Theoretical Analysis on Influence of Defects on AC Transport in Metallic Single-Walled Carbon Nanotubes
Single-walled carbon nanotubes (SWNTs) are expected to be potential elements in next-generation electronic devices, owing to their excellent electric transport properties. Therefore, their DC transport properties have extensively been investigated experimentally and theoretically. Toward applications, however, the understanding of AC transport properties is also needed. The AC response of pristine metallic SWNTs has already been examined theoretically , and the phase difference between current and bias voltage was found to change from inductive response to capacitive one as the DC conductance decreases. Recently, we have studied the influence of a single vacancy on the AC transport , and found that (1) the electron scattering by the defect states induces capacitive response and (2) the phase difference depends on the vacancy position. In the present work, we analyze the AC transport in metallic SWNTs with vacancies focusing on its dependence on the tube diameter and the presence of another vacancy to understand the influence of vacancies more deeply. We adopt the Keldysh nonequilibrium Greenís function method within the wide-band limit approximation and nearest-neighbor π-orbital tight-binding approximation [1-3]. Our results reveal that large-diameter metallic SWNTs with a vacancy behave more capacitively than small-diameter ones . This correlates with the fact that the large-diameter SWNTs with a vacancy have scattering states for electrons around the vacancy whose energy distribution is sharper than that in the small-diameter ones. In the cases of SWNTs including two vacancies, an inductive peak appears at the Fermi level where the resonant tunneling occurs due to the electron-wave interference between the two vacancies. More interestingly, sharp capacitive peaks appear adjacent to the inductive peaks. The origin of this behavior is under investigation.  T. Yamamoto et al., Phys. Rev. B 82, 206404 (2010);  D. Hirai et al., Appl. Phys. Express 4, 075103 (2011);  D. Hirai et al., Jpn. J. Appl. Phys., in press.