Site-selective imaging in scanning tunneling microscopy of graphite: The nature of site asymmetry

In the absence of tip-sample mechanical interaction, graphite images show very different behaviors from those of typical images. Contrary to the current interpretation of site asymmetry, where protrusions in images are considered as B-site atoms, we have found that one can selectively image A- or B-site atoms depending on the bias polarity. Furthermore, the corrugation amplitude is extremely small. If measurements are performed under tip-sample mechanical interaction, one observes larger corrugations and in-phase images at opposite polarities. A new interpretation of site asymmetry is proposed.