ALKALI-METAL CHAINS ON THE GAAS(110) SURFACE

It has been reported that Cs adsorbed at low coverages on GaAs(110) forms linear one-dimensional chains which usually appeared in pairs. The alkali metal atoms in the two chains (called primary and secondary) are shifted along the [110BAR] direction by an amount a/unroofed-radical 2, where a is the bulk cubic lattice constant of GaAs. It is an unresolved issue as to why the chains appear in pairs adjacent to each other. We suggest that the primary and secondary chains can be really viewed as a single chain with a zig-zag structure. One can then compare the calculated total energy for a single linear chain (of double the density) with the total energy for a zig-zag chain. The latter is found to be a lower energy structure in agreement with the scanning tunneling microscopy observations. Physical interactions responsible for stabilizing the zig-zag structure are briefly discussed.