From Insulator to Conductor to Superconductor

The electronics industry would be a big beneficiary of any practical applications to come out of fullerene studies. Scientists already know that, depending on its form, a buckyball solid can have different electrical properties. Unadulterated solid buckminsterfullerene is an insulator at low temperatures; an electric current can't flow through it like it does in a metal. Yet in 1991 researchers at Bell Labs used doping techniques to turn a buckyball solid into not only an electrical conductor but a superconductor as well. Superconductivity is one of the most sought after properties in the world of high technology. It means that a material offers no electrical resistance and lets a current flow through it unhindered. Superconducting materials have already been developed, but their big drawback to widespread commercial application, especially for such futuristic projects as magnetically levitated trains, is that they have to be cooled to temperatures not that much higher than absolute zero before they will work. The cooling requirements alone may be prohibitively expensive. Materials that become superconducting at higher and higher temperatures have long been sought.

What the Bell Labs scientists did was turn C60 into an artificial metal by slipping potassium atoms into the spaces between buckyballs that had been crystallized into a solid thin film (see Figure 9.6). In a report of their work in Physics Today, they described how the transformation from insulator to conductor took place right before their eyes: the pristine yellow color of the pure C60 thin film gradually turned a metallic gray with increased exposure to a potassium vapor. As the color changed, the thin film became more and more electrically conductive. The surprise came when the researchers chilled this artificial metal to about 18 degrees Kelvin (18 degrees above absolute zero) and discovered that it became superconducting. This transition temperature was surprisingly high by superconducting standards. An interesting aside is that in the mid-1960s scientists were able to make a superconductor by sandwiching potassium atoms between flat sheets of graphite, but its transition temperature was only a few tenths of a degree above absolute zero. Somehow, curling up graphite sheets into closed hollow spheres confers special superconducting properties on buckyballs when they are doped with an alkali metal.

A strange thing occurred, however, during the Bell Labs experiments. If the researchers exposed the carbon film too long to the potassium vapor, it lost its superconducting powers and reverted back to being an insulator. The researchers theorized that there is a limit to how



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