains superconducting at temperatures up to minus-366 degrees, or about 94 degrees above absolute zero.
That is still considerably colder than the temperatures that so-called high-temperature superconductors can work at, but the researchers believe that they can tweak the buckyball crystal to raise the superconducting temperature an additional 80 degrees or more.
That would be warm enough for the buckyball superconductors to work while cooled by liquid nitrogen instead of much more expensive liquid helium, greatly improving the prospects of eventual practical applications.
“I believe the prospects are quite interesting,” said Dr. Olle Gunnarsson, a physicist at the Max Planck Institute for Solid State Research in Stuttgart, Germany, who wrote an accompanying commentary to the Nature article. “It’s very hard to know how high it will go.”
In 1991, scientists at Bell Labs, then part of AT&T, first discovered that buckyballs molecules made of 60 carbon molecules arrayed in the shape of a soccer ball can be turned into superconductors when mixed with potassium. Buckyballs are named after R. Buckminster Fuller, designer of the geodesic dome that they resemble.
Superconductors can carry electrical current without any resistance.
But the buckyballs switched back into insulators at temperatures above minus-427 degrees, and further research nudged the transition temperature to minus-400.
Even that was a scientifically noteworthy accomplishment conventional superconductors up to that point had topped out at about minus- 418.
High-temperature superconductors, which work by a different mechanism still not understood by scientists, have transition temperatures as warm as minus-164 degrees. High- temperature superconductors, made of copper oxides, tend to be brittle, however.
Buckyball superconductors could potentially be more amenable for some applications, but buckyballs also have complications: they tend to degrade in the presence of oxygen.
In the latest efforts, instead of adding potassium, the layer of aluminum oxide along with three gold electrodes form an electronic device known as a field-effect transistor on the surface of a pure crystal of buckyballs.
The potassium had contributed electrons that carried electric current by hopping from buckyball to buckyball.
Theoretical calculations had indicated that buckyballs might be better superconductors if one could remove electrons instead of adding them, but buckyballs tend to cling onto their electrons very tightly.
With the transistor, the researchers were able to apply an electric field that could draw electrons away from the buckyballs or drive additional electrons onto them. By varying the strength of the electric field, they found that the buckyballs reached their highest superconducting temperature when, on average, three electrons were missing per buckyball.
Calculations indicate the temperature can be increased further if the buckyballs could be spaced farther apart by wedging other molecules between them.
“That would be a pretty dramatic result,” said Dr. Robert C. Haddon, who led the original buckyball superconductor research at Bell Labs and is a now professor of chemistry and chemical engineering at University of California at Riverside.
Dr. Bertram Batlogg, leader of the Bell Labs team, said: “The principle is quite straightforward. It’s going to be a matter of clever chemistry to implement this idea.”
With the electric field of the transistor changing the electrical properties of the buckyballs, Dr. Gunnarsson imagined using them as a simple switch.
Turn the electric field on, and current flows through the superconducting buckyballs. Turn the field off, and the current is cut off.
“At this point, we haven’t thought much about potential applications,” said Dr. J. Hendrik Schˆn, a physicist at Bell Labs and a member of the research team.
By attaching transistors to other organic crystals, the researchers hope to find other superconductors at work at perhaps even higher temperatures.
In earlier research, the technique had already previously yielded a laser. “We are playing on the theme very successfully,” Dr. Batlogg said. “This way of exploring materials for new phenomena is what’s different, what’s particularly fruitful.”