Guo-meng Zhao and Yong Sheng Wang of the University of Houston in Texas found subtle signs of superconductivity. It wasn't zero resistance, but it's the closest anyone's got so far. "I think all the experimental results are consistent with superconductivity," Zhao says. "But I cannot rule out other explanations."
At the moment no superconductor will work above about 130 kelvin (-143 ¡C). But if a material could carry current with no resistance at room temperature, no energy would be lost as heat, meaning faster, lower-power electronics. And electricity could be carried long distances with 100 per cent efficiency.
Zhao and Wang studied the effects of magnetic fields on hollow fibres of carbon known as "multiwall carbon nanotubes". Each nanotube is typically a millionth of a metre long, several billionths of a metre in diameter and with walls a few atoms thick. The nanotubes cling together in oblong bundles about a millimetre in length.
The researchers did not see zero resistance in their bundles. They think this is because the connections between the tiny tubes never become superconducting. But they did see more subtle signs of superconductivity within the tubes themselves.
For example, when the researchers put a magnetic field across a bundle at temperatures up to 400 kelvin (127 ¡C), the bundle generated its own weak, opposing magnetic field. Such a reaction can be a sign of superconductivity. And when the team cooled the bundles from even higher temperatures then turned the external field off, they stayed magnetised. A current running around within the tubes could generate this lingering field if there wasn't any resistance to make it fade away.
While each effect could have a more prosaic explanation, they varied in similar ways as the temperature of the bundles changed. The correlation suggests superconductivity was responsible, Zhao and Wang argue in a paper to be published in Philosophical Magazine B. However, their argument doesn't convince Paul Grant, a physicist with the Electric Power Research Institute in Palo Alto, California. "Generally, superconductivity is such a dominating effect that when it occurs it just shouts out at you," Grant says. "It doesn't appear in these indirect ways."
Superconductivity theories do not forbid the phenomenon at very high temperatures, says Sasha Alexandrov, a theoretical physicist at Britain's Loughborough University. A material becomes superconducting when its electrons pair up. Normally such negatively charged particles would repel each other, but in a positively charged crystal structure, vibrations called phonons help them get together. In carbon nanotubes, the frequency of these vibrations is very high, which, in theory at least, means superconductivity at higher temperatures. "The results on the magnetic response are very intriguing, and favour the explanation they present," Alexandrov says. "It's certainly possible," agrees David Caplin, head of the Centre for High Temperature Superconductivity at Imperial College, London.
To decide whether or not the nanotubes really are superconductors, you need to measure the resistance through a single tube, Alexandrov says. "To be convinced, I'd like to see zero resistance."
Author: Adrian Cho More at: www.arxiv.org/abs/cond-mat/0111268
New Scientist issue: 1st December 2001
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