  The effects of charge doping on the structural deformation and on the
electronic structure of armchair single wall carbon nanotube (SWNT) bundles are
investigated through first-principles calculations. In particular, we select a
(6; 6) SWNT as an example and we calculate a mechanical deformation in the SWNT
bundles as a function of gate voltage, which could serve as a basis of the
electromechanical actuators in an artificial muscle. We find that the
magnitudes of the actuation responses such as strain and stress of the (6; 6)
SWNT bundle in the case of hole doping are substantially larger than those of
electron doping. The (6; 6) SWNT bundle also exhibits a low-symmetry and opens
an energy band gap of about 0.41 eV around the charge neutral condition, which
allows a semiconductor-to-metal transition in the electron-doping regime when
the relative shift of the Fermi energy goes up to 0.60 eV, above which the
Young modulus increases.
