Shell structure in metallic carbon nanotubes

We studied the shell structure in metallic carbon nanotubes. The report on experimental observation of the orbital degeneracy over a large number of stadtes were made. The existence of degeneracy in real nanotube devices does not follow trivially from the symmetry properties of two dimensional graphite. A sizable orbital mismatch was found in most studies. We found that in a large percentage of shells, the two orbitals are degenerate with a possible level mismatch not exceeding one tenth of the shell spacing. Our findings also demonstrated that the electrons are reflected without mode mixing at the contacts. By studying magnetic field dependence orbitals form a triplet state starting from zero magnetic field. We also observed unexpected low energy excitations when a four electron shell is completely filled, both in the sequential tunneling and in the inelastic co-tunneling regimes.