The Path of Least Resistance
Four past and present Wigner fellows have investigated high-temperature superconducting materials.
Specht encountered little resistance to his passionate interest in science, however. His high-school physics teacher convinced him that he "could be a physicist and still be cool."
Since becoming a Wigner fellow in 1987, Specht has used X rays to study high-temperature superconductors, which offer almost zero resistance to the flow of electricity. These superconductors include materials partly developed at ORNL that will likely be used in high-temperature superconducting wires and cables expected to become commercial this decade.
Specht obtained a B.S. degree in physics from Berkeley and a Ph.D. degree in physics from the Massachusetts Institute of Technology. His Ph.D. thesis research consisted of experiments in which X rays were scattered from thin films to study their structural changes during melting. He performed these studies at synchrotron X-ray facilities at Stanford University and Brookhaven National Laboratory. ORNL's Cullie Sparks, who at the time was conducting experiments at the Brookhaven synchrotron, encouraged Specht to apply for a Wigner Fellowship.
"As a Wigner Fellow, I studied superconductivity and X-ray scattering from surfaces," Specht says.
In the early 1990s ORNL researchers developed the RABiTS™ (rolling-assisted biaxially textured substrates) technique for making substrates for high-temperature superconductors. Because tapes made from nickel alloys on which thin films were deposited varied in composition and structure, Specht and other researchers learned for the first time how to characterize RABiTS tapes.
Specht developed two new X-ray scattering instruments to assess the quality of RABiTS superconductors. One instrument measures the crystal orientation in buffer layers along long tapes. The other monitors the growth of films in situ after precursor material is deposited on the nickel alloy substrate.
"These measurements are important," Specht says. "They show whether the proper crystal orientation is in place for transferring the texture from the nickel alloy substrate through the buffer layers to the superconducting oxide layer on top." The top layer is usually yttrium-barium-copper oxide, or YBCO. Specht's work set the standard for an emerging field of study.
John Budai has also been involved in developing new experimental, X-ray synchrotron techniques since becoming a Wigner fellow in 1984. Budai likewise has applied these techniques to various materials, including oxide films in high-temperature superconductors. While examining YBCO films grown on silver foils in 1990, he discovered they were aligned in particular crystal orientations with respect to the metal. Budai developed an explanation for this alignment and described how superconductivity could be significantly enhanced by growing films on roll-textured metal foils. His work helped motivate the development of "textured templates," the concept behind RABiTS research.
During his 20-year career at ORNL, Budai has also used X-rays to study quasicrystals, ion-implanted nanocrystals, and mesoscale microstructures. "I now use microdiffraction to make three-dimensional movies of grain growth in polycrystalline aluminum," he says.
A native of Burlington, Vermont, Budai majored in physics and math at Dartmouth College and earned his Ph.D. in physics at Cornell University, where he helped build one of the first instruments at the CHESS synchrotron facility. There he met ORNL's Ben Larson and Cullie Sparks, who were using CHESS for their research. They told Budai about the Wigner Fellowship.
After a postdoctoral stint at Bell Labs, Budai accepted a Wigner position with Larson in ORNL's Solid State Division. "Over the years, I've been involved in developing X-ray techniques at each new-generation synchrotron source," Budai says, listing facilities at Cornell and Stanford universities and then Brookhaven and Argonne national laboratories. Recently, he has been involved in developing synchrotron X-ray microdiffraction techniques at Argonne's Advanced Photon Source for "mesoscale" materials studies. Budai recently used X-ray microdiffraction to revisit his previous studies of superconducting materials.
Thomas Maier has joined the staff of ORNL's Computer Science and
Mathematics Division (CSMD) after completing a very productive
two years as a Wigner fellow. He submitted at least a half
dozen scientific papers. He wrote a 50-page review of a computational
method of simulating a system of strongly interacting electrons,
including those in high-temperature superconducting materials.
Maier has been enjoying the opportunity to use the Cray X1, a high-performance computer at ORNL, in his search for a mechanism to explain how high-temperature superconductivity works. With this powerful tool he showed in a systematic study that the widely accepted two-dimensional Hubbard model is able to describe high-temperature superconductivity.
Maier earned all his degrees, including his Ph.D. in physics in 2001, at the University of Regensburg in his hometown in Germany. His doctoral thesis about correlations of interacting electrons netted him the Röntgen Prize for young scientists.
He came to ORNL as a Wigner Fellow in March 2003 following two years of postdoctoral computational condensed matter research with Professor Mark Jarrell in the University of Cincinnati's Department of Physics. Maier's work there on simulations of high-temperature superconductors resulted in 10 first-author publications in top physics journals.
Maier now works with Thomas Schulthess, a friend and colleague of Professor Jarrell and a researcher in CSMD's Computational Materials Sciences Group. The two approach high-temperature superconductivity from different directions. They hope that together they can arrive at a more complete picture of this phenomenon.
Maier takes a many-body approach in which he describes 1023 electrons in terms of quantum mechanics, such as the probability that any particular electron is in a certain position for a certain time. As superconductors are chilled to low enough temperatures, their electron systems, which have been disordered, "undergo a phase change" and exhibit collective behavior. Maier views each electron both as a single particle with a charge and spin and as a wave. High-temperature superconductors are close to being magnetic, so Maier thinks that magnetic fluctuations are the keys to understanding high-temperature superconductivity.
Giant Proximity Effect
Gonzalo Alvarez, a current Wigner fellow, and his Ph.D. thesis advisor, Elbio Dagotto, recently simulated on the computer the "giant proximity effect" where a high-temperature superconductor placed close to a normal material makes the normal material superconducting. "Our prediction inspired an important experiment that demonstrated the existence of this effect," Alvarez says. Ivan Bozovic performed this experiment at Brookhaven National Laboratory, and Alvarez and Dagotto published an article on the experiment in the December 2004 issue of Physics World.
Alvarez, who has B.S. and M.S. degrees in physics from the University of Montevideo in Uruguay, completed his doctoral research on magnetic semiconductors with Professor Dagotto, recently named an ORNL-University of Tennessee Distinguished Scientist. As a result of his thesis work in physics at the National High Magnetic Field Laboratory at Florida State University, Alvarez received an Outstanding Dissertation in Magnetism Award from the American Physical Society.
Alvarez has written a computer code that allows simulation of magnetic, semiconducting materials that may be useful in creating faster, smaller, and cheaper devices for storing, retrieving, and processing information—future technologies that may prove irresistible.
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