Weak localization in pregraphitic carbon fibers

High-resolution resistivity and magnetoresistance measurements have been performed as a function of temperature on pregraphitic carbon fibers. The resistivity exhibits a negative temperature coefficient (d{ρ}/dT<0) from room temperature down to 1.5 K. It is shown that such behavior can hardly be explained by a thermal excitation of carriers in the liquid-helium temperature range within the framework of the simple two-band model, usually applied to graphitic and pregraphitic structures. The magnetoresistance present a negative component more pronounced at low temperature. It is found that the magnetic field acts to restore the expected behavior of the resistivity in the low-temperature range, in the sense that the resistivity becomes gradually temperature independent. These observations as well as the experimental results obtained at higher temperature are consistent with the weak-localization theory for two-dimensional systems.