First-principles study of the electrical conductance of telescopically aligned carbon nanotubes

We perform a comparative study for the quantum transport of telescoping carbon nanotubes, where the (5,5) and (10,10) nanotubes are coaxially aligned, using first-principles local-density-functional and tight-binding calculations. In both calculations, the intertube conductance initially increases as the hybridized length in the contact region increases, and then decreases, exhibiting a maximum conductance. However, the calculated conductances from first principles are generally smaller than those from the single $\ensuremath{\pi}$-orbital tight-binding model. In the first-principles calculations, we obtain the maximum intertube conductance that does not exceed ${G}_{0}\phantom{\rule{0.3em}{0ex}}(=2{e}^{2}∕h)$, while individual tubes have two conducting channels, giving the conductance of $2{G}_{0}$. On the other hand, the single $\ensuremath{\pi}$-orbital tight-binding model gives the maximum conductance close to $2{G}_{0}$, similar to previous calculations. Using a double-wall nanotube, we examine the effect of interwall interactions on conductance and find that the ${\ensuremath{\pi}}^{*}$ states of the inner and outer tubes are strongly coupled in the tight-binding model, allowing for an extra conducting channel, while the ${\ensuremath{\pi}}^{*}$ channel is closed in the first-principles calculations.