Short-range order in two-dimensional binary alloys

Using variable-temperature scanning tunneling microscopy, the degree of short-range order of purely two- dimensional (2D) binary alloys on triangular lattices [Sn(1-x)-Si-x, Pb(1-x)-Si-x, and Pb(1-x)-Ge-x, all showing the root3x3 R30 degrees reconstruction onto Si(111) or Ge(111)] has been determined quantitatively via a statistical analysis of the atomic positions. The experimental data, also in comparison with Monte Carlo simulations, generally indicate that an effective nearest-neighbor repulsion between substitutional Si (Ge) adatoms explains with good accuracy the short-range-order features observed. This finding implies the occurrence of an ordered phase of the alloys for x = 0.33 and, in the case of a 1:1 ratio of Sn(Pb) and Si(Ge) adatoms on the surface (x = 0.5), demonstrates that the investigated alloys are very good practical realizations of a frustrated antiferromagnetic 2D Ising system on a triangular lattice. The implications of the observed short-range order for the electronic properties of these alloys (as a function of x) are discussed.