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Negative Magnetoresistance in Activated Carbon Fibers Heat-Treated Above 2000°C

  • A. W. P. Fung (a1), Z. H. Wang (a2), M. S. Dresselhaus (a1) (a2), G. Dresselhaus (a3) and M. Endo (a4)...

Abstract

Activated carbon fibers (ACFs) were heat-treated at temperatures above 2000°C to study both the effect of heat treatment on the order development in ACFs and the effect of granularity on the transport properties of granular materials in general. The electrical conductivity σ(T) and Magnetoresistance (MR) were measured as a function of temperature for ACFs Made of two different precursors and heat-treated at different temperatures. While the field dependence of the observed negative MR could be fit to the two-dimensional weak localization (2D WL) theory at each measurement temperature, σ(T) showed only a weak temperature dependence, inconsistent with the ln (T) dependence predicted by the same theory. Even More interesting is the observation of a negative MR, which is a quantum-Mechanical phenomenon, near room temperature. It is thought that the grain boundaries might be responsible for such deviations from the standard 2D WL theory.

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[1] Fung, A. W. P., Dresselhaus, M. S., and Endo, M., Phys. Rev. B, (1993), accepted.
[2] Rao, A. M., Fung, A. W. P., Dresselhaus, M. S., Dresselhaus, G., and Endo, M., J. Mat. Res., 7, 1788 (1992).
[3] Altshuler, B. L. and Aronov, A. G., in Electron-Electron Interactions in Disordered Systems, ed. by Efros, A. F. and Poliak, M., ch. 1, p. 1 (Elsevier Science Publishing, New York, 1985).
[4] Dresselhaus, M. S., Dresselhaus, G., Sugihara, K., Spain, I. L., and Goldberg, H. A., Graphite Fibers and Filamenets (Springer, Berlin, Heidelberg, 1988).

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