  The electronic band structure of armchair carbon nanotubes may be
considerably modified by potentials with angular dependence. Different angular
modes V_q ~ cos(q*theta) have been studied within a tight-binding scheme. Using
symmetry arguments, we demonstrate a bandgap opening in these metallic
nanotubes when certain selection rules are satisfied for both potential and
nanotube structure. We estimate the bandgap opening as a function of both the
external potential strength and the nanotube radius and suggest an effective
mechanism of metal-semiconductor transition by combination of different forms
of perturbations.
