The effect of surface defects upon the order-disorder phase transition of a (001) surface was studied by using Monte Carlo simulations which were based upon the asymmetrical dimer model. It was found that the transition became broad in a system with a low density of type-C defects. The temperature dependences of the long-range order parameter, the local-order parameter, and the intensity of the low-energy electron diffraction peak, were investigated. In order to analyze local ordering quantitatively, on the basis of scanning tunnelling microscopic images, a so-called mesoscopic ordering index was introduced. The critical temperatures (mid-point of the transition regions of the long-range order parameter, mesoscopic ordering index and low-energy electron diffraction intensity) were found to be almost the same. The observed temperature dependence of the low-energy electron diffraction peak was attributed to the combined effects of defects and of the coherence length.

Y.Nakamura, H.Kawai, M.Nakayama: Physical Review B, 1997, 55[16], 10549-60