Localization, Coulomb interactions, and electrical heating in single-wall carbon nanotubes/polymer composites

Low-field and high-field transport properties of carbon nanotubes/polymer composites are investigated for different tube fractions. Above the percolation threshold f_c∼0.33%, transport is due to hopping of localized charge carriers with a localization length ξ∼10-30 nm. Coulomb interactions associated with a soft gap Δ_CG∼2.5 meV are present at low temperature close to f_c. We argue that it originates from the Coulomb charging energy effect which is partly screened by adjacent bundles. The high-field conductivity is described within an electrical heating scheme. All the results suggest that using composites close to the percolation threshold may be a way to access intrinsic properties of the nanotubes by experiments at a macroscopic scale.