All2

Surface Science 244 (1991) 135-148 North-Holland

Chemisorption-induced surface segregation and order-disorder transition Y. Teraoka Department of Physics, University of Osaka Prefecture, Sakai 591, Japan

Received 21 June 1990; accepted for publication 2 October 1990

135

The effects of chemisorption on surface segregation and order-disorder transition in ordering alloys A~B l_x are investigated by use of two models, in the previously treated model, an adsorption layer is assumed to consist of chemisorbed M atoms and vacancies. On the other hand, in our new model, an alloy surface layer is assumed to be a pseudo-ternary alloy, which consists of A atoms, A atoms with chemisorbed M atoms, and B atoms. As an example, foe binary alloys with a (100) surface are investigated in detail within a framework of the Bragg-Williams approximation. Our new model has been found to explain qualitatively experimental data on CuAu with an fcc(100) surface.

Surface Science 244 (1991) 149-159 149 North-Holland

Diffraction from stepped surfaces in thermal equilibrium N.C. Bartelt, T.L. Einstein and Ellen D. Williams Department of Physics, University of Marylam~ College Park, MD 20742-4111, USA Received 29 June 1990; accepted for publication 14 September 1990

We have performed Monte Carlo simulations of the diffraction from simple two-dimensional models of vicinal surfaces in order to aid interpretation of measured diffraction profiles. At low temperature, we find the sharp diffraction features predicted from the analogy of stepped surfaces with two-dimensional incommensurate phases. These sharp features vanish only near the r o u g h ~ temperature of the low-index surface corresponding to the terraces between steps. If one fits experimental data ha~ng sharp diffraction features to models of step disorder which do not include the ordering influence of step wandering, one can severely overestimate the amount of disorder. We emphasize that long-range correlations in step positions are more important than the local order in step edge structure or step ~ t m r a ~ for in te~re t i~ sharp diffraction features from steps. After much ~ tfffort, it has become well-established that asymptotically the h~h t -he igh t correlations for rough surfaces diverge logarithmicldly (with a prefactor having a universal component at low temperature). We show explicitly how to use diffraction data to access this behavior for stepped surfaces. In the process, we evaluate the accuracy of a popular approximate expression for the diffracted int~asity.

160 Surface Science 244 (1991) 160-176 North-Holland

Causal-surface Green's function method J.B. Pendry, A. Pr~tre, P.J. Rous The Blackett Laboratory, Imperial College of Science Technology and Medicine, Prince Consort Roaa~ London SW7 2BZ, UK

and Luis Martin-Moreno Cavendish Laboratory, University of Cambridge, Madingley Roa~ Cambridge CB3 0HE, UK Received 10 August 1990; accepted for publication 11 October 1990

We present a new real-space approach to the calculation of electronic properties of materials within a framework sufficiently flexible to handle both electronic structure and electron transport. We employ a causal algorithm to construct the Green's function, G, of a local potential of arbitrary shape, starting on some surface on wkich G is known, and e ~ the surface until it has swept out the entire volume of interest. Hence the "causal-surface Green's function method" or CSOFM. By evaluating G in real-space we