Reflection by defects in a tight-binding model of nanotubes

We use a transfer-matrix method to study defects in a tight-binding model of carbon nanotubes. We calculate the reflection coefficient R for a simple barrier created by a pointlike defect of strength E in armchair (N-a,N-a) and zigzag (N-a,0) nanotubes for the whole range of energy omega and arbitrary number of conducting channels. We find that R scales at the Fermi level (i.e., omega = 0) as R = s(E/t)(2)/N-a(2) (t being the hopping parameter), where s approximate to 1/6 (for the armchair nanotubes) and s approximate to 1/2 (for the zigzag nanotubes). We also perform a similar calculation for a "5-77-5" defect and find the results to be like the ones obtained for a strong point defect with E=6t. [S0556-2821(98)05114-5].