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 elements: yttrium, barium, copper, and oxygen.

“At YBCO grain boundaries, calcium atoms replace some of the barium and copper atoms,” said the paper’s lead author, Robert Klie of BNL’s Center for Functional Nanomaterials (CFN).

“Where the atoms are tightly packed, a calcium atom replaces a larger barium atom, relieving the strain. In loosely packed areas, the calcium replaces a smaller copper atom, which relaxes strained areas

that are nearby.” The substitutions 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 of Tokyo.

— Karen McNulty Walsh