Cold
plastic gives electrons free ride
By
Kimberly Patch,
Technology Research NewsEditor's note: this research has been withdrawn by the scientists.
Materials become superconducting when they lose all resistance to electricity, allowing electrons to pass through them without losing energy to heat. This is a potentially useful trait in electronics, because superconducting circuits would use relatively little electricity and go very fast without heating up.
Although many metals are superconductors, the trait won't be practical for use in common computer applications however, until researchers figure out how to make substances superconducting in warmer temperatures or maintain circuits at the extremely cold temperatures the phenomenon currently needs.
A group of scientists from Lucent Technologies' Bell Laboratories has added a new material to the slate of known superconductors. The team has shown that a plastic can be superconducting.
The researchers used a field effect transistor made of the organic polymer film polythiophene, and caused it to superconduct electrons at temperatures below 2.35 Kelvin, or -270.8 Celsius.
"Since polythiophene is an attractive material for electronics applications and we [had] already fabricated polythiophene field effect transistors, we were curious [about the possibilities of turning] this material from insulating to superconducting," said Hendrick Schön, a scientist in Bell Laboratories' condensed matter physics department. "At high densities in low temperature we observed superconductivity," he said.
The researchers made the material superconducting when it was in a highly ordered film, meaning it's individual molecules were neatly lined up. "We found that the superconductivity can be suppressed by disorder in the film. Therefore the structural organization of the material is very important," Schön said.
In contrast, metals are generally doped, or mixed with other materials, to become superconducting. "The advantage [with plastic] is that only the carrier concentration is varied,” and varying the concentration, or order of a polymer is relatively easy to do, he said. "We used a very simple deposition technique -- solution casting," said Schön.
This type of casting can be achieved at low temperatures without special processing requirements like vacuums. It is also easy to put on flexible substrates, he added.
The work is "impressive. [The researchers] have extended dramatically the range of systems in which superconductivity is observed," said Dale J. VanHarlingen, a physics professor at the University of Illinois at Urbana-Champaign. "This is making all of us rethink what materials are likely to exhibit superconductivity."
It looks like the superconducting mechanism in polymers is different from that of other known superconductors.
When metals superconduct, the electrons travel in a certain type of Cooper pair. What's unusual about a Cooper pair is the electrons balance each other with opposite and equal momentum. At low temperatures the Cooper pairs are in a very low energy state, which allows them to accelerate together in the presence of an electric field without scattering. Scattering is what causes electrons to lose energy to heat in materials that are not superconductors.
In plastics, "it is likely that the symmetry and mechanism of the [electrons] pairing will be different from that of the conventional... superconductors," said VanHarlingen, who added that the researcher's greatest challenge will be "understanding how the microstructure of the films affects the superconducting properties, and how to control it."
The advantage of plastic is it is easy to work with,VanHarlingen said. "A lot is known about how to process and pattern polymer film into sophisticated circuits. This implies that it may be possible to develop a superconducting electronic technology based on polymers,” he said.
Along those lines, "the most exciting aspect [of plastic superconductors] is the ability to tune the material from semiconducting to superconducting by gating," said VanHarlingen. Gating, or applying current through the third electrode of a field-effect transistor to control the current running through the other two, has "long been studied in superconductors, but the effects are not as striking as demonstrated here," he said.
The researchers are working to further tune the material's transition from insulating to superconducting using an electrical current, said Schön.
Now that they have proved plastic can superconduct, the Bell researchers are also working toward a goal many metallic superconductor researchers have been working on for years -- making them superconduct at higher temperatures. They are working to increase the transition temperature by making the polymers even more ordered and by exploring the superconducting properties of different kinds of organic polymers, said Schön.
It will probably be a long time before superconducting plastic can be used in practical applications, said Schön. "I believe it will take much more than two years to become practical. So far we are working on the lab scale and trying to compare the first small superconducting circuits," he said.
The study of these materials, after all, is still in its infancy, said VanHarlingen. "There's definitely potential for some interesting devices," he said, adding, however that the low temperatures still required for superconductivity could prohibit widespread practical uses.
Schön's research colleagues were Ananth Dodabalapur Zhenan Bao, space and Christian Kloc, Of Bell Laboratories, Ortwin Schenker of the University of Konstanz in Germany, and Bertram Batlogg of Bell Laboratories and the Swiss Federal Institute of Technology (ETH). They published the research in the March 8, 2001 issue of Nature. The research was funded by Lucent Technologies and the University of Konstanz in Germany.
Timeline: > 2 years
Funding: Corporate, University
TRN Categories: Semiconductors and Materials
Story Type: News
Related Elements: Technical paper, "Gate-induced Superconductivity into a Solution-processed Organic Polymer Film," Nature, March 8, 2001.
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April 11, 2001
Page One
Glass mix sharpens holograms
Material bends microwaves backwards
Shaky chip makes for bug-eyed bots
Cold plastic gives electrons free ride
Holographic technique stresses interference
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