Abstract:
In the Coulomb blockade regime, we find the
quantum-mechanical orbitals in carbon
nanotubes to be doubly degenerate over a
large number of states. The experimental
observation of the orbital degeneracy
indicates adiabatic reflection at the
nanotube-metal contacts and high quality of
the nanotube material. A pair of electrons
occupying the degenerate orbitals is found to form a triplet state even at zero magnetic field. Finally, we observe unexpected low-energy excitations at complete filling of a four-electron shell.