Two-level roughness model

P2(N,M,r) make evaluation difficult if many different states are present, but we restrict the discussion to a two-state model, because that already shows the most important features. We consider two cases roughness on a surface with two levels and a reconstruction with two antiphase domains. For more complex roughness models, we refer to the literature [41]. 

Figure 3.4.2.34 Model of surface roughness of the sharp Bragg component is constant, involving only two levels. The characteristic but the diffuse component has a width that island size corresponds to a specific corre- depends on the correlation length short for lation length and leads to a two-component the system on the left and long for the one line shape. For a fixed coverage 9, the height on the right.

Figure 3.4.2.35 A crystal truncation rod for amplitude, does not depend on the rough-the two-level model with 0 = 0.3. Along the ness if both components are integrated. In rod, the relative weight of the Bragg and practise, often only the Bragg component diffuse components changes. The total inte- can be measured, leading to a loss in inten-grated intensity, and thus the structure factor sity in the tails of the rod.

Another illustrative application of the two-state model is for a (2x1) reconstructed surface. On such a surface, two domains are possible that are shifted by one bulk lattice spacing (antiphase domains) (see Figure 3.4.2.37). The treatment of this system is fully analogous to that of the surface with two levels of roughness. The... 

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