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August 18, 2010

Roller Coaster Superconductors and Fractals Boost Superconductors

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1. Superconductors can carry electricity without resistance, so they are more efficient than copper wires. However, to attain the superconducting state, these materials have to be cooled below a critical temperature, so-called transition temperature, at which point normal electrical resistance disappears. Developing superconductors with higher transition temperatures is one of physics’ greatest quests. Now, researchers at the Carnegie Institution’s Geophysical Laboratory, with colleagues,* have unexpectedly found that the transition temperature can be induced under two different intense pressures in a three-layered bismuth oxide crystal referred to as “Bi2223.” The higher pressure produces the higher transition temperature. They believe this unusual two-step phenomena comes from competition of electronic behavior in different kinds of copper-oxygen layers in the crystal.


Under normal pressure, the optimally doped Bi2223’s transition temperature is -265°F (108K). The scientists subjected doped crystals of the material to a range of pressures up to 359,000 times the atmospheric pressure at sea level (36.4 Giga Pascal), the highest pressure yet for magnetic measurements in cuprate superconductors. The first higher transition temperature happened at 100,666 atmospheres (10.2 GPa).

“After that, increasing pressures ended up with lower transition temperatures,” remarked Chen. “Then to our complete surprise at about 237,000 atmospheres (24 GPa) the reduction of the transition temperature stopped. Under even more pressure, 359,000 atmospheres, the transition temperature rose to -215°F (136K). That was the highest pressure our measuring system could detect.”

The researchers think that 237,000 atmospheres might be a critical point where pressure suppresses one behavior and enhances superconductivity.

“The finding gives new perspectives on making higher transition temperature in multilayer cuprate superconductors. The research may offer a promising way of designing and engineering superconductors with much higher transition temperatures at ambient conditions,” concluded coauthor Viktor Struzhkin also of Carnegie.


2. Bianconi and colleagues obtained an extremely detailed picture of the superconductor's structure, with high scattering intensity corresponding to greater structural order.

What they found was that this intensity followed a power-law distribution, in other words that the superconductor was made up of a small number of very high-ordered regions and larger numbers of disordered regions. This, they say, is the hallmark of a scale-free distribution, which is typical of a fractal pattern – with the oxygen stripes forming a similar structure on all scales up to 400 µm

The researchers found that this fractal distribution increases the temperature up to which the lanthanum copper oxide remains superconducting. They altered the transition temperature of the sample by heat treatment and then recorded its X-ray diffraction image. Carrying out this process at five different transition temperatures, they found that the higher this temperature the more closely the intensity pattern resembled a power law.


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