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 ρ(T) obeying a power-law temperature dependence, ρ(T)∝T^-β. In the insulating regime (high annealing temperatures), the ρ(T) behavior ranges from Mott-like three-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.