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Hidden order found in cuprates may help explain superconductivity
James
E. Kloeppel, Physical Sciences Editor
217-244-1073; kloeppel@illinois.edu
2/12/04
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CHAMPAIGN,
Ill. — Like the delicate form of an icicle defying gravity during
a spring thaw, patterns emerge in nature when forces compete. Scientists
at the University of Illinois at Urbana-Champaign have found a hidden
pattern in cuprate (copper-containing) superconductors that may help
explain high-temperature superconductivity.
Superconductivity, the complete loss of electrical resistance in some
materials, occurs at temperatures near absolute zero. First observed
in 1911 by Dutch physicist Heike Kamerlingh Onnes, the mechanism of
superconductivity remained unexplained until 1957, when Illinois physicists
John Bardeen, Leon Cooper, and J. Robert Schrieffer determined that
electrons, normally repulsive, could form pairs and move in concert
in superconducting materials below a certain critical temperature.
For more than a decade, scientists have been baffled by superconductivity
in the copper oxides, which occurs at liquid-nitrogen temperatures and
does not seem to behave according to standard BCS theory. A tantalizing
goal, which would have enormous implications for electronics and power
distribution, is to achieve superconductivity at room temperature. A
large piece of the puzzle has been to understand how the coherent dance
of electrons that gives rise to superconductivity changes when the material
is heated.
In a paper to appear in the journal Science, as part of the Science
Express Web site, on Feb. 12, researchers at Illinois show that when
heated, the orderly superconducting dance of electrons is replaced,
not by randomness as might be assumed, but by a distinct type of movement
in which electrons organize into a checkerboard pattern. The experimental
findings imply that the two types of electron organization, coherent
motion and spatial organization, are in competition in the copper oxides
– an idea that may break the logjam on the mystery of high-temperature
superconductivity.
“Heating a normal superconductor above its critical temperature
results in a normal metallic behavior, but heating a high-temperature
superconductor above its critical temperature results in a non-metallic
state of electrons called the pseudogap state,” said physics professor Ali Yazdani, a Willett Faculty Scholar at Illinois and senior
author of the paper. “We have examined for the first time the
motion of electrons in this mysterious pseudogap state on the nanometer
scale.”
Yazdani and graduate students Michael Vershinin and Shashank Misra used
a scanning tunneling microscope to map electron waves in cuprate superconductors
at high temperatures. “Comparing maps of electron waves in both
the superconducting and the pseudogap state, we have found that electrons
in the pseudogap state organize into a checkerboard pattern,”
Yazdani said. “This pattern appears to be the result of competing
forces felt by the electrons, such as Coulomb repulsion because of their
charge and magnetic interactions resulting from their spins.”
Regardless of the specific cause of the local ordering, “our experimental
observations provide new constraints on the potential theoretical description
of the pseudogap state in the cuprates and how it transforms into superconductivity
when we cool the cuprate samples,” Yazdani said.
Pattern formation of electron waves in high-temperature copper-oxide
superconductors has long been anticipated theoretically, and Illinois
physics professor Eduardo Fradkin contributed to the theoretical work.
However, the experimental discovery of such pattern formation was made
possible by a new generation of STM designed by Yazdani’s group
to operate at temperatures above the superconducting transition temperature.
Collaborators on the pattern-formation project also included colleagues
at the Central Research Institute of Electric Power Industry in Japan.
The National Science Foundation, Office of Naval Research and the U.S.
Department of Energy funded the work.
