A Curvier Form of Carbon

by Paul Preuss
Earlier this year, after a team of theorists led by Marvin Cohen and Steven G. Louie had calculated the properties of a new form of carbon from first principles, it was announced that the novel material had been synthesized by a team of experimentalists led by Alex Zettl.

Cohen, Louie, and Zettl are with Berkeley Lab's Materials Sciences Division and are professors of physics at the University of California at Berkeley. Their work marks a striking advance in the scientific study of fullerenes, the closed carbon structures of which the best known example is the 60-atom "buckyball." Other fullerenes with fewer than 60 carbon atoms have been fleetingly observed, but carbon-36 is the first to be made in bulk.

Jeffrey Grossman and Michel C�t� of the Cohen-Louie team began by determining that a fullerene with a shape something like a rugby football was likely to be the most stable of several possible 36-atom configurations. Working with colleagues at the National Energy Research Scientific Computing Center (NERSC), C�t� and Grossman applied Cohen's pseudopotential method to calculate the electronic densities and other properties of the material's possible crystal structures.

A striking difference between C-36 crystals and those of C-60 is that C-36 molecules are covalently bonded—much more tightly bound than buckyballs in a crystal. C-36 is also much more chemically reactive. Perhaps most interesting is that carbon-36 may lose all electrical resistance at temperatures far higher than any other carbon structure—perhaps even at temperatures in the range that superconducting copper-oxide ceramics have achieved.

"The highest-temperature superconductor is the home run that everybody is trying to hit," says Cohen. "Even if the carbon-36 materials don't achieve this, they give us a new class of solids to help develop our knowledge about this field."

C-60 doped with alkali metals can be a superconductor at up to 40 degrees Kelvin, a phenomenon thought to be associated with the curvature of the balls. Carbon-36 fullerenes are more tightly curved than buckyballs, suggesting that, if suitably doped, they could become superconducting at far higher temperatures.

The 18th century mathematician Leonhard Euler established that every closed polygon made with hexagons and pentagons must have exactly 12 pentagonal faces. Unlike C-60, whose soccer-ball shape is the smallest possible structure in which the 12 do not touch, every atom in the carbon-36 molecule is at the vertex of one or two pentagons. "Clustering of pentagons creates severely strained atomic sites," says C�t�.

The strained or "bent" bonds between atoms in fullerenes may expose electron orbitals normally unaffected by the vibrational modes of other forms of carbon. The more electron orbitals that the atomic vibrations known as phonons can affect, the greater the potential for electron-phonon coupling and the greater the prospects for superconductivity.

"However," says Grossman, "the superconducting temperature depends on other factors besides the electron-phonon coupling, so it's hard to make quantitative predictions from theory."

Whether or not C-36 proves to be a high-temperature superconductor, its discovery holds great promise for scientific advance through closer coordination of theory, computation, and experiment.

"We're no longer saying, 'here's a model of something you may never see,'" says Grossman. "Because of increased computing power and highly developed algorithms, we can now construct realistic physical models and then make predictions from first principles about real materials which can be made."

On 25 June, Charles Piskoti, Alex Zettl, and Jeff Yarger announced the isolation of bulk samples of C-36, dissolved from purified soot. C�t� and Grossman of the Cohen-Louie group emphasize the value of their close working relationship with Zettl's experimental group. Says Grossman, "The cross-fertilization has helped us all."

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