Magnetotransport at the metal-insulator transition in fluorine-intercalated graphite fibers

We have investigated the transport properties of high-fluorine-concentration fluorine-intercalated vapor-grown graphite fibers (${\mathrm{C}}_{2.9}$F and ${\mathrm{C}}_{3.0}$F). As previously reported, the transport properties of dilute fluorine-intercalated graphite fibers (${\mathrm{C}}_{\mathit{x}}$F with x≥3.6) exhibit a weak disorder regime with a strong carrer-carrier interaction correction. These corrections to the metallic properties of the ${\mathrm{C}}_{3.6}$F fibers emphasize the presence of intercalation-induced disorder, which is believed to be associated with the semi-ionic bonding of fluorine in carbon. With increasing fluorine concentration (x≤3.0), the fibers undergo a transition from a metallic to an insulating regime. The temperature dependence of the resistivity of the fibers in the insulating regime is no longer logarithmic at low temperature as is measured for less concentrated samples (x≥3.6), but rather fits a two-dimensional Mott law quite closely. From the temperature dependence of the resistivity and the nonlinear I-vs-V relationship at high electric fields, a localization length on the order of ξ=1000 Å is extracted. This very large localization length shows that the fibers with ${\mathrm{C}}_{2.9}$F and ${\mathrm{C}}_{3.0}$F stoichiometries lie close to the metal-insulator transition. For the ${\mathrm{C}}_{2.9}$F and the ${\mathrm{C}}_{3.0}$F fibers, the transverse magnetoresistance is negative at low fields, but becomes positive at higher fields, whereas the longitudinal magnetoresistance is positive for all fields. Both the transverse and longitudinal magnetoresistance saturate at high field and low temperature. These features are explained by the superposition of an orbital quantum-interference-induced negative magnetoresistance and a spin-polarization-induced positive magnetoresistance.