Hydrocarbon Superconductor Discovered

Picene becomes a superconductor below 18 kelvin

Photo: Yosihiro Kubozono

3 March 2010—In a development that if independently confirmed could lead to an entirely new class of superconductors, a group of Japanese scientists—led by Professor Yoshihiro Kubozono of Okayama University—is reporting that a simple hydrocarbon, picene, exhibits superconductivity below 18 kelvin, a relatively high temperature. The Japanese team details its findings in this week’s issue of Nature .

Superconductivity is a phenomenon whereby the resistance of a material to the flow of electricity vanishes. If confirmed, this will be the first time in a decade that a new organic, high-temperature superconductor has been discovered.

”The results look quite interesting, and I would expect [them] to cause a stir in the superconductivity community,” says Jeff Lynn, a physicist at the National Institute of Standards and Technology (NIST), in Gaithersburg, Md., who investigates superconductivity.

Kubozono and colleagues have found that picene becomes a superconductor when it is laced with potassium or rubidium and then chilled. Picene is an organic compound found in crude oil; it is made up of 22 carbon atoms and 14 hydrogen atoms. It looks like five benzene rings—common organic molecules—fused together in a staggered line.

”Most people…wouldn’t think of hydrocarbons as being electrically interesting,” says the University of Liverpool’s Matthew Rosseinsky and Durham University’s Kosmas Prassides, who coauthored a commentary in Nature explaining the significance of the work. ”This is exciting news for superconductivity researchers and should stimulate extensive work on the electronic properties of other acenes [the family of aromatic compounds to which picene belongs].”

The Japanese team doped the molecules with potassium or rubidium in a process called intercalation, in which the metal atoms are inserted between parts of the picene molecule. They created a thin film of the intercalated picene and built a field-effect transistor to test for superconductivity.

”We started to investigate metal doping into picene crystals because we expected metallic behavior. Actually, we observed superconducting transition at 18 K,” says Kubozono.

Superconductivity experts around the world are intrigued. ”In the global picture, the discovery probably won’t affect how we think about superconductors, as the pairing mechanism is likely conventional, but it certainly provides a new research avenue that should produce interesting results,” says Lynn, who adds that NIST will start experimenting with picene and related compounds.

But not everybody is convinced that what Kubozono and his colleagues are seeing is real superconductivity. Announcements of new superconductors are often met with skepticism by superconductivity researchers, because many compounds claimed to be superconductors have turned out not to be so.

”There is definitely a transition, but is it superconductivity? I am not sure,” says theorist and superconductivity expert Chandra Varma of the University of California, Riverside.

The feeling is echoed by leading superconductivity scientist Robert Cava at Princeton University, who says, ”I think it’s too early to tell whether they have really isolated a new superconducting compound.”

So what’s next? Superconductivity researchers around the world will likely seek to confirm the Japanese group’s work and understand picene more.

As for the team reporting the discovery, Kubozono says, ”Currently we are investigating other metal-atom intercalation into picene crystals to search for new superconductors. Furthermore, we are synthesizing new organic molecules which consist of more benzene rings or are expanded two-dimensionally, in order to intercalate metal atoms into their crystals.”

About the Author

Saswato R. Das is a science reporter in New York City. In the March 2010 issue he wrote about how Russian scientists had solved the mystery of superinsulators.

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