New research reveals subtlety of superconductivity
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ARGONNE, Ill. (March 20, 2007) — Argonne 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 temperature, up to 164 K (about 165 below zero F), 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 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, 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 the Advanced
Photon Source (APS) at Argonne,
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 and Peter Lee. 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 the U.S. Department of Energy's Office of Basic
Energy Science,
the U.S. National Science Foundation and the Japan
Society for the Promotion of Science.
Argonne National Laboratory brings
the world's brightest scientists and engineers together to find exciting and
creative new solutions to pressing national problems in science and technology.
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and applied scientific research in virtually every scientific discipline. Argonne
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and federal, state and municipal agencies to help them solve their specific
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the U.S.
Department of Energy's Office
of Science.
For more information, please
contact Steve McGregor (630/252-5580 or media@anl.gov)
at Argonne.
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