  An external magnetic field is found to have strong effects on the electronic
structure of carbon nanotubes. A field-induced metal-insulator transition is
predicted for all pure nanotubes. In a weak field, nanotubes exhibit both large
diamagnetic and paramagnetic responses which depend on the field direction, the
position of the Fermi energy, the helicity, and the size of the nanotube.
Universal scalings are found for the susceptibility as functions of the Fermi
energy, the temperature, and the size of nanotubes. These results are in
agreement with experiments.
