  A comprehensive first-principles theoretical study of the electronic
properties and half-metallic nature of finite zigzag carbon nanotubes is
presented. Unlike previous reports, we find that all nanotubes studied present
a spin-polarized ground state, where opposite spins are localized at the two
zigzag edges in a long-range antifferomagnetic configuration. Relative
stability analysis of the different spin states indicate that, for the shorter
segments, spin-ordering should be present even at room temperature. The energy
gap between the highest occupied and the lowest unoccupied molecular orbitals
of the finite systems is found to be inversely proportional to the nanotubes
segments length, suggesting a route to control their electronic properties.
Similar to the case of zigzag graphene nanoribbons, half-metallic behavior is
obtained under the influence of an external axial electric field.
