Negative magnetoresistance of pyrolytic carbon and effects of low-temperature electron irradiation

Pyrolytic carbon that exhibits negative magnetoresistance is irradiated wit h 2 MeV electrons at temperatures below 35 K, and the changes in electronic transport properties such as zero-magnetic field resistivity, Hall coeffic ient, and magnetoresistance are measured as a function of electron fluence. With increasing electron fluence, the zero-field resistivity decreases, wh ile the Hall coefficient and the absolute values of negative magnetoresista nce increase. The experimental data are analyzed assuming that the densitie s of electrons and holes vary with the magnetic field. The analysis shows t hat the densities increase with the square of the magnetic field; the resul t is in good agreement with the Bright theory, in which the two-dimensional Landau levels are assumed to be broadened due to defect scattering. Both i ntrinsic defects and irradiation-produced defects act as electron accepters . The addition of accepters increases the ratio of hole density to electron density, p/n, resulting in the enhancement of negative magnetoresistance. The present results clarify that the negative magnetoresistance in pyrolyti c carbon is caused by the existence of acceptor defects and the two-dimensi onal Landau levels, which are broadened by the defects. In addition, they s uggest that the intrinsic acceptor defects in pyrolytic carbon are presumab ly vacancies. [S0163-1829(99)05439-9].