Scientists at the U.S. Department of Energy’s Brookhaven National Laboratory, in collaboration with researchers at the Rutherford Appleton Laboratory in the United Kingdom and Tohoku University in Japan, have discovered evidence supporting a possible mechanism for high-temperature superconductivity that had previously appeared incompatible with certain experimental observations.

The scientists were studying a material composed of lanthanum, barium, copper and oxygen (LBCO). The parent compound, LCO, which lacks the barium, is not a superconductor (a material through which current moves with no resistance), nor even a regular conductor. This is because the repulsive forces between the like-charged electrons on adjacent copper atoms keep them separated and somewhat locked into position – unable to carry a current. In this arrangement, the electrons on adjacent atoms align themselves so that their spins alternate in an up, down, up, down fashion, producing a condition scientists call antiferromagnetism.



When barium is substituted for some of the lanthanum atoms, however, the material starts to exhibit superconductivity. Barium has one fewer electron than lanthanum to contribute to atomic interactions, so adding barium adds electron “holes,” or the absence of electrons, to the system. The more barium, the more holes, and the greater the superconductivity – until the composition reaches a point where there is exactly one barium atom for every eight copper atoms: Then the superconductivity disappears. Above this point, as more “holes” (barium atoms) are added, the superconductivity reappears but then gradually declines.



From previous experiments, the scientists know that, at the point where the superconductivity disappears, narrow antiferromagnetic regions of the material are separated by regions with holes, forming alternating “stripes.” Other scientists have proposed that such stripes might be important for superconductivity if the stripes could flow – that is, if the stripes made of electron holes could wiggle through the crystal lattice. But trying to study these materials in their superconducting form is “like looking at the stripes on a waving flag from a great distance using soda bottles as binoculars,” Tranquada said. Because everything is fluid, the pattern is hard to identify.



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