VARIABLE-RANGE HOPPING CONDUCTION AND NEGATIVE MAGNETORESISTANCE OF DISORDERED CARBONS AT LOW-TEMPERATURE

Temperature dependence of the electrical conductivity in the range from room temperature down to 1.31 K was studied for three kinds of cokes heat treated at 900°C, bamboo char heat treated at 900°C and glassy carbon heat treated at 1000°C. Localized states as evidenced by variable-range hopping conduction (conductivity proportional to exp T -1 4) was observed for these carbons at temperatures below 4 K. Room temperature Hall coefficient was studied for these carbons. Magnetoresistance was measured for the samples in the temperature range between 1.31 and 4.2 K. Positive magnetoresistance related to variable-range hopping conduction was observed for a bamboo char to increase with increasing magnetic field and with decreasing temperature. The mechanism of conduction can be interpreted by use of a model of density of states suggested by Davis and Mott for amorphous semiconductor. For cokes and glassy carbon negative magnetoresistance superposes with the positive magnetoresistance. Dependence of conductivity and of magnetoresistance on heat-treatment was investigated in the temperature range from 1.31 to 4.2 K for cokes heat treated at 1100, 1300 and 1500°C. Conductivity was found to be constant against temperature for these samples. With increasing heat-treatment temperature the contribution of negative magnetoresistance was observed for samples heat treated at 1500°C. © 1979.