Newswise — With
an advanced imaging technique and a savvy strategy,
researchers at Cornell University's Laboratory of
Atomic and Solid State Physics (LAASP) have shown
how adding charge-carrying atoms like oxygen to a
superconductor can increase the material's ability
to conduct electricity overall and -- paradoxically
-- to decrease it in localized spots.
The discovery, published in the Aug. 12 issue of Science ,
could lead to the eventual development of more effective
scientists, led by Cornell professor of physics
Davis, used a specialized scanning tunneling microscope
(STM) in the basement of Cornell's Clark Hall for
the research. They identified for the first time
the locations of individual oxygen atoms within
a particular superconductor's molecular structure
and used that information to examine how the atoms
affect current flow in their immediate vicinity.
It's a small but vital step, they say, toward understanding
how superconductors work.
Superconductors are materials that conduct electricity
with virtually no resistance. The materials, in this
case copper-based compounds (cuprates) doped with
charge-carrying atoms like oxygen and cooled to extremely
low temperatures, are widely used in fields from
medicine to the military. But the physics behind
them is still not well understood, making the ultimate
goal of creating a room-temperature superconductor
Researchers have long suspected that dopant atoms
-- crucial for conductivity because they attract
electrons and leave the positively charged gaps that
allow current to flow without resistance -- are actually
counterproductive because they create electronic
disorder at the atomic level. But until now, no one
had been able to look closely enough at the atomic
structure to confirm the correlation.
The researchers at Cornell tackled the problem by
preparing samples of a cuprate superconductor doped
with different concentrations of oxygen atoms. Using
the STM, which can measure current in areas less
than a nanometer wide -- the width of three silicon
atoms -- they mapped the materials according to how
well or poorly current flowed in each point on the
plane. The locations of the oxygen atoms, they found,
correlated with the areas of energy disorder they
had already identified.
"Now we can put the dopant atoms into the image
and ask, are they correlated with the electronic
disorder directly?" said Davis. "When the dopants
are far away, electron waves are homogeneous." When
the dopant atoms are near the conducting plane, though,
the waves become drastically heterogeneous, causing
the superconductivity to break down.
Think of the compound's electrons as dancers moving
together in a carefully choreographed production,
"Superconductivity is made by pairing two electrons.
It's like a dance -- not a waltz, but a distributed
dance like a contra dance," Davis said. "If you put
stones in the middle of the dance floor you disturb
the pattern. And once you've destroyed all the pairs,
you've destroyed the superconductivity."
But (and here the contra dance analogy breaks down
a little) the stones -- in this case, the dopant
atoms -- are prerequisites for the dance. So taking
them out isn't an option.
"These atoms have to be working in two different
ways -- one way on average and another way locally," said
Kyle McElroy, a postdoctoral researcher at the University
of California-Berkeley and co-author of the paper. "One
of the big questions is why different cuprate families
superconduct at different temperatures. There's a
spread of four to five times the transition temperature.
Why do these transition temperatures change so much,
and what is governing that?"
Experts predict that the worldwide market for superconductors
will reach $5 billion by the year 2010 from about
half that in 2000 -- if growth continues linearly.
But if scientists can learn to make materials that
superconduct at higher temperatures, the market could
"This kind of information is a necessary step toward
understanding first the mechanism of high temperature
superconductivity and, next, how to raise the transition
temperatures," said James Slezak, co-author of the
paper and a graduate student in physics at Cornell.
The paper's other authors include D.H. Lee of the
University of California-Berkeley, H. Eisaki of the
National Institute of Advanced Industrial Science
and Technology, Ibaraki, Japan, and S. Uchida of
the University of Tokyo.