Excitation spectrum of carbon nanotube quantum dots

We have studied the excitation spectrum of nanotube quantum dots. At low temperatures, the stability diagrams show very regular Coulomb blockade diamonds. We have observed four-fold shell structure both in the electron addition and excitation energy spectrum. The measured nanotube spectra are in good agreement with theoretical predictions based on the nanotube band structure. In suspended nanotubes, additional harmonic excitations appear at low energies that we attribute to phonon assisted tunneling in the nanotube quantum dot. Our results permit the complete identification of the electron quantum states in nanotube quantum dots.