Portland, Ore. -- Nanotubes already offer ballistic transport of electrons at room temperature--and, when cooled to near absolute zero, single-walled carbon nanotubes lose all resistance to current flow and become superconductors. Now EEs in Japan have demonstrated that careful attention to bonding the ends of multiwalled nanotubes can increase the temperature at which they superconduct, potentially enabling quantum computing at the molecular level.
"We report that entirely end-bonded multiwalled carbon nanotubes can exhibit superconductivity with a transition temperature as high as 12 Kelvin, which is approximately 30 times greater than single-walled nanotubes," the researchers said in a report on their work.
Junji Haruyama, an associate professor in the department of electrical engineering and electronics at Aoyama Gakuin University (Japan), led the team. The team included fellow professor Nobuyuki Kobayashi and researcher assistant Izumi Takesue, as well as professors Shigeo Maruyama and Shohei Chiashi at the University of Tokyo and professors Hisanori Shinohara and Toshiki Sugai at Nagoya University.
According to the Bardeen-Cooper-Schrieffer (BCS) theory, superconductivity occurs when electrons overcome the mutual Coulomb repulsion forces between them and instead form Cooper pairs by virtue of the absence of lattice vibrations--phonons--near absolute zero. Rather than Coulomb repulsion, carbon nanotubes inhibit superconductivity because of the repulsive force of a state called a Tomonaga-Luttinger liquid.
Haruyama's team, however, has overcome the Tomonaga-Luttinger liquid repulsion that destroys Cooper pairs by arraying concentric multiwalled nanotubes so that contact is made by gold bonded to the ends of every concentric tube. Normal electrodes attach only to the outermost nanotube in the concentric set of walls in a multiwalled nanotube. By careful bonding techniques, ensuring that every wall of the multiwalled nanotubes are in direct physical contact with a gold electrode, the researchers found that the temperature required for superconducting goes up.
The team prepared the ends of multiwalled nanotubes by trimming them with ultrasound and evaporating gold into the end so that metal bonds to all the concentric nanotube walls. The researchers said they increased the temperature of superconduction in proportion to the number of nanotubes directly bonded to gold, here to 12K. Now the team plans to attempt increasing the number of concentric nanotubes even further, hoping to raise the temperature of superconductivity even higher.