Difference in electric conduction of monocrystal and polycrystal graphite
单晶石墨、多晶石墨电导行为的差异

In this paper, we report the difference in electric conduction of monocrystal and polycrystal graphite. Under a magnetic field of 8.15T, the positive magnetoresistance ratio of monocrystal and polycrystal graphite samples are found to be 69400%, 170% respectively at 4.5K. The polycrystal graphite presents accordingly the insulator-like character in a wide interval of temperature under 0 and 8T applied magnetic field, but the magnetic-field-correlated semimetal-insulator-like Transition has been reported for the monocrystal graphite (highly oriented pyrolitic graphite, HOPG). On analysis, it is revealed that large difference in electric conduction originates predominantly from the Coulomb interaction in HOPG graphite layers in contrast with the crystal interface scattering in the polycrystal graphite.