  We describe in greater detail the exactly solvable microscopic model we have
developed for analyzing the strain-mediated interaction of vacancy lines in a
pseudomorphic adsorbate system (Phys. Rev. Lett., to appear). The model is
applied to Ga/Si(112) by extracting values for the microscopic parameters from
total-energy calculations. The results, which are in good agreement with
experimental observations, reveal an unexpectedly complex interplay between
compressive and tensile strain within the mixed Ga-Si surface layer.
