August 29, 2008

Single Crystal Superconductors and Quantum Traffic Jam analysis of Superconductor electrons

Single Crystal Superconductor - clear scientific study of new Iron Superconductor
Research groups have created versions of a class of widely studied superconducting compounds that are each one continuous crystal, rather than composed of many crystalline grains. These single-crystal materials are important achievements because they display better properties than polycrystalline types and are easier to study.

The major result published in the first paper is a determination of the highest magnetic field the superconducting state can withstand (called the upper critical field) and an evaluation of how anisotropic it is. Some superconductors are extremely anisotropic, and so fully understanding them requires good measurements of this behavior.

In their second paper,they showed that their single-crystal growth method can be applied to another iron arsenide compound, SrFe2As2 (where "Sr" is strontium).

In the third paper, the researchers discuss a new member of the iron arsenides. The compound is CaFe2As2 ("Ca" is calcium) and it had never before been identified as a member of that particular crystallographic family. CaFe2As2 undergoes exceptionally clear changes to its structure and magnetic behavior at 170K (-150F).

The fourth paper in the series further documents their study of CaFe2As2, detailing exactly why the material can be classified as a superconducting iron arsenide. The researchers found that at modest pressures the structural and magnetic changes that occur at 170 K are suppressed and the material becomes a superconductor.

"This means that, from a basic science point of view, CaFe2As2 offers a clean model system that seems to encompass all of the salient features of these compounds (structural, magnetic and superconducting phase transitions) and that its behavior can be tuned with pressure," Canfield said. "This is a very exciting discovery that may help guide the way to understanding this new family of superconductors."

Quantum Traffic Jam in Superconductors
Scientists at the U.S. Department of Energy's Brookhaven National Laboratory, in collaboration with colleagues at Cornell University, Tokyo University, the University of California, Berkeley, and the University of Colorado, have uncovered the first experimental evidence for why the transition temperature of high-temperature superconductors — the temperature at which these materials carry electrical current with no resistance — cannot simply be elevated by increasing the electrons' binding energy.

Doping superconductors creates holes and it is like taking some cars off the highway during rush hour. All of a sudden, the traffic starts to move.

The research shows that what is believed to be required to increase the superconductivity in these systems — stronger magnetic interactions — also pushes the system closer to the 'quantum traffic-jam' status, where lack of holes locks the electrons into positions from which they cannot move

Better theory for Superconductor Electrical Grid designs
John R. Clem, a physicist at the U.S. Department of Energy’s Ames Laboratory, has developed a theory that will help build future superconducting alternating-current fault-current limiters for electricity transmission and distribution systems. Clem’s work identifies design strategies that can reduce costs and improve efficiency in a bifilar fault-current limiter, a new and promising type of superconducting fault-current limiter. “I was able to theoretically confirm that planned design changes to the current bifilar fault-current limiter being developed by Siemens and American Superconductor would decrease AC losses in the system,” said Clem. “My calculations are good news for the future of the device.”
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