Electronic transport in carbon nanotube ropes and mats

We suggest that conduction in carbon nanotube ropes or mars is best underst ood in terms of a heterogeneous model, involving regions of metallic conduc tion together with hopping or tunnelling through small electrical barriers corresponding to defects of various types. Such a model gives a good accoun t of the measured resistivity, in particular the crossover from nonmetallic to metallic sign for the resistivity temperature dependence seen in many s amples, in analogy to our similar model for conducting polymers. In agreeme nt 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 pea ks, and semiconductor contributions as possible causes of these nonlinearit ies.