Studies of the Hall coefficient and of the magneto-resistance of a donor doped soft carbon in the temperature range 1.5°–300° K

Abstract

Using the experimental arrangement developed by Inagaki, Komatsu and Zanchetta the temperature dependence of the Hall coefficient, magnetoresistance and resistance were investigated in the range 1.5°–300°K for soft carbon samples heattreated from 1400° to 3200°C, after introduction of donors. Sufficiently large concentrations of sodium and of potassium were introduced to shift the Fermi level well above the band overlap; subsequent driving out of the donors by oxidation and reheat was done in steps and measurement performed for various degrees of doping. Selected data are presented in a number of graphs. The behavior of the Hall coefficient for HTT > 2000° C agrees well with expectations for the graphitic model with a band overlap. However for HTT 2000°C and below the results indicate the presence of a basically different structure with coalescing bands. The magnetoresistance is composed of two parts which seem to be additive. The regular positive magnetoresistance present in graphitized materials, decreases when graphite layers are spread apart and/or when the Fermi level moves out from the overlap region. The negative magnetoresistance part appears only when microcrystals become sufficiently small, does not depend on spreading of the layers nor on the position of Fermi level, saturates at higher magnetic fields and increases in absolute valne with decrease of temperature down to about 4°–10°K where a sharp turn-around is observed. The temperature coefficient of the resistance varies as the Fermi level moves across the band overlap. Problems in the interpretation of these results are discussed.