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As shown in Fig. 7, such surfaces, also called high index surfaces, show a periodic succession of terraces and steps of monoatomic height. Although they can be specified by their corresponding Miller indices [(557) in Fig. 7b], this notation is not very convenient since it does not indicate, at first sight, the geometrical structure. We will, rather, use a notation introduced by Lang et al. 8]: [p(lmn) x (l'm'n')] in which [mn and ['m'n' are, respectively, the Miller indices of the terraces and of the ledges and p gives the number of atomic rows in the terrace parallel to the edge.

Finally, we also describe some other techniques (photoelectron diffraction, surface extended X-ray absorption fine structure) from which structural information can be deduced. 1 Two-Dimensional Lattices One of the symmetry transformations allowing us to generate two-dimensional periodic structures is translation (Fig. 2). The transformation T = na + mb n, m = 0, ± 1, ± 2, ... connects the origin to a geometrically equivalent site and an infinite twodimensional lattice is thus obtained. The parallelogram with sides a and b is called the primitive or unit cell.

The other symmetry transformations are: - rotational symmetry with an angle ¢ = 2n/n (n = 1,2,3,4,6), n = 5 being forbidden, at least for perfectly periodic structures. - reflection symmetry across a line. 1 Surface Crystallography 45 Adatom Kink Advacancy Ledge Fig. 1. TLK model • • • • i • Fig. 2. Two-dimensional unit cell These different point operations yield ten two-dimensional crystallographic point groups. There are only five possible two dimensional lattices: the five Bravais lattices, drawn in Fig.

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Acoustic Pressures in Case of Soap Bubbles by Barus C.


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