At temperatures below 15 K, the Meissner effect, superconducting gap and fluctuation supercurrent have been observed in 0.4 nm carbon nanotube / zeolite composites. The measured superconducting behaviors display smooth temperature variation due to fluctuations, with a mean-field Tc = 15 K.
The three separately measured superconducting behaviors can be consistently explained within a unified theoretical framework. The three effects are separately explained below.
(1) At low temperatures (below 15 K), the nanotube / zeolite composites exhibit a very special characteristic of expelling the magnetic field. The tendency of expelling the magnetic field becomes stronger as the temperature is lowered. Such an effect is the so-called Meissner effect, and is the acid test for the existence of superconductivity.
(2) Superconducting gap arises because superconducting electrons differ from the usual conducting electrons in that they are paired. Once the superconducting pairs are formed, it requires a certain amount of energy (gap energy) to break up the pairs into the usual non-superconducting electrons. The energy required to break up the pairs is called the superconducting gap. A superconducting gap is also regarded as a convincing evidence for the existence of superconductivity. We have found that the temperature dependence of the superconducting gap is consistent with that for the Meissner effect.
(3) We have also observed the fluctuation supercurrent, which is related to electrical conduction by the paired electrons in the superconducting state. This last observation is only possible when there is no imperfection in the nanotubes or very weak imperfections. So we have fabricated samples in which the nanotubes are only 500 Angstroms in length, so as to reduce the possibility that they might have imperfections.
Work is still in progress on this project.
Z. K. Tang, L. Y. Zhang, N. Wang, X. X. Zhang, G. H. Wen, G. D. Li, J. N. Wang, C. T. Chan, and Ping Sheng, "Superconductivity in 4 Angstrom Single-Walled Carbon Nanotubes" Science 292 (2001), 2462-2465.