Charge and spin density waves in the electronic structure of graphite: application to analysis of STM images

Abstract

On the basis of the electronic structure of a graphite monolayer, represented in the unrestricted Hartree-Fock approximation by an extended Hubbard Hamiltonian, we interpret the data on observed scanning tunneling microscopy (STM) images of bulk graphite, graphite monolayers on Pt(111) and graphite intercalation compounds. The well-known (and puzzling) pattern of graphite STM images, with only three of the six atoms of each carbon hexagon visible, is tentatively explained by the intrinsic features of the electronic structure of a graphite monolayer, without invoking well-known explanations attributing the observed effect to structural differences between the sites and to interlayer interactions. In particular we construct a phase diagram for graphite in the space defined by the magnitude of on-site and nearest-neighbor electron repulsions. The conditions for insulating charge and spin density wave solutions are delineated. A charge density wave state, which we estimate is reasonable for the graphite monolayer, would give the 3-fold STM image. A spin density wave state, which we think somewhat less likely, will also give unequal tunneling currents from A and B sites of graphite monolayer, provided the STM tip carries a local magnetic moment. 39 refs., 3 figs.