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Superconductivity is one of those nearly magical properties that seem to defy all intuition for how the physical world ought to work. In a superconductor, electric currents flow without resistance—an electron passes unimpeded through the material like a torpedo through some frictionless ocean. After discovering the phenomenon in 1911 Dutch physicist Heike Kamerlingh Onnes showed that an electric current in a closed superconducting loop of mercury would keep flowing long after the driving potential was removed; he demonstrated his discovery by carrying such a persistent current from the Netherlands to England.
Since then physicists have discovered superconductors based on other metals and even ceramics. The latest entry is one rooted in a hydrocarbon, which superconducts at a relatively high temperature compared with elemental metals.
The finding comes from a team of Japanese researchers, who report in the March 4 issue of Nature that the hydrocarbon molecule picene, ordinarily a semiconductor, becomes a potential superconductor when interspersed with an alkali metal such as potassium or rubidium. (Scientific American is part of Nature Publishing Group.) And when picene (C22H14) is doped with potassium, it superconducts at a relatively toasty 18 kelvins (–255 degrees C). Although the transition temperature is far below that of superconductors based on the ceramiclike copper oxides, where electricity can flow without resistance at around 160 kelvins, or –113 degrees C, the discovery could lead to the development of new, high-temperature superconductors rooted in organic chemistry.
Yoshihiro Kubozono, a professor of solid-state chemistry and physics at Okayama University in Japan and a study co-author, says that picene is the first example of a superconducting hydrocarbon. (Researchers have had similar success coaxing strictly carbon-based compounds to superconduct, for instance by doping the fullerene C60 with potassium.) Hydrocarbons are better known for their combustibility than their electric properties: Gasoline is a blend of hydrocarbons, including the familiar octane (C8H18); the natural gas that heats about half of U.S. homes is mostly methane (CH4).
Picene occurs naturally in coal tar and is found in residues from petroleum refining, but Kubozono and his colleagues synthesized the compound for the new study. The picene molecule is flat, and it forms crystals comprising stacked layers. Cooking the compound for days with an alkali metal such as potassium introduced metallic atoms between the hydrocarbon layers, which enhanced its conductivity in the planes parallel to the picene slabs.
Kubozono says that he and his colleagues are now doping picene with other metals—in the new study, the group reports trying sodium and cesium, in addition to potassium and rubidium—to further boost the material's superconducting temperature. They are also experimenting with other hydrocarbons to see which ones can superconduct, Kubozono says.
Brian Maple, a physicist at the University of California, San Diego, says the research is an encouraging step in expanding the realm of superconductivity, particularly in the high-temperature regime. "This is just another example of how prevalent superconductivity is amongst a broad range of materials if you can make them into metals," he says. "Globally, I think this just shows that one should be very optimistic about finding superconductors with higher transition temperatures by looking in directions that heretofore one would have thought wouldn't be very promising."
