Persistent orbital degeneracy in carbon nanotubes

The quantum-mechanical orbitals in carbon nanotube quantum dots are doubly degenerate over a large number of states. We argue that this experimental observation indicates that electrons are reflected without mode mixing at the nanotube-metal contacts. Two electrons occupying a pair of degenerate orbitals (shell) are found to form a triplet state starting from zero magnetic field. Finally, we observe unexpected low-energy excitations at complete filling of a four-electron shell.