The (7 × 7) surface is considered to have a triangular checkerboard structure in which the layer stacking sequences $\cdots \mathrm{AaBbA}$ and $\cdots \mathrm{AaCcA}$ alternate in neighboring triangular subunits in conformity with ion-backscattering results. The topological requirements of joining double layers at the subunit boundaries lead to the prediction of arrays of dimers and of deep (6.3-Å) holes which are consistent with results of electron diffraction and of tunneling microscopy, respectively.

• Received 21 March 1983

DOI:https://doi.org/10.1103/PhysRevB.28.2305