Departmental Papers (MSE)

Document Type

Journal Article

Date of this Version

January 2005

Comments

Postprint version. Copyright American Physical Society. Published in Physical Review B, Volume 71, Article 155410, 2005, 11 pages.
Publisher URL: http://link.aps.org/abstract/PRB/v71/e155410

Abstract

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. 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.

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Date Posted: 10 May 2005

This document has been peer reviewed.