COULOMB-GAP MAGNETOTRANSPORT IN GRANULAR AND POROUS CARBON STRUCTURES

The possibility of observing a Coulomb gap due to carrier-carrier interactions in granular metallic systems is investigated, with particular emphasis on porous carbon structures such as carbon aerogels and activated carbon fibers (ACF's), both of which contain ultrafine pores and grains, the size of which can be controlled by the sample preparation procedures. The exp[-(T0/T)1/2] temperature dependence observed at low temperature in the dc electrical conductivity of both materials is characteristic of granular metallic systems. A variable-range hopping mechanism near a Coulomb gap around the Fermi level is proposed to circumvent the difficulties of previous tunneling and hopping models. The temperature dependence of the magnetoresistance in fields up to 15 T is measured for both aerogels and ACF's to further confirm the effect of the Coulomb gap. The results not only qualify porous materials as a medium for studying localization phenomena, but also shed some light on the complex problem of transport near the percolation threshold in granular metallic systems in general.