Pressure and doping dependence of electronic properties of carbon nanotube ropes

We have performed DC transport and optical measurements on purified and doped thick films of single walled carbon nanotubes (SWNT) in the temperature range 4–300 K. The resistivity under hydrostatic pressure at room temperature shows nonmonotonic variation which is a manifestation of the symmetry change first seen in Raman scattering by Venkateswaran et al. (Phys. Rev. B 59, 10928 (1999)). Despite the non-metallic temperature dependence of the dc resistivity the optical conductivity shows Drude-response in agreement with the presence of metallic tubes in the mat. Measuring DC and optical conductivity on the same sample allowed us to separate the contribution of the intrinsic rope resistivity and that of the contact regions.