Pushing the boundaries of high-temperature superconductors
Yimei Zhu (left) and Robert Klie
Click here for a high resolution photograph.
A collaboration led by scientists at BNL has
revealed a new mechanism that explains why
adding calcium to a high-temperature superconductor
increases its current-carrying capacity. The
findings refute the current explanation and open
the door for similar additives with potentially
better current-boosting abilities. The study, which
was supported by the Office of Basic Energy Sciences
within DOE’s Office of Science, is published
in the May 26, 2005, edition of Nature.
In theory, high-temperature superconductors
conduct electricity with no resistance. But the most
practical, inexpensive high-temperature superconducting
materials — those suitable for applications
such as electronic devices and power lines — are
made of many tiny crystalline grains. The boundaries
between grains act like barriers to electric
charge carriers, impeding the flow of current.
This is the case for the superconducting material
in this study, known as YBCO for its constituent
copper, and oxygen.
“At YBCO grain
atoms replace some
of the barium and
copper atoms,” said
the paper’s lead
author, Robert Klie
of BNL’s Center for
“Where the atoms
are tightly packed,
a calcium atom
replaces a larger
barium atom, relieving
In loosely packed areas,
the calcium replaces
a smaller copper
atom, which relaxes
that are nearby.”
regulate the atomic structure at the boundaries,
providing additional “pathways” for electric
charge carriers to pass from grain to grain.
“This finding is surprising because we thought
only calcium could improve the grain-boundary
conductivity of YBCO, but our discovery means
that similarly sized elements could be equally or
more effective,” said Klie.
Klie and CFN’s Yimei Zhu, one of the paper’s
co-authors, made the discovery using the scanning
transmission electron microscope at Oak
Ridge National Laboratory.
As part of this ongoing collaborative research,
the YBCO sample was fabricated at the University
of Göttingen in Germany and its electronic
properties were previously measured at Brookhaven
by Zhu’s group. The collaboration also includes
researchers from Vanderbilt University, the
University of California at Davis, and The University
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