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 $f_c\sim 0.33\%,$ transport is due to hopping of localized charge carriers with a localization length $\xi \sim 10-30\hspace{0.5em}\mathrm{nm}.$ Coulomb interactions associated with a soft gap ${\Delta }_{\mathrm{CG}}\sim 2.5\hspace{0.5em}\mathrm{meV}$ 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.