We find strong evidence for a metal-insulator (MI) transition in macroscopic single wall carbon nanotube (SWNT) 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 ${\mathrm{H}}_2{\mathrm{SO}}_4$-doped samples exhibit small negative MR, and the resistivity is low and only weakly temperature-dependent. Stepwise dedoping by annealing in vacuum induces a MI transition. Critical behavior is observed near the transition, with $\rho \left(T\right)$ obeying a power-law temperature dependence, $\rho \left(T\right)\propto T^{-\beta }$. In the insulating regime (high annealing temperatures), the $\rho \left(T\right)$ behavior ranges from Mott-like three-dimensional (3D) variable-range hopping (VRH), $\rho \left(T\right)\propto \exp \left[{(T_0∕T)}^{-1∕4}\right]$, to Coulomb-gap (CGVRH) behavior, $\rho \left(T\right)\propto \exp \left[{(-T_0∕T)}^{-1∕2}\right]$. Concurrently, MR$\left(B\right)$ becomes positive for large $B$, exhibiting a minimum at magnetic field $B_{min}$. The temperature dependence of $B_{min}$ can be characterized by $B_{min}\left(T\right)=B_c(1-T∕T_c)$ for a large number of samples prepared by different methods. Below a sample-dependent crossover temperature $T_c$, MR$\left(B\right)$ 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.

• Received 1 June 2004

DOI:https://doi.org/10.1103/PhysRevB.71.155410