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Zusammenfassung:
The apparent contradiction of one-dimensional adsorbate chains on Si(111) having a 3¥2 unit cell and yet a 3¥1 diffraction pattern is resolved for the example of Ba/Si(111)-(3¥2). Random registry shifts between adsorbate chains are observed in tunneling microscopy, with very short interchain correlation lengths. Fourier analysis provides a natural explanation for a pseudo-(3¥1) diffraction pattern. Within density-functional theory such registry shifts can occur with essentially negligible energy cost, leading to entropy-driven, virtually perfect disorder. Substrate states of high symmetry and one-dimensional character are inferred to promote this phenomenon.
