New research reveals subtlety of superconductivity
Date: Tuesday, March 20, 2007 @ 20:12:51 MST
Diagram shows the typical T' structure of PLCCO superconductors, consisting of CuO2 sheets separated by thin sheets of rare-earth oxide. Credit: Argonne National Laboratory
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
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
Source: Argonne National Laboratory