That explanation sufficed until 1986, when physicists discovered new materials that became superconductors at temperatures above 100 kelvins. These "high-temperature superconductors" were made of layers of copper alloys sandwiched between layers of nonconducting material that were laced, or "doped," with trace amounts of material that could contribute a few extra electrons to the mix.
Physicists quickly realized their existing theories of superconductivity could not explain what was happening in the new materials. For one thing, the undoped versions of the compounds didn't conduct electricity at all. Their electrons -- due to their desire to repel one another -- tended to lock themselves a comfortable distance away from their neighbors. This locked pattern was dubbed "Mott localization," which gives rise to an insulating state.
In 2008, the search for answers took another turn when a second class of high-temperature superconductors was discovered. Dubbed the pnictides, these new iron-based superconductors were also layered and also needed to be doped. But unlike their copper cousins, undoped pnictides were not Mott insulators.
"Mott localization doesn't occur in the undoped pnictides, but there is considerable evidence that the electrons in these materials are near the point where Mott localization occurs," Si said. "This proximity to Mott localization endows the system with strong quantum magnetic fluctuations, which we believe underlie the high-temperature superconductivity in the pnictides."
In all high-temperature superconductors, the iron or copper atoms in the conducting layers form a grid-like, checkerboard pattern.
In work published earlier this year, Si and colleagues replaced arsenic atoms in one of the intervening layers of a pnictide with slightly smaller phosphorous atoms. This subtle change brought the iron atoms in the checkerboard a tad closer together, and that
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