Atomically Resolved Scanning Tunneling Spectroscopy on Si(001)-(2×1) Asymmetric Dimers

The tunneling spectrum of Si(001) reveals a significant difference in the electronic structure of the up and down atoms, which are separated by less than 0.25 nm. In particular, the peaks correlated with the unoccupied states at the conductance band edge could be determined in detail. The intensities of these peaks show periodic modulations coinciding with the periodicity of the $\left(2\times 1\right)$ reconstruction of the asymmetric dimers along the $\left[\right[\left(\overline{1}10\right)$ direction. These results prove directly the theoretically predicted charge transfer from the down to the up atom.