Physicists in Japan have shown that "entirely end-bonded" multi-walled carbon nanotubes can superconduct at temperatures as high as 12 K, which is 30 times greater than for single-walled carbon nanotubes. The discovery has been made by a team led by Junji Haruyama of Aoyama Gakuin University in Kanagawa. The superconducting nanotubes could be used to study fundamental 1D quantum effects and also find practical applications in molecular quantum computing (Phys. Rev. Lett. 96 057001).
Superconductivity is the complete absence of electrical resistance and is observed in certain materials when they are cooled below a superconducting transition temperature (Tc). Physicists agree that superconductivity relies on getting electrons to overcome their mutual Coulomb repulsion and form “Cooper pairs”. In the Bardeen-Cooper-Schrieffer (BCS) theory of low-temperature superconductivity, the electrons are held together because of their interactions with phonons — lattice vibrations in the material.
However, 1D conductors like carbon nanotubes — rolled up sheets of graphite just nanometres in diameter — are not naturally superconducting. One reason for this is the presence of so-called Tomonaga-Luttinger liquid (TLL) states in the material, which cause the electrons to repulse each other and so destroy Cooper pairs.
Now, however, Haruyama and colleagues have designed a system in which there is a superconducting phase that can compete with the TLL phase and even overcome it — a feat hitherto believed impossible. The system consists of an array of multi-walled carbon nanotubes, each of which consists of a series of concentric nanotube shells. Electrical contacts made of metal are bonded to the tubes so they touch the top of all the shells. Conventional “bulk junction” contacts, in contrast, touch only the outermost shell of a tube and along its length.
Haruyama and co-workers grew their multiwalled nanotubes from a template of porous alumina. Next, they cut the tops off the nanotubes using ultrasound or etching techniques and then evaporated a gold electrode onto the exposed ends of the tubes. In this way, nearly all of the nanotube shells were made electrically active.
The Japan team find that the end-bonded nanotubes lose all resistivity at temperatures below 12 K. According to the researchers, this is because the TLL states are suppressed so that superconductivity can appear. Moreover, the Tc depends on the numbers of electrically activated shells and the physicists will now try to increase this figure by making more or all of the shells active.
