Metal-Insulator Transition in Doped Single-Wall Carbon Nanotubes

We find strong evidence for a metal-insulator (MI) transition in macroscopic single wall carbon nanotube conductors. This is revealed by systematic measurements of resistivity and transverse magnetoresistance (MR) in the ranges 1.9-300 K and 0-9 Tesla, as a function of p-type redox doping. Strongly H2SO4-doped samples exhibit small negative MR, and the resistivity is low and only weakly temperature dependent. Stepwise de-doping by annealing in vacuum induces a MI transition. Critical behavior is observed near the transition, with ρ(T) obeying power-law temperature dependence, ρ(T) ∝ T -β. In the insulating regime (high annealing temperatures) the ρ(T) behavior ranges from Mott-like 3-dimensional (3D) variable-range hopping (VRH), ρ(T) ∝ exp[(-T0/T)-1/4], to Coulomb-gap (CGVRH) behavior, ρ(T) ∝ exp[(-T0/T)-1/2]. Concurrently, MR(B) becomes positive for large B, exhibiting a minimum at magnetic field Bmin. The temperature dependence of Bmin can be characterized by Bmin(T) = Bc(1 - T/Tc) for a large number of samples prepared by different methods. Below a sample-dependent crossover temperature Tc, MR(B) is positive for all B. The observed changes in transport properties are explained by the effect of doping on semiconducting SWNTs and tube-tube coupling.