NGLE graphite crystals are known to be good conductors along the graphite planes and very poor ones across (ratio of resistivities 104 or higher. '} Coulson' and Wallace' have shown that a graphite crystal along the planes is a semiconductor with zero activation energy, the only difference with a metal consisting in a decrease of the density of states to zero for both the filled and un6lled bands as one approaches their common limit. Although the tight binding approximation does not yield a completely satisfactory description of all the properties of a graphite crystal, the result mentioned seems well established. The theory is, however, developed only for the case of in6nitely large crystals.
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Thompson et al. (1950) studied this question.