A radical theory of high-temperature superconductivity--electric conduction with no resistance at comparatively high temperatures--has won its first experimental support. In today's Science, researchers describe how these superconductors are completely different from conventional low-temperature superconductors. Experts say the findings are the first convincing evidence that superconductive compounds work according to two entirely different mechanisms.
There's no mystery to low-temperature superconductors. When a superconducting material is cooled to a few degrees above absolute zero, some of its electrons hang out in pairs and "condense" into a single quantum state--similar to how steam condenses to water when cooled. Arranged like that, electrons are difficult to scatter and can move with no resistance. A few years ago, some physicists suggested that high-temperature superconductors--which operate at about -200 degrees Celsius--worked backwards: most of the electrons pair, but only a few condense. An ingenious explanation for this odd behavior was provided by the controversial charge-stripe theory, which holds that charge and spin can be separated on the electron.
Zhi-Xun Shen of Stanford University and his colleagues wanted to test some of the predictions of the new theory. They bombarded a high-temperature superconductor called bismuth-strontium-calcium-copper oxide with photons. By measuring the energy and momentum of electrons knocked out of their orbits by the photons, the researchers were able to confirm that all electrons were paired. Unlike in low-temperature superconductors, the pairings were somehow taking place between electrons of completely different momenta. Moreover, the typical momentum transfer between the electrons and the material was exactly that predicted by charge-stripe theory.
If that momentum had been missing, stripe theory would be dead, says Steven Kivelson, a physicist at the University of California, Los Angeles, and an originator of the theory. The experiment isn't the last word on stripe theory, he says, but the evidence of paired electrons proves that standard theory fails to explain high-temperature superconductivity.