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Supraconductivité - meilleure compréhension
du phénomène
http://www.sciencemag.org/sciencexpress/recent.shtml
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.
