Warmer Superconducting Buckyballs

The marvel of superconductors is their ability to carry electricity without any loss. A superconducting ring, for example, can theoretically sustain a looping current indefinitely. The catch, however, is that most superconducting materials work only when they are supercold: traditional superconductors must be chilled to within a few degrees of absolute zero, and even so-called high-temperature superconductors--ceramics made with copper and oxygen--need to reach the temperature of liquid nitrogen, or negative 196 degrees Celsius, before they start to hum. But new hope for even higher temperature superconductors has emerged in the form of carbon soccer balls called buckminsterfullerenes, or, simply, buckyballs: scientists from Bell Laboratories report in today's issue of Nature that it may be possible to create buckminsterfullerenes that are superconducting at practical temperatures.

In the early 1990s, scientists found how to make buckyballs superconducting by doping them with metal atoms. The metals, trapped inside the buckyball's carbon sphere, release free electrons that carry current. But the highest temperature at which these metal-doped buckyballs became superconducting was about negative 255 degrees Celsius. Now Bertram Batlogg and his colleagues have raised that threshold by another 40 degrees--simply by doping the buckyballs in a different way. Instead of adding electrons with metal dopants, they removed them by injecting "holes" to molecules of C₆₀--buckyballs containing 60 carbon atoms (see image). Holes, too, carry current and act much like electrons, except that they have a positive charge. When the scientists added about three holes per molecule of C₆₀, the buckyballs became superconducting at negative 221 degrees Celsius. And they suspect they can raise that temperature even higher--perhaps well above that of liquid nitrogen--by using some inert dopant to push the individual buckyballs farther apart.