Tip-surface interactions in scanning tunneling microscopy

The tip-surface interactions in scanning tunneling microscopy (STM) of the Si(100) surface are investigated with ab initio total energy pseudopotential calculations. The results of the calculations lead to a new understanding of the microscopic STM measurement process. It is found that under typical conditions the influence of the tip is large enough to effectively flip a dimer on this surface. This leads to a reinterpretation of the ``symmetric'' dimer STM image as an asymmetric dimer configuration that flips as it follows the motion of the scanning tip.