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WEAK LOCALIZATION EFFECTS IN FLUORINEINTERCALATED GRAPHITE FIBERS S.L. DI VITTORIO AND M.S.DRESSELHAUS, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 V. BAYOT, L.PIRAUX AND J-P. ISSI, Universite Catholique de Louvain, Louvain-laNeuve, Belgium M. ENDO, Shinshu University, Nagano 380, Japan T. NAKAJIMA, Kyoto University, Sakyo-ku, Kyoto 606, Japan

ABSTRACT The intercalation of fluorine into graphite introduces defects into the highly crystalline pristine fibers. These defects are studied using temperature-dependent resistivity and magnetoresistance measurements. A logarithmic increase in resistivity at low temperature is observed, whereas the high temperature behavior is metallic. At weak magnetic fields and low temperatures, a negative magnetoresistance is observed, which becomes positive at high fields. These effects are explained using the two theories of weak localization and hole-hole interaction. In the light of TEM pictures of the microstructure of the fluorinated fibers, the origin of the defects in the intercalated fibers is discussed.

INTRODUCTION Fluorine-intercalated graphite exhibits new properties that differ sharply from all previously studied graphite-intercalation compounds (GIGs). In most GICs,[1] as the intercalate concentration is increased, the conductivity for dilute compounds increases sharply with respect to pristine graphite. This increase in conductivity is explained by an increase in carrier density, as carriers are transferred from the intercalate into the graphene layers. At higher intercalate concentrations, the conductivity oadecreases by a small amount from the peak in conductivity, because the decrease in mean scattering time dominates over the increase in carrier concentration. Both effects are also observed in fluorine GIGs, as shown in Fig. 1. However, the total increase in conductivity through fluorine intercalation is small compared to most GIGs, whereas the subsequent decrease in conductivity is larger than in most GIGs by several orders of magnitude.[1] Two effects account for this unusual behavior. At high fluorine concentrations, the shift to a more covalent solid decreases the carrier concentration and contributes to the rapid decrease in conductivity. This mixed bonding behavior of fluorine in graphite[2] contrasts with the usual fully ionic or fully covalent character of most GIGs. Also, the process of fluorine intercalation into the graphite is believed to induce a large density of defects into the graphene layers (isolated layers of the graphite structure). In this paper, we use resistivity and magnetoresistance measurements to study the effect of defects in fluorine-intercalated fibers. Fibers are used, instead of highly oriented pyrolytic graphite (HOPG), because of the larger magnitude of the defect-induced effects in fibers. Resistivity and magnetoresistance measurements are used to show evidence for the presence of defects in the intercalated fibers. The characterization and origin of the disorder, for which transport measurements do not pr