1. Ørsted Laboratory, Niels Bohr Institute, Universitetsparken 5, DK-2100 Copenhagen, Denmark
2. Department of Physics, University of Warwick, Coventry CV4 7AL, UK

Correspondence to: David Henry Cobden² Correspondence should be addressed to D.H.C. (e-mail: Email:  d.h.cobden@warwick.ac.uk).

Abstract

The connection of electrical leads to wire-like molecules is a logical step in the development of molecular electronics, but also allows studies of fundamental physics. For example, metallic carbon nanotubes¹ are quantum wires that have been found to act as one-dimensional quantum dots^{2, 3}, Luttinger liquids^{4, 5}, proximity-induced superconductors^{6, 7} and ballistic⁸ and diffusive⁹ one-dimensional metals. Here we report that electrically contacted single-walled carbon nanotubes can serve as powerful probes of Kondo physics, demonstrating the universality of the Kondo effect. Arising in the prototypical case from the interaction between a localized impurity magnetic moment and delocalized electrons in a metallic host, the Kondo effect has been used to explain¹⁰ enhanced low-temperature scattering from magnetic impurities in metals, and also occurs in transport through semiconductor quantum dots^{11, 12, 13, 14, 15, 16, 17, 18}. The far greater tunability of dots (in our case, nanotubes) compared with atomic impurities renders new classes of Kondo-like effects^{19, 20} accessible. Our nanotube devices differ from previous systems in which Kondo effects have been observed, in that they are one-dimensional 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.