  The remarkable transport properties of carbon nanotubes (NTs) are determined
by their unique electronic structure (1). The electronic states of a NT form
one-dimensional electron and hole subbands which, in general, are separated by
an energy gap (2,3). States near the energy gap are predicted to have a large
orbital magnetic moment much larger than the Bohr magneton (4,5). The moment is
due to electron motion around the NT circumference. This orbital magnetic
moment is thought to play a role in the magnetic susceptibility of NTs (6-9)
and the magneto-resistance observed in large multi-walled NTs (10-12). However,
the coupling between magnetic field and the electronic states of an individual
NT has not been experimentally quantified. We have made electrical measurements
of relatively small diameter (2 - 5 nm) individual NTs in the presence of an
axial magnetic field. We observe energy shifts of electronic states and the
associated changes in subband structure. Our results quantitatively confirm
predicted values for orbital magnetic moments in NTs.
