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Subtlety of superconductivity revealed
Argonne, IL, 21 March 2007:
Argonne National
Laboratory scientists helped lead the superconducting revolution 20 years
ago this month with their landmark solution of the structure of the most widely
known high-temperature superconductor YBa2Cu3O7. Now, they have solved another
tantalizing superconductivity mystery: how a subtle change in the structure of
so-called electron-doped superconductors switches the phenomenon of
superconductivity on and off.
Superconductivity is the loss of all resistance
to the flow of electric current at very low temperature, a surprising phenomenon
with the potential to save enormous quantities of energy if it can be applied to
the electric power grid. Twenty years ago, a new class of materials that
superconduct at dramatically higher temperatures, up to 164K, was discovered,
promising widespread energy-saving applications. Most of these superconductors
are “hole-doped,” so named because their superconductivity is triggered by
removing electrons (adding “holes”) to an insulating magnetic compound. A few of
the high-temperature superconductors, however, are “electron-doped,” requiring
the addition of electrons to produce superconductivity.
The mystery of these electron-doped
superconductors is that in addition to electron doping, they must be heated to a
high temperature during their manufacture to enable them to superconduct. No one
could understand why the heat treatment was necessary; it did not seem to alter
the structure or composition of the material, yet it dramatically transformed
the material from an insulator to a superconductor.
“Our discovery opens the door to understanding
how electron-doped superconductors work,” said Stephan Rosenkranz (MSD), an
Argonne scientist on the experimental team. “We didn't realize the interplay of
structure and superconductivity was so subtle. But now that we know what is good
for superconductivity, we can vary the amount of the good and bad stuff in
systematic ways to find out what makes them tick.”
The research team lead by scientists from
Argonne, the University of Tennessee, and Brigham Young University found that
heating the electron-doped superconductor
Pr1-xLaCexCuO4 repaired subtle flaws in the
microscopic structure of the material. These flaws are so delicate that their
repair by heating escaped detection for nearly two decades. The Argonne team
found them by effectively looking with two magnifying glasses. They correlated
measurements of copper atom positions using X-rays at Argonne's Advanced Photon
Source (APS) with measurements of the oxygen atom positions by neutrons at the
National Institute for Standards and Technology Center for Neutron Research.
The combination of these two measurements
revealed a small change in the placement of both copper and oxygen atoms taking
place during the heat treatment, leading to a perfect structure and
superconductivity. Furthermore, the change is fully reversible: The material
could be cycled from the flawed to the perfect structure, switching the
superconductivity off or on.
The X-ray experiments for this work were led by
Rosenkranz and Argonne's Peter Chupas (XSD) and Peter Lee (XSD). They used the
high-intensity X-ray beams produced by the APS to determine the precise location
and type of each atom in the crystal structure. Branton Campbell, another member
of the research team and former postdoctoral researcher at Argonne, now at
Brigham Young University, compared this technique to putting an object on a
table, hitting it with baseballs thrown from different angles, and then using
the marks left where the bounced balls struck the surrounding walls to figure
out what the object looks like. Other members of the experimental team include
Pengcheng Dai from the University of Tennessee and Oak Ridge National
Laboratory, Hye-Jung Kang, now at the National Institute of Standards and
Technology, and scientists from Tokyo's Central Research Institute of Electric
Power Industry, who made the samples.
The detailed results of these findings were
published in the
Nature Materials paper “Microscopic Annealing Process and its Impact on
Superconductivity in T'-Structure Electron-Doped Copper Oxides,” which is
available online. Funding for this research was provided by DOE's Office of
Basic Energy Science, the U.S. National Science Foundation and the Japan Society
for the Promotion of Science.
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