Theory and observation of highly asymmetric atomic structure in scanning-tunneling-microscopy images of graphite

Images of the (0001) surface of graphite observed in the scanning-tunneling microscope (STM) show a strong asymmetry in the tunneling current between neighboring carbon atoms in the hexagonal ring. The magnitude of this asymmetry is seen to be almost independent of the polarity and to decrease slightly with increasing amplitude of the bais voltage for voltages below 1 V. A theory is developed that explains this anomaly as a purely electronic effect, arising from the symmetry of the states scanned by the STM, which dominates over the topography of graphite. The predicted trends for the bias-voltage dependence of the asymmetry are confirmed experimentally.