  We have studied electron transport in clean single-walled carbon nanotube
quantum dots. Because of the large number of Coulomb blockade diamonds
simultaneously showing both shell structure and Kondo effect, we are able to
perform a detailed analysis of tunneling renormalization effects. Thus
determining the environment induced level shifts of this artificial atom. In
shells where only one of the two orbitals is coupled strongly, we observe a
marked asymmetric gate-dependence of the inelastic cotunneling lines together
with a systematic gate dependence of the size (and shape) of the Coulomb
diamonds. These effects are all given a simple explanation in terms of
second-order perturbation theory in the tunnel coupling.
