SEATTLE, WASHINGTON--Materials that lose their electrical resistance at a whisper above absolute zero generally don't garner much attention. But when a French-Russian team reported that carbon nanotubes become superconductors at a measly 0.55 kelvin here earlier this month, researchers took notice. The reason: Calculations show that nanotubes filled with so-called buckyballs could do much better and perhaps even superconduct at room temperature.
Previous studies have shown that crystals of buckyballs--carbon spheres officially known as fullerenes--can superconduct at temperatures as high as 52 kelvin. Theory suggests that if fullerenes are lined up in wirelike rows, the threshold for superconductivity would rise dramatically. That's because lowering the number of immediate neighbors to each fullerene increases a quantum-mechanical property known as the density of states--a situation favorable to a higher temperature superconductor. In recent years, researchers in Japan and elsewhere have made rows of fullerenes by packing them inside nanotubes like peas in a pod. Now the race is on to see if these peapods will superconduct at a high temperature.
But detecting superconductivity in nanotubes--even empty ones--has been tough. In 1999, a group led by Mathieu Kociak and Helene Bouchiat at the University of Paris, South, in Orsay, France, reported in Science (28 May 1999, p. 1508) that ropes of 100 or so nanotubes could carry supercurrent between two superconducting electrodes. In this early study, the electrons traveled in pairs through the nanotubes, as they do in all superconductors, but poor contact with the electrodes caused electrical losses that kept the experiment from confirming that superconductivity was taking place.
At the annual meeting of the American Physical Society and in the 12 March issue of Physical Review Letters, Kociak and his colleagues at the French national research agency CNRS and the Russian Academy of Sciences in Chernogolovka showed that empty nanotubes can also carry electron pairs between nonsuperconducting electrodes (in this case, metal pads made from a sandwich of aluminum oxide, platinum, and gold). The results confirmed that it was the superconductivity in the tubes that was driving electron pairs together.
"It's impressive work," says David Tománek, a nanotube expert at Michigan State University in East Lansing. "This is the first direct evidence that nanotubes superconduct." Now the question is whether Kociak's team can pull off the same feat with nanotubes packed with fullerenes. "Fullerene peapods should give you room-temperature superconductivity," says Tománek; however, they could also lead to a type of magnetic behavior that would undermine superconductivity completely. The winner will likely be known in the next few months.