Nanotubes (Dutch team uses nanostructures to produce logic gates, an oscillator and electron beams; Can carbon nanotubes handle high-energy particles?) may be made only of carbon, but their unusual shape and size give them a host of intriguing electronic properties, the most recently discovered being superconductivity.

Physicists in Hong Kong have shown that nanotubes with exceptionally small diameters become superconducting at relatively high temperatures. The researchers used their own technique to synthesize single-walled nanotubes inside the channels of zeolite crystals. The tubes, which have diameters of only 4 ¥ 10-8 cm, appear to become superconducting at about 15 K - much higher than the liquid helium temperatures normally associated with superconductivity.

These latest results have come soon after a report from French and Russian researchers that reveals that small bundles of single-walled nanotubes superconduct - albeit at a low temperature of 0.55 K.

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 (phonons) - a property that is essential for superconductivity.

Some scientists associate this extreme curvature with the superconductivity of fullerenes (large carbon molecules). Alkali metal-doped fullerenes superconduct at up to 40 K and electron hole-doped fullerene crystals superconduct at 52 K. The group will now try doping the nanotubes to see if it can increase the superconducting temperature even further.

Further reading

M Kociak et al. 2001 Phys. Rev. Lett. 86 2416.
Z K Tang et al. 2001 Science 292 2462.