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 (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 H2SO4-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(T) obeying a power-law temperature dependence, rho(T)proportional to T-beta. In the insulating regime (high annealing temperatures), the rho(T) behavior ranges from Mott-like three-dimensional (3D) variable-range hopping (VRH), rho(T)proportional to exp[(T-0/T)(-1/4)], to Coulomb-gap (CGVRH) behavior, rho(T)proportional to exp[(-T-0/T)(-1/2)]. Concurrently, MR(B) 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(T)=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(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.