• Ultrathin CNTs Show Superconductor Characteristics
In 1995, the Berkeley group, headed by Prof. Steven Louie and Prof. Marvin Cohen, has predicted that very thin nanotubes can be superconducting, due to the enhanced coupling between the electrons and the vibrations of the carbon lattice when the carbon sheet is rolled up into a thin tube. In fact, the prediction is that the thinner the tube, the higher the superconducting transition temperature. The predicted transition temperature is in order of magnitude agreement with what discovered later in the 0.4 nm single-walled carbon nanotubes which first observed by Dr. N. Wang using a high-resolution electron microscope.
The research group at the Physics Department first observed:
(1) - Superconductivity from 0.4 nm carbon nanotubes
(2) - Signature of one dimensionality on the superconducting behavior
At low temperatures, the Meissner effect, superconducting gap and fluctuation supercurrent in 0.4 nm single-walled carbon nanotubes have been observed. The measured superconducting behaviors display smooth temperature variation due to one-dimensional fluctuation, with a mean-field Tc = 15 K.
We have confirmed the superconductivity in 4 angstrom nanotubes by observing three effects:
(1) At low temperatures (below 15 K), the nanotubes 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 superconductivity. (Fig. 1)
(2) We have observed the superconducting gap. Superconducting electrons differ from the usual conducting electrons in that they are always paired instead of moving individually. Once the superconducting pairs are formed, it requires a certain amount of energy to break up these pairs into the usual conducting electrons. The energy required to break up the pairs is called the superconducting gap. Superconducting gap is also regarded as a convincing evidence for the existence of superconductivity. (Fig. 2)
(3) We have observed the 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.
Dr. N. Wang has developed a novel sample preparation technique and setup nanomeasurement equipment, and therefore realized the direct measurement of the super conductivity of 0.4 nm carbon nanotubes. This is the first direct evidence to illustrate the superconductivity of such small single-walled carbon nanotubes.
Fig. 1 - 3. Supercurrent transition, domain structures and Meissner effect in 0.4nm SWNTs.
Learn more in our publication:
• Z. K. Tang, Lingyun 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.