Unusual conductance suppression in metallic carbon nanotubes*

Clean metallic carbon nanotube usually show a finite ``dip'' in the conductance as a function of gate voltage. We have observed an extra conductance suppression as larger gate voltages are applied, forming a hump-like shape in the conductance versus gate voltage curve. We have performed numerical calculations using the recursive Green's function technique on a tight-binding model to identify the origin of the hump. Our results show that the hump formation is associated with the unique combination of the linear dispersion of metallic carbon nanotubes and the presence of extrinsic impurities such as adsorbates. The width and position of the hump strongly depend on the densities of positive and negative impurities but show no dependence on the shape of the impurity potential, indicating why the hump is a rather ubiquitous phenomenon for metallic nanotubes.