Half-filling SU(4) Kondo state in carbon nanotubes: Numerical results

Charge transport measurements in carbon nanotube quantum dots by P. Jarillo-Herrero et al. [Nature 434 (2005) 484] have detected a strongly enhanced Kondo temperature TK∼8.0 K. This Kondo state was associated with the SU(4) symmetry of the Hamiltonian at quarter-filling for an orbitally double-degenerate single-occupied electronic shell, indicating the simultaneous Kondo screening of charge and spin. Pure SU(4) symmetry can only be achieved when the orbital quantum number is preserved upon tunneling. So-called channel mixing can destroy the SU(4) state, transforming it into an SU(2)-type Kondo state (the so-called two-level SU(2) state). In this work, numerical calculations of charge transport show in detail the transition between these two Kondo states. Our results indicate that the SU(4) state seems to be quite robust, surviving to considerable amount of channel mixing. Moreover, a curious behavior of the conductance is revealed close to the two level SU(2) state