  Scanning tunneling microscopy (STM) reveals unusual sharp features in
otherwise defect free bismuth nanolines self-assembled on Si(001). They appear
as subatomic thin lines perpendicular to the bismuth nanoline at positive
biases and as atomic size beads at negative biases. Density functional theory
(DFT) simulations show that these features can be attributed to buckled Si
dimers substituting for Bi dimers in the nanoline, where the sharp feature is
the counterintuitive signature of these dimers flipping during scanning. The
perfect correspondence between the STM data and the DFT simulation demonstrated
in this study highlights the detailed understanding we have of the complex
Bi-Si(001) Haiku system.
