Nonlinear transport and localization in single-walled carbon nanotubes

We have measured the electrical transport properties of mats of single-wall ed carbon nanotubes (SWNT) as a function of applied electric and magnetic f ields. We find that at low temperatures the resistance as a function of tem perature R(T) follows the Mott variable range hopping (VRH) formula for hop ping in three dimensions. Measurement of the electric field dependence of t he resistance R(E) allows for the determination of the Bohr radius of a loc alized state a = 700nm. The magnetoresistance (MR) of SWNT mat samples is l arge and negative at all temperatures and fields studied. The low field neg ative MR is proportional to H-2, in agreement with variable range hopping i n two or three dimensions. 3D VRH indicates good intertube contacts, implyi ng that the localization is due to the disorder experienced by the individu al tubes, The 3D localization radius gives a measure of the 1D localization length on the individual tubes, which we estimate to be >700 nm. Implicati ons for the electron-phonon mean free path are discussed.