Persistent Four-Electron Shell Filling in Carbon Nanotubes

We report on our measurements of electron conductivity in single-walled carbon nanotube devices in the Coulomb Blockade regime. Persistent four-fold grouping of the Coulomb blockade peaks was observed over an extended range of gate voltages. This grouping reflects the four-fold degeneracy of the electron levels (two spin directions and two valleys). We discuss the energy levels, excited states, and inelastic co-tunneling events extracted from the I(V) spectroscopy of the system.