Practical realisation of a two-dimensional frustrated Ising system

Scanning Tunneling Microscopy and Monte-Carlo simulations quantitatively demonstrate that the class of the so called mosaic or gamma phases of 1/3 ML Sn0.5Si0.5/Si(111), Pb0.5Si0.5/Si(111), Pb0.5Ge0.5/Ge(111) (all showing the same root3 x root3 R30degrees reconstruction) are very good practical realisation of a two dimensional antiferromagnetic Ising system. The two-dimensional lattice of these systems is triangular (six nearest neighbours) and this leads to frustration, that is to the absence of a unique ground state phase. The Ising model is used to make realistic predictions about the short range order features and the electronic properties of this class of surfaces at low temperatures.