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 ${f}_{c}\ensuremath{\sim}0.33%,$ transport is due to hopping of localized charge carriers with a localization length $\ensuremath{\xi}\ensuremath{\sim}10\ensuremath{-}30 \mathrm{nm}.$ Coulomb interactions associated with a soft gap ${\ensuremath{\Delta}}_{\mathrm{CG}}\ensuremath{\sim}2.5 \mathrm{meV}$ 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.