Secrets of High-Temperature Superconductors

Feb. 8, 2007
Certain oxides of copper called cuprates doped with small amounts of other elements become superconducting at temperatures to 134°K, significantly warmer than the near-absolute-zero temperature of liquid helium needed to chill down other superconductor types.

Doping disrupts the crystal structure of the copper oxide, creating "holes" where electrons ought to be, and this somehow facilitates superconductivity. Physicists have puzzled over the fact that at a certain low level of doping, many cuprates cease to superconduct, yet at levels above and below this, superconductivity returns. Previous experiments on these high-temperature superconductors showed that electrons bind together in pairs. The energy needed to pull a pair apart (energy gap) depends on direction; a plot of energy versus direction forms a cloverleaf pattern.

Now researchers at Brookhaven National Laboratory and Cornell University have found this same cloverleaf-shaped energy gap in a nonsuperconducting cuprate sample, even at temperatures near absolute zero. The discovery could help shed light on how high-temperature superconductors work.

The group used photoemission spectroscopy to measure the energy and momentum of electrons in the nonsuperconducting sample. A separate experiment characterized a piece of the same crystal with a specially built scanning tunneling microscope so sensitive it can detect the arrangement of electrons in the material. The measurements are said to be the first of a cuprate's electronic structure in which the material's superconductivity did not interfere.

This particular blend of cuprate dubbed LBCO ceases to superconduct after it loses one-eighth of its electrons. Remaining electrons in the material arrange themselves in alternating "stripes" about four atoms wide, and this apparently inhibits superconductivity. Interestingly, both measurement methods reveal the same low-energy electronic signatures, though the reason is unclear. One researcher speculates that the difference between superconducting and nonsuperconducting cuprates lie in the way electrons form pairs. Electrons in certain samples may simply pair too strongly for superconductivity to work.

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