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Superconductors go fractal

Oxygen atoms arrange themselves in a self-similar pattern

Superconductors go fractal

A new experiment using powerful X-ray beams has found a surprising pattern lurking in a superconductor, a material that conducts electricity without energy-sapping resistance. In a particular kind of superconductor, oxygen atoms are physically arranged as a fractal, showing the same pattern at small and large scales. 

Fractals have been spotted in places as diverse as broccoli, Englands coastline and financial markets. Here, the fractal pattern boosts the efficiency of the superconductor, scientists report August 12 in Nature.

The new study is experimental physics at its best, says physicist Jan Zaanen of Leiden University in the Netherlands, who wrote an accompanying article in the journal. A new machine comes on line, and it produces a surprise nobody expects.

Though the researchers dont yet know how the pattern forms or why it enhances superconductivity, they hope the discovery will help in the quest to develop superconductors that work at room temperature, says study coauthor Antonio Bianconi of Sapienza University of Rome. Physicists have been pushing to make superconductivity happen at higher temperatures, but the top performers are still stuck about halfway between absolute zero and room temperature.

Looking at a copper-oxide superconductor that can perform at approximately -233 degrees Celsius, Bianconi and his team developed a new technique to determine the detailed structure of its atoms. They bombarded the superconductor with powerful X-rays generated at the European Synchrotron Radiation Facility in Grenoble, France. The resulting diffraction pattern revealed atoms locations.

The team knew the material was made like a layered cake, with layers of superconducting copper oxide alternating with spacer layers. At higher temperatures, oxygen atoms tend to roam around in the spacer layer. But when temperatures drop, they settle down. These oxygen atoms and the electrons they bring to what would otherwise be vacancies are thought to contribute to the drop in resistance that accompanies superconductivity. But until now, no one had been able to see the structure with high resolution.

Bianconi and his team got a shock when they realized the pattern formed by the once-roaming oxygen atoms was fractal. The pattern looked the same at the 1-micrometer scale as it did at the 400-micrometer scale.

This self-similarity was completely unexpected in superconductors, Bianconi says. We were very astonished. We couldnt believe our eyes, he says. This is not an area where we expected to see a fractal pattern.

To see whether the fractal pattern was important, the team interfered with it by heating and then quickly cooling the superconductor. Crystals with stronger fractal patterns performed better as a superconductor at higher temperatures than those with weaker fractal patterns. The fractal pattern enhanced the superconductors performance, the team concluded.

The finding is very interesting, since it provides a much-welcomed fresh view of the high temperature superconductivity problem, comments physicist Elbio Dagotto of the University of Tennessee in Knoxville and the Oak Ridge National Laboratory.

Figuring out why the fractal pattern forms in these copper-oxide crystals and how it influences the superconductivity are the next big questions, Bianconi says. Once the details are uncovered, researchers could control the arrangement of oxygen atoms to design better copper-oxide superconductors perhaps even those that operate at room temperature.