  We show that high-harmonic generation (HHG) in carbon nanotubes (CNTs) can be
efficiently controlled using the Aharanov-Bohm (AB) effect. When a static
magnetic field (B) is applied along the tube, electronic wave functions acquire
complex phases along the circumferential direction (AB effect), which modifies
the band structure. In particular, when the magnetic field is applied to
metallic CNTs, which can be regarded as one-dimensional massless Dirac systems,
realistic values of B lead to a nonzero gap in the THz regime. We demonstrate
that such change from gapless to gapped Dirac systems drastically increases the
HHG intensity in the THz regime. In the gapless Dirac system, the velocity of
each electron never changes under the electric field, and thus there is no HHG.
On the other hand, the gap opening activates both the interband and itraband
currents, which strongly contribute to HHG. Our work demonstrates a unique way
to manipulate HHG in nanotubes by tuning electronic wave functions using the
magnetic field and the tube structure.
