Conquering one of the biggest challenges in the study of high-temperature (high-Tc) superconductors, scientists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory have grown crystals of one such material that are large enough to directly measure the material's magnetic properties. These measurements, published online on August 2 by Nature Physics, cast considerable doubt on some assumptions commonly made in trying to understand the role magnetism plays in these materials' ability to carry current with no resistance. Such materials promise more-efficient, lower-cost energy transmission if they can be made to operate under real-world conditions.
"Many theorists believe that magnetism is important for high-temperature superconductivity, although they don't agree on how it is important," said Brookhaven physicist John Tranquada, who led the research team. Figuring out this puzzle has been complicated by the fact that techniques used to measure materials' magnetic properties require good-quality, large, single crystals - and growing such crystals of high-Tc materials has been a long-term challenge.
Smaller crystals work well for studies of electronic properties, however, so those properties have been characterized for select high-Tc superconductors. Since magnetic properties in conventional metallic conductors are a direct result of those materials' electronic properties, theorists have used the same well-established mathematical approach for deriving magnetism from electronic measurements in high-Tc materials. The Brookhaven team's success at finally growing large crystals of a well-studied high-Tc material offered the first opportunity to directly test the assumption that this approach is valid.
"The calculations based on the material's electronic properties - which change dramatically as the material is cooled and transitions from its electrically resistive state to become a superconductor - predicted there would be a similar large change in magnetic characteristics below the transition temperature (Tc)," said Brookhaven physicist Guangyong Xu. "But our direct measurements of the magnetic properties showed surprisingly little change. This implies that the model the theorists have been using to describe these magnetic properties is incomplete."
It's not that the magnetic properties are completely unrelated to the electronic properties; they are both still part of the same system, the scientists emphasize. Magnetism, after all, comes from the relative arrangements of the directions in which electrons spin, like a collection of tiny bar magnets.
"It could be that the magnetism somehow drives the electronic structure, rather than the other way around - or that something underlying both magnetism and electronic structure influences both but in different ways," Xu said.
"You can think of it as the foreground and the background of a painting," Tranquada suggested. "We are interested in the superconductivity, which is what stands out - the foreground. And we know electrons are involved in that by pairing up to carry current with no resistance. But are those same electrons defining the magnetic properties? Or do other, 'background' electrons define the magnetism?"
The magnetic measurements showed that some of the magnetic characteristics of the original "parent" compound - which is an insulator - remain when the material becomes a superconductor. This suggests that there may be two kinds of electrons: some moving around like waves to carry the current while others remain in relatively fixed positions to produce the magnetism.
Defining these characteristics will be important as scientists search for or try to design new materials that act as superconductors at temperatures appropriate for real-world applications, such as high-efficiency power transmission lines.
"If the dual existence of localized and free-flowing electrons is important, we want to look for other materials that have those characteristics, but transition to superconductivity at even higher temperatures," Tranquada said.
This research was funded by the Office of Basic Energy Sciences within DOE's Office of Science.
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