Localization in single-walled carbon nanotubes

We demonstrate that in low temperature semiconductor-like regions the electrical resistance of single-walled carbon nanotube mats is highly nonlinear with a temperature-dependent threshold field for the onset of nonohmic conduction. The modest applied electric field completely suppresses the upturn in resistance and recovers metallic behavior over the entire temperature range 2.2 K < T < 300 K. The transport data indicate low-temperature localization of charge carriers arise from disorder on the nanotube bundles themselves and not from granularity casued by weak interbundle connections. The temperature-independent localization radius a is determined to be approximately 330 nm.