A SIMPLE WAVE-FUNCTION INTERFERENCE MODEL FOR THE ELECTRON-DENSITY SUPERSTRUCTURES OBSERVED DURING SCANNING TUNNELING MICROSCOPY OF PERTURBED GRAPHITE SURFACES

Scanning tunneling microscope images of graphite surfaces sometimes reveal patterns of increased and decreased electron density superimposed on the normal graphite image. Mizes and Foster [Science 244 (1989) 559] first suggested that these superstructures are due to interference between normal and scattered electron wave functions. However, the calculated STM images they presented concentrated on interference patterns that should accompany scattering from adsorbates of 1-3 atoms in size, patterns that, as the authors stated, did not closely resemble their experimental data. Subsequent work on electron-density superstructures [J. Xhie, K. Sattler, U. Muller, N. Venkateswaran and G. Raina, J. Vac. Sci. Technol. B9 (1991) 883] indicates that Mizes and Foster's explanation has not been accepted as conclusive. When the scattered wave concept is applied within a more simplified model (in which the orientation of the scattering site is ignored, and only the effects of interference between allowed wave functions is considered), the connection between theory and experiment is made clearer. STM images of electron-density superstructures are compared to interference patterns calculated using this model, and it is found that the simplest possible perturbations of the normal graphite wave functions (i.e. constructive and destructive interference with a single wave function component) reproduce the most commonly observed superstructures, including such details as the apparent lateral displacement of lattice positions.