Unique dynamic appearance of a Ge-Si ad-dimer on Si(001)

We carry out a comparative study of the energetics and dynamics of Si-Si, G e-Ge, and Ge-Si ad-dimers on top of a dimer row in the Si(001) surface, usi ng first-principles calculations. The dynamic appearance of a Ge-Si dimer i s distinctively different from that of a Si-Si or Ge-Ge dimer, providing a unique way for its identification by scanning tunneling microscopy (STM). I ts "rocking" motion, observed in STM, actually reflects a 180 degrees rotat ion of the dimer, involving a piecewise-rotation mechanism. The calculated energy barrier of 0.74 eV is in good agreement with the experimental value of 0.82 eV.