Time-dependent electron transport in carbon nanotubes with impurities

Impurity effects on time-dependent electron transport through armchair carbon nanotubes connected with normal-metal leads are investigated theoretically. The system shows on-resonance and off-resonance behaviors according to different nanotube lengths. The impurity suppresses the current for the on-resonance case, while it enhances the current for the off-resonance case. Additional resonant peaks caused by photon-assisted tunneling are found when microwave fields are applied. The effects of the impurity strength are also studied, and one new peak induced by photon-assisted tunneling is found. The dependence of the current on the microwave frequency clearly shows the electron-hole symmetry broken.