  Using semiclassical Boltzmann equation we have studied theoretically an
effect of a direct current (DC) generation in undoped armchair carbon nanotube
(CN) by mixing two coherent electromagnetic waves with commensurate frequencies
i.e omega and 2 omega . We compared the results of the armchair with that of
the zigzag carbon nanotubes for the same conditions (i.e. when the normalized
current is plotted against the amplitude). Quantitatively they agree with each
other except that the absolute value of the peaks of the current for zigzag is
about 1.1 times that of the armchair. We noticed that the current is negative
similar to that observed in zigzag CNs describing the same effect. However it
is interesting to note that graph of normalized current against omega tau
showed that the armchair is greater than that of the zigzag for about 1.1 times
which is opposite. We also observed that when the phase shift theta lies
between pi divided by 2 and 3 pi divided by 2 there is an inversion and the
current becomes positive. We suggest the use of this approach for the
generation of terahertz radiation and also for the determination of the
relaxation time of electrons in carbon nanotubes.
