Theory of the diamagnetic susceptibility of condensed ring systems: Reinterpretation of diamagnetism of pregraphitic carbons

Abstract

The equations of the London theory for the magnetic anisotropy of aromatic molecules can be solved in the case of condensed ring systems forming ribbons of infinite length but of finite width. The magnetic susceptibility calculated at low temperatures has the same character as that of the London anisotropy generally attributed to "ring currents." However, the diamagnetism decreases with increasing temperature because of the splitting of the levels in the magnetic field, and the change of occupation number of these levels which is due to the transfer of the electrons across the energy gap. The theoretical results are analogous to the susceptibilities measured on pregraphitic carbons formed by small crystallites (20-150 \AA{}) before the layer has taken on its three-dimensional graphitic arrangement. This permits us to reinterpret the diamagnetism of these compounds which was previously attributed to a Landau free-electron diamagnetism.