ITHACA, N.Y. -- With an advanced imaging technique and asavvy strategy, researchers at Cornell University's Laboratory ofAtomic and Solid State Physics (LAASP) have shown how addingcharge-carrying atoms like oxygen to a superconductor can increase thematerial's ability to conduct electricity overall and -- paradoxically-- to decrease it in localized spots.
The discovery, published inthe Aug. 12 issue of Science, could lead to the eventual development ofmore effective superconductors.
The scientists, led by Cornellprofessor of physics J.C. Séamus Davis, used a specialized scanningtunneling microscope (STM) in the basement of Cornell's Clark Hall forthe research. They identified for the first time the locations ofindividual oxygen atoms within a particular superconductor's molecularstructure and used that information to examine how the atoms affectcurrent flow in their immediate vicinity. It's a small but vital step,they say, toward understanding how superconductors work.
Superconductorsare materials that conduct electricity with virtually no resistance.The materials, in this case copper-based compounds (cuprates) dopedwith charge-carrying atoms like oxygen and cooled to extremely lowtemperatures, are widely used in fields from medicine to the military.But the physics behind them is still not well understood, making theultimate goal of creating a room-temperature superconductor elusive.
Researchershave long suspected that dopant atoms -- crucial for conductivitybecause they attract electrons and leave the positively charged gapsthat allow current to flow without resistance -- are actuallycounterproductive because they create electronic disorder at the atomiclevel. But until now, no one had been able to look closely enough atthe atomic structure to confirm the correlation.
The researchersat Cornell tackled the problem by preparing samples of a cupratesuperconductor doped with different concentrations of oxygen atoms.Using the STM, which can measure current in areas less than a nanometerwide -- the width of three silicon atoms -- they mapped the materialsaccording to how well or poorly current flowed in each point on theplane. The locations of the oxygen atoms, they found, correlated withthe areas of energy disorder they had already identified.
"Now wecan put the dopant atoms into the image and ask, are they correlatedwith the electronic disorder directly?" said Davis. "When the dopantsare far away, electron waves are homogeneous." When the dopant atomsare near the conducting plane, though, the waves become drasticallyheterogeneous, causing the superconductivity to break down.
Think of the compound's electrons as dancers moving together in a carefully choreographed production, Davis said.
"Superconductivityis made by pairing two electrons. It's like a dance -- not a waltz, buta distributed dance like a contra dance," Davis said. "If you putstones in the middle of the dance floor you disturb the pattern. Andonce you've destroyed all the pairs, you've destroyed thesuperconductivity."
But (and here the contra dance analogy breaksdown a little) the stones -- in this case, the dopant atoms -- areprerequisites for the dance. So taking them out isn't an option.
"Theseatoms have to be working in two different ways -- one way on averageand another way locally," said Kyle McElroy, a postdoctoral researcherat the University of California-Berkeley and co-author of the paper."One of the big questions is why different cuprate familiessuperconduct at different temperatures. There's a spread of four tofive times the transition temperature. Why do these transitiontemperatures change so much, and what is governing that?"
Expertspredict that the worldwide market for superconductors will reach $5billion by the year 2010 from about half that in 2000 -- if growthcontinues linearly. But if scientists can learn to make materials thatsuperconduct at higher temperatures, the market could skyrocket.
"Thiskind of information is a necessary step toward understanding first themechanism of high temperature superconductivity and, next, how to raisethe transition temperatures," said James Slezak, co-author of the paperand a graduate student in physics at Cornell.
The paper's otherauthors include D.H. Lee of the University of California-Berkeley, H.Eisaki of the National Institute of Advanced Industrial Science andTechnology, Ibaraki, Japan, and S. Uchida of the University of Tokyo.
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