Carbon nanotubes are still a rich source on new physics
Physicists have long known that charge carriers such as electrons and 'holes' can travel along carbon nanotubes in either a clockwise or anticlockwise direction, providing an extra degree of freedom that is not found in other electronic systems. However, it had been assumed that there was little or no interaction between this orbital motion and the intrinsic magnetic moment (or 'spin') of the charge carriers, which can be either up or down. However, Paul McEuen and co-workers1 at Cornell University have now shown this assumption to be wrong.
The Cornell researchers placed a 500-nm-long nanotube between two electrodes and used another two electrodes to create a quantum dot that confined the charge carriers at either the right or the left side of the nanotube. By measuring the current through the nanotube as a function of various voltages and the strength of a magnetic field applied along the length of the nanotube, they were able to probe the interactions between orbital motion and spin in this system. If there was no interaction, the four different possible combinations of clockwise/anticlockwise and up/down would all have the same energy in the absence of the magnetic field. However, the Cornell team found that they had different energies, which is a clear sign that orbital motion and spin are coupled in nanotubes. The new results suggest that it might be possible to use electric fields to control the spins of charge carriers in nanotubes, which could have applications in 'spintronics'.
In similar experiments, Vikram Deshpande and Marc Bockrath2 of Caltech have seen a Wigner crystal — a novel crystalline phase in which the effects of the long-range Coulomb forces between charge carriers overwhelm their kinetic energy — in a nanotube for the first time. The Wigner crystal has been seen in two-dimensional systems before, but never in one or three dimensions, usually because defects and impurities get in the way.
Kuemmeth, F. Ilani, S. Ralph, D. C. & McEuen, P. L. Coupling of spin and orbital motion of electrons in carbon nanotubes. Nature 452, 448–452 (2008).
Deshpande, V. V & Bockrath, M. The one-dimensional Wigner crystal in carbon nanotubes. Nature Physics 10.1038/nphys895 (2008).
Rights and permissions
About this article
Cite this article
Rodgers, P. Twists and turns. Nature Nanotech (2008). https://doi.org/10.1038/nnano.2008.92