Understanding the origin of superconductivity -- the ability of some materials to conduct electricity without losing energy -- will help scientists improve magnetic resonance imaging (MRI) and the efficiency of electric power transmission, and build smaller, more powerful electronic devices.
"Scientists usually assume that superconductivity arises from electrons coupling in pairs," said Yimei Zhu, a physicist at Brookhaven's Advanced Electron Microscopy Facility and lead author of the study. "Though this is the case for most superconductors, it has not been shown yet how electrons contribute to superconductivity in magnesium diboride. So we decided to look more closely at this material's electronic structure."
Since the discovery of superconductivity in MgB2, Brookhaven theoretical scientists led by physicists James Davenport and Guenter Schneider have made extensive calculations involving interactions between electrons or between electron "holes," which are empty locations that could be filled by electrons. According to one of the most prevalent theories, superconductivity in MgB2 arises from interactions between holes. Also, because MgB2 is made of alternating planes of boron and magnesium atoms aligned parallel to one another, these holes are expected to interact more easily within the planes than between adjacent planes.
"Compared to other superconductors, MgB2 has a relatively simple structure," said Johan Tafto, a physicist at the University of Oslo and one of the team members. "So scientists hope to get more insight into superconductivity by focusing their attention on a simple compound rather than on more complex ones."
To test the theoretical predictions about MgB2, the scientists examined the electron and hole structure of the substance using two complementary techniques. In the first technique, called x-ray absorption spectroscopy, the scientists used very intense x-rays generated by the National Synchrotron Light Source (NSLS) at Brookhaven and a unique NIST x-ray detector. When the x-rays enter the sample, the electrons inside the sample absorb the x-rays and are ejected out of their original positions.
"When these ejected electrons fall into the holes, they reveal the number and density of these holes in the MgB2 sample," said Daniel Fischer, a physicist at NIST who has been working with the x-ray absorption technique for the last 18 years at the NSLS.
The second technique, called electron energy loss spectroscopy, uses state-of-the-art transmission electron microscopes (TEMs) at Brookhaven. Unlike optical microscopes, which use visible light, an electron microscope projects electrons toward the sample. These electrons transfer some of their energy to electrons in the sample, which bump around the sample atoms and reveal the positions of electronic holes in the MgB2 sample.
"We needed to use both techniques because they complement each other very well and lead to a very accurate determination of the distribution and number of electron holes in magnesium diboride," said Zhu, who leads Brookhaven's TEM group and has been investigating the electronic structure of materials at the nanoscale (one billionth of a meter) for the last 20 years.
The results agree with the theoretical predictions by showing that interactions between holes in the boron planes do occur in MgB2, and that superconductivity stems from such interactions. Said Tafto, "As we gain more understanding of the properties of magnesium diboride at the atomic level, I am confident that, in the near future, we will be able to relate them to macroscopic properties such as superconductivity -- and maybe explain the origin of superconductivity in general."
This work was funded by the U.S. Department of Energy, which supports basic research in a variety of scientific fields, and the U.S. Department of Commerce.
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The U.S. Department of Energy's Brookhaven National Laboratory (http://www.
Note to local editors: Yimei Zhu lives in Stony Brook, New York.