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Super Finding on Superconductivity

Scientists take a crucial step toward creating new materials with zero resistance at room-temperature
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J. W. Stewart
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QUASIPARTICLE CLOUD appeared over zinc impurities in a high Tc superconductor, confirming several theories about their underlying physics.

That fuses blow and wires burn out is as inevitable as taxes and death. But that wouldn¿t be the case if scientists could create materials that are superconducting at room temperature. Then, even hair-thin wires might carry loads of current without resistance or losing any energy to heat. And a host of new technologies, among them magnetically levitated high-speed trains, might become a reality.

In fact, scientists have recently come one step closer to realizing room-temperature superconductors¿by learning more about what is, for now, the next best thing: so-called high critical temperature, or Tc, superconductors. These copper oxide ceramics, discovered in 1986, are far more practical than metal alloy superconductors. Whereas the metal alloys must be cooled to temperatures near absolute zero before their electrical resistance drops to zero, the ceramics need not be quite so cold.

And new results have confirmed that these ceramics are different in another crucial aspect. Seamus Davis and his colleagues at the University of California at Berkeley, Lawrence Berkeley National Laboratory and the University of Tokyo have shown that a different mechanism accounts for superconductivity in high Tc ceramics. They reported their findings in the February 17th issue of Nature.

BSCCO

BSCCO is a complex, layered copper oxide that becomes superconducting at relatively high critical temperatures.

Davis and company knew that in metals, the property arose from vibrations in the crystal lattice. In ceramics, they suspected strongly interacting paired electrons moving through copper oxide layers¿but needed proof. For that, they decided to disturb the copper oxide layers in a high Tc superconductor called BSCCO, which also contains layers of bismuth oxide, strontium oxide and calcium.

The group replaced the copper atoms in BSCCO¿the sources of any paired electrons¿with zinc impurities sure to scatter the electrons. They watched what happened using a home-built scanning tunneling microscope (STM). Davis and his colleagues designed this instrument to operate at temperatures as low as 0.25 degree above zero. An ultrafine tip, measuring only a few atoms wide, passes over a sample surface, generating an electric current through which electrons can "tunnel." This tunneling causes slight displacements in the tip, which are translated into topographic images.

The images the STM generated in this experiment revealed bright, cross-shaped clouds of quasiparticles, electron excitation states that collectively act like a free electron, above the zinc atoms. These clouds extended out for about 10 angstroms, and their four-fold symmetry indicated the presence of so-called d-waves, functions of the quasiparticles¿ angular momenta. These d-waves¿predicted by theory¿were the evidence the group needed to verify that high critical temperature superconductivity is mediated by strongly interacting paired electrons.

second cloud

SECOND CLOUD, rotated 45 degrees from the first and three times as large, was not predicted by theory.

"This is the first demonstration of quasiparticle imaging and tunneling spectroscopy at individual impurity atoms in complex materials like the cuprate-oxides," Davis adds. "The experimental design is simple¿put one impurity atom at an important site and see what happens¿but the technique is so powerful it opens completely new avenues of research, including the potential to develop exotic new materials. We¿ve shown that even materials that are structurally and electronically very complex can be studied one atom at a time."

Davis¿s images revealed additional information. Aside from confirming the actual physics of high Tc superconductivity, the pictures also validated the so-called Swiss Cheese model, which describes the sizes and shapes of nonsuperconducting regions around the zinc impurities. And the pictures showed a fainter second, cross-shaped, quasiparticle cloud over the zinc impurities, which was rotated 45 degrees and three times larger than the first.

This mysterious feature is only one of several puzzles the group hopes to solve in future months. They plan to continue their work using different ceramic high Tc superconductors and impurities other than zinc. They will also test a new superconducting tip made for their STM. This tip, the first of its kind, is made from niobium and operates in a magnetic field as powerful as 7.2 Tesla. "No one knows the precise recipe for making new higher temperature superconductors," Davis says. But he is sure going to try to find it.

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