Orbital Kondo effect in carbon nanotubes

TL;DR

This paper reports the observation of a purely orbital Kondo effect in carbon nanotubes, demonstrating that orbital degrees of freedom can replace spin in Kondo phenomena, especially under magnetic field tuning.

Contribution

It introduces the first experimental evidence of an orbital Kondo effect in carbon nanotubes, expanding understanding of Kondo physics beyond spin-based systems.

Findings

Orbital Kondo effect observed in carbon nanotubes.

Magnetic field tunes spin-polarized states into orbital degeneracy.

Enhanced Kondo resonance with SU(4) symmetry when degeneracies coexist.

Abstract

Progress in the fabrication of nanometer-scale electronic devices is opening new opportunities to uncover the deepest aspects of the Kondo effect, one of the paradigmatic phenomena in the physics of strongly correlated electrons. Artificial single-impurity Kondo systems have been realized in various nanostructures, including semiconductor quantum dots, carbon nanotubes and individual molecules. 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 electrons. In principle, however, the role of the spin could be replaced by other degrees of freedom, such as an orbital quantum number. Here we demonstrate 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 tunneling. When orbital and spin degeneracies are simultaneously present, 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.