Kondo physics in carbon nanotubes

The connection of electrical leads to wire-like molecules is a logical step in the development of molecular electronics, but also allows studies of fu ndamental physics. For example, metallic carbon nanotubes(1) are quantum wi res that have been found to act as one-dimensional quantum dots(2,3), Lutti nger liquids(4,5), proximity-induced superconductors(6,7) and ballistic(8) and diffusive(9) one-dimensional metals. Here we report that electrically c ontacted single-walled carbon nanotubes can serve as powerful probes of Kon do physics, demonstrating the universality of the Kondo effect. Arising in the prototypical case from the interaction between a localized impurity mag netic moment and delocalized electrons in a metallic host, the Kondo effect has been used to explain(10) enhanced low-temperature scattering from magn etic impurities in metals, and also occurs in transport through semiconduct or quantum dots(11-18). The far greater tunability of dots (in our case, na notubes) compared with atomic impurities renders new classes of Kondo-like effects(19,20) accessible. Our nanotube devices differ from previous system s in which Kondo effects have been observed, in that they are one-dimension al quantum dots with three-dimensional metal (gold) reservoirs. This allows us to observe Kondo resonances for very large electron numbers (N) in the dot, and approaching the unitary limit (where the transmission reaches its maximum possible value). Moreover, we detect a previously unobserved Kondo effect, occurring for even values of N in a magnetic field.