Carbon nanotubes can now add superconductivity to their repertoire of novel electronic and mechanical properties. Ping Sheng and co-workers at Hong Kong University of Science and Technology have found that the nanotubes exhibit superconducting behaviour below 20 kelvin, confirming that resistance-free current can flow through pure carbon (Z K Tang et al 2001 Science 292 2462).
Sheng and colleagues detected superconductivity in single-walled carbon nanotubes – which are rolled up sheets of graphite – just 0.4 nanometres in diameter. “We believe this is the first time that superconductivity has been seen in individual carbon nanotubes”, Sheng told PhysicsWeb. Superconductivity has been seen in carbon nanotubes before, but it was due to the ‘proximity effect’. This is an exotic phenomenon in which two superconductors can induce resistance-free current in certain materials sandwiched between them.
The nanotubes showed three telltale signs of superconductivity: the Meissner effect, a superconducting gap and a supercurrent. In the Meissner effect, a superconductor placed in a magnetic field expels magnetic flux from its interior. “This is the acid test for superconductivity”, Sheng told PhysicsWeb. The team used a SQUID magnetometer to measure the magnetic susceptibility of the carbon nanotubes, which is directly related to this internal flux.
The nanotubes were placed in a magnetic field after they had been cooled to 1.8 kelvin, and the temperature was then raised to 50 kelvin. This process was repeated for magnetic fields ranging in strength from 0.02 tesla to 5 tesla. Below 10 kelvin, magnetic flux inside the nanotubes fell steadily as the field grew stronger, and was close to zero at 5 tesla. This effect was still evident as the temperature approached 20 kelvin. This closely matches the predicted behaviour of the Meissner effect.
The electrons in conventional conductors move individually, but superconducting electrons move in pairs. “The energy needed to separate the paired electrons is known as the superconducting gap”, explains Sheng. This gap is further evidence of superconductivity. The third effect the team observed was a ‘supercurrent’. “This flow of paired electrons is only possible in defect-free nanotubes”, says Sheng, “so we made nanotubes just 50 nanometres long to reduce the chance of imperfections, enabling us to detect the supercurrent”.
The data collected by Sheng and colleagues are consistent with the Bardeen-Cooper-Schreiffer theory of superconductivity, which states that vibrations of the crystal lattice – known as phonons – aid the free flow of paired electrons in superconductors. “We have fulfilled a prediction made in 1995 that superconductivity would occur in nanotubes due to enhanced coupling between phonons and electrons”, says Sheng, “and our results are in the predicted range”.
