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 xi=10-30 nm. Coulomb interactions associated with a
soft gap Delta_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.Comment: 4 pages, 5 figures, Submitted to Phys. Rev.
