Microscopic model for dimer buckling on Si(001)

Examination of scanning tunneling microscopy (STM) images for the dimerized Si(001) surface, particularly at low temperature, suggests that dimers can exist in either a buckled or an unbuckled geometry, with both mechanically stable but the former lower in energy. A simple model for such behavior has been constructed. Each dimer is represented by a single angular degree of freedom (``tipping'' angle). Interactions couple dimers nonlinearly to their nearest neighbors, and linearly to surface defects. The system potential energy has a simple algebraic form in the angles. Stable tipping patterns have been computed, and conform to the qualitative features of STM observations. The model should facilitate study of possible surface phase transitions on Si(001) by conventional computer-simulation methods.