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When
electrons pair up in alternating layers of copper oxide
(checkerboard), superconductivity happens. Researchers
found that in materials that superconduct at high
temperatures, magnetic excitations (large peak) play a
key role in pairing the electrons. The revealing
evidence was gathered with neutron probes directed at three
carefully aligned crystals of the superconducting material
known as PLCCO.
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GAITHERSBURG,
Md.—When it
comes to superconductivity, magnetic excitations may top
good vibrations.
Writing
in the July 6, 2006, issue of Nature,* scientists
working at the Commerce
Department’s National Institute of Standards and Technology
(NIST) Center for Neutron
Research (NCNR) in collaboration with physicists from the
University of Tennessee (UT)
and Oak Ridge National Laboratory (ORNL) report strong evidence
that magnetic
fluctuations are key to a universal mechanism for pairing
electrons and enabling
resistance-free passage of electric current in high-temperature
superconductors.
An
important missing piece in the puzzle of high-temperature
superconductivity, the finding should boost efforts to develop
a variety of useful technologies now considered impractical
for conventional superconductors, which work at markedly
lower temperatures. Examples include loss-free systems for storing and
distributing electric energy, superconducting digital routers for high-speed
communications, and more efficient generators and motors.
The team was led by Pengcheng Dai, a UT-ORNL joint professor.
“Our
results unify understanding of the role of magnetism in
high-temperature
superconductivity and move the research community one step
closer to understanding the underlying pairing mechanism
itself,” says NIST physicist Jeffrey Lynn, a member
of the collaboration. Better understanding of the mechanism
of high-temperature superconductivity may lead to the discovery
of new materials in which electrical resistance vanishes
at even warmer temperatures.
Objects
of intense scientific and technological interest since
their discovery in 1986, high-temperature superconductors
work their magic in ways different than materials that
become superconducting at significantly colder temperatures,
as first observed in 1911. In these conventional superconductors,
vibrations in the materials’ atomic latticework mediate
the pairing process that results in the unimpeded flow of
electrons.
Scientists have ruled out vibrations, or phonons, as the
likely electron matchmaker in high-temperature superconducting
compounds. And while they have assembled important clues
over the last two decades, researchers have yet to pin down
the electron-pairing mechanism in the high-temperature superconductors.
“Various
experiments and theories have suggested that this resonance—this
sharp magnetic excitation—may
be the glue needed to explain high-temperature superconductivity,
but key pieces of evidence were missing,” explains
lead author Stephen Wilson, a UT graduate student.
Previous
work by other researchers had determined that magnetism played
a role in one of two major classes of high-temperature
superconductors—those engineered
with holes, or occasional vacancies where electrons normally
would reside. But, until this work, carried out at NCNR
and ORNL’s High Flux Isotope
Reactor, the underlying pairing mechanism in the other class—materials
doped with an excess of electrons—eluded detection.
Using
neutron probes, which are extremely sensitive to magnetism,
the team was the first to observe a magnetic resonance in
an electron-doped high-temperature superconductor, in a carefully
engineered compound known as PLCCO. More importantly, the
resonance energy was found to obey a well-established relationship
universal to high-temperature superconductors, irrespective
of type.
This
demonstrated a fundamental link between magnetism and the
superconducting phase, the researchers report. These observations
and findings should open new avenues of research into the
exotic properties of high-temperature superconductors, they
write.
NIST,
the National Science Foundation and the Department of Energy
supported the research.
As a
non-regulatory agency of the U.S. Department of Commerce’s Technology
Administration, NIST promotes U.S. innovation and industrial
competitiveness by advancing measurement science, standards,
and technology in ways that enhance economic security and
improve our quality of life.
*S.D.
Wilson, P. Dai, S. Li, S. Chi, H.J. Kang, J.W. Lynn. 2006.
Resonance in the electron-doped high-transition-temperature
superconductor Pr0.88LaCe0.12CuO4-d . Nature.
July 6, 2006
NOTE TO EDITORS: The University of Tennessee
Office of Communications has issued a complementary news release
at http://pr.tennessee.edu/news. Contact Jay Mayfield, (865)
974-9409, jay.mayfield@tennessee.edu.
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