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July 2, 2001
Volume 79, Number 27
CENEAR 79 27 p.8
ISSN 0009-2347
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Small diameter is the key to high superconducting temperature


Carbon nanotubes may be made only of carbon, but their unusual shape and size give them a host of intriguing electronic properties, the most recently discovered one being superconductivity.

Hong Kong University of Science & Technology physicists Ping Sheng, Ning Wang, Zi-Kang Tang, and colleagues now have convincing experimental evidence that nanotubes with exceptionally small diameters exhibit superconductivity at relatively high temperatures [Science, 292, 2462 (2001)].

The Hong Kong researchers used a technique they developed to synthesize single-walled nanotubes inside the channels of zeolite crystals (C&EN, Nov. 6, 2000, page 9). The tubes, which have diameters of only 4 Å, appear to become superconducting at about 15 K.

These latest results come on the heels of another recent report, from French and Russian researchers, that small bundles of single-walled nanotubes superconduct, albeit at a low temperature of 0.55 K [Phys. Rev. Lett., 86, 2416 (2001)].

The Hong Kong group's results agree with predictions made by University of California, Berkeley, physics professors Marvin L. Cohen and Steven G. Louie.

Their calculations indicate that the smaller the tube diameter, the higher the superconducting temperature. This is due to the greater curvature of the tube, which increases the interaction between electrons and lattice vibrations known as phonons--a property essential for superconductivity.

It's to this extreme curvature that some scientists attribute the superconductivity of nanotubes' fullerene cousins. Alkali metal-doped fullerenes superconduct at temperatures up to 40 K, and electron hole-doped fullerene crystals superconduct at 52 K (C&EN, Dec. 4, 2000, page 12).

Mildred S. Dresselhaus, a physics professor at MIT, calls the new research "noteworthy," observing that the authors may have evidence of so-called one-dimensional superconductivity. This has been an elusive phenomenon that may have numerous exotic properties because electrons are confined to one dimension.

"This may be the best example" of 1-D superconductivity, Cohen notes.

The group will now try doping the nanotubes to see if they can increase the superconducting temperature even further.

TINY TUBES Superconducting nanotubes inside zeolite pores (inset), against backdrop of zeolite crystals.

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