Electronic transport in carbon nanotube ropes and mats

We suggest that conduction in carbon nanotube ropes or mats is best understood in terms of a heterogeneous model, involving regions of metallic conduction together with hopping or tunnelling through small electrical barriers corresponding to defects of various types. Such a model gives a good account of the measured resistivity, in particular the crossover from nonmetallic to metallic sign for the resistivity temperature dependence seen in many samples, in analogy to our similar model for conducting polymers. In agreement with this model, the thermopower has been observed to be more metal-like than resistivity, but does show pronounced nonlinearities as a function of temperature. We investigate electron-phonon effects, density of states peaks, and semiconductor contributions as possible causes of these nonlinearities.