1. Kavli Institute of Nanoscience, Delft University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
2. Present addresses: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, USA (J.K.); Laboratorio Nazionale TASC-INFM, I-34012 Trieste, Italy (S.D.F.)

Correspondence to: Pablo Jarillo-Herrero¹ Correspondence and requests for materials should be addressed to P.J. (Email: pablo@qt.tn.tudelft.nl).

Abstract

Progress in the fabrication of nanometre-scale electronic devices is opening new opportunities to uncover deeper aspects of the Kondo effect¹—a characteristic phenomenon in the physics of strongly correlated electrons. Artificial single-impurity Kondo systems have been realized in various nanostructures, including semiconductor quantum dots^{2, 3, 4}, carbon nanotubes^{5, 6} and individual molecules^{7, 8}. The Kondo effect is usually regarded as a spin-related phenomenon, namely the coherent exchange of the spin between a localized state and a Fermi sea of delocalized electrons. In principle, however, the role of the spin could be replaced by other degrees of freedom, such as an orbital quantum number^{9, 10}. Here we show that the unique electronic structure of carbon nanotubes enables the observation of a purely orbital Kondo effect. We use a magnetic field to tune spin-polarized states into orbital degeneracy and conclude that the orbital quantum number is conserved during tunnelling. When orbital and spin degeneracies are present simultaneously, we observe a strongly enhanced Kondo effect, with a multiple splitting of the Kondo resonance at finite field and predicted to obey a so-called SU(4) symmetry.

1. Kavli Institute of Nanoscience, Delft University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
2. Present addresses: Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4307, USA (J.K.); Laboratorio Nazionale TASC-INFM, I-34012 Trieste, Italy (S.D.F.)

Correspondence to: Pablo Jarillo-Herrero¹ Correspondence and requests for materials should be addressed to P.J. (Email: pablo@qt.tn.tudelft.nl).

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