Brookhaven Lab Direct Magnetic Measurements Question Assumptions About High-Tc Superconductors
03 August 2009
|A single crystal of BSCCO showing the typical size achieved by the Brookhaven team. Source: BNL. Click to enlarge.|
Scientists at the US Department of Energy’s (DOE) Brookhaven National Laboratory (BNL) have grown crystals of a high-temperature (high-Tc) superconductor material that are large enough to directly measure the material’s magnetic properties. These measurements, published online on 2 August 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.
The Brookhaven team studied the copper-oxide superconductor that has undergone the most extensive electronic analysis of any high-Tc material. Abbreviated as BSCCO, the material contains bismuth, strontium, and calcium in addition to copper and oxygen (Bi2Sr2CaCu2O8+δ).
“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). 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.—Brookhaven physicist Guangyong Xu
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.—John Tranquada
This research was funded by the Office of Basic Energy Sciences within DOE’s Office of Science.
Making and Analyzing the Crystals. “It’s very easy to produce the small crystals required for electronic studies, but it is very difficult to grow the large BSCCO single crystals with millimeter thickness that are required for magnetic analysis,” said Brookhaven physicist Genda Gu, an expert in crystal growth. “We developed a special technique and operated two specialized furnaces continuously—24 hours a day/7 days a week—or two years to grow the large crystals used in this study.” “No one else in the world has matched this feat,” said team leader John Tranquada.
To measure the fluctuating magnetism in these crystals, the scientists used a neutron scattering spectrometer at the Rutherford Appleton Laboratory in the UK. As beams of neutrons are scattered off the crystal sample, detectors pick up subtle changes in the deflected particles’ energy and momentum to reveal information about the sample’s magnetic properties. Vibrations of the crystal lattice can obscure the magnetic fluctuations, so careful analysis is needed to separate the “signal” from this background. The scientists performed additional measurements at the Institut Laue-Langevin in France to verify the identity of the magnetic fluctuations.
Finally, to check the magnetic measurements against predictions made by calculations using data from electronic studies, the scientists looked for changes in the magnetic response over a range of temperature, from the non-superconducting state to below the transition temperature (Tc) where the material becomes a superconductor.
Unlike the dramatic changes observed in electronic behavior as the material is cooled below the transition temperature, there were only minor changes in magnetic behavior. This finding challenges the validity of the most popular theoretical models currently used to predict magnetic properties from electronic measurements.
Guangyong Xu, G. D. Gu, M. Hücker, B. Fauqué, T. G. Perring, L. P. Regnault & J. M. Tranquada (2009) Testing the itinerancy of spin dynamics in superconducting Bi2Sr2CaCu2O8+δ. Nature Physics doi: 10.1038/nphys1360
What are the odds.
I lookes JUST like a quarter.
Posted by: ToppaTom | 03 August 2009 at 10:27 PM
Need for high temp super conductors for energy distribution is eliminated by distributed power networks. Removing the distance between single point power stations to consumers zeros the need for low loss transmission systems. While these may still be needed for large consumers (e.g. industry) - residences with RPUs have no need for expensive grid materials.
Brookhaven does good work in multiple areas - haven't heard much from their HIRC program lately.
Posted by: sulleny | 04 August 2009 at 02:45 PM
To answer my own question and correct the misspelled acronym - RHIC has turned gold ions into perfect liquid matter. Indicating that at sub-sub atomic levels quarks and binding forces take on properties of fluids and not gas.
All portends a future where alchemy is within the realm of plausibility.
Posted by: sulleny | 05 August 2009 at 11:42 PM
There are other uses for High temperature superconductors than transmission, where small scale devices are almost practical (but still lab-based) and important. Imangine being able to store 100kWhr of electricity in something the size of a bucket (including a liquid nitrogen flask, if needed). That would knock the socks off all current and projected battery-electric vehicle technology.
The fact that the magnetic properties of this material don't follow conventional understanding may be a hurdle to developing such devices.
Posted by: Floatplane | 12 August 2009 at 01:43 PM