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We investigated disorder and structural defects in a one-dimensional quasiperiodic superstructure of a thin Ag film on GaAs(110) surfaces by scanning tunneling microscopy. The superstructure forms sequences with long and short separations exhibiting a self-similarity. We demonstrate that the modu- lation can be described best with a Fibonacci sequence and deviations are due to structural defects. We identify dislocations, phason defects, and domain walls. The static stress field of dislocations is found to be a source of phason defects. PACS numbers: 61.44.Br, 61.16.Ch, 68.35.Dv Quasicrystals possess a well-ordered nonperiodic ar- rangement of atoms with long range rotational symme- tries. This unique structure has attracted considerable attention and its detailed determination has been the is- sue of numerous studies. Deviation from the ideal quasi- crystal structure, i.e., structural defects and disorder in quasicrystals, affect, however, to a large degree their properties, such as heat conduction, diffusion constants, phase transitions, and electron transport properties (1- 8). Therefore knowledge about defects and their relation to the quasiperiodic structure is a key to the understanding of quasicrystals and possibly even of their formation. To date defects have been investigated experimentally nearly exclusively in three- and two-dimensional quasicrystals (2,9,10) by transmission electron microscopy. To our knowledge disorder and defects in one-dimensional quasi- crystals have been addressed only theoretically, likely due to the lack of suitable model systems for experimental real space investigations. In this Letter we investigate structural defects and dis- order on a new type of one-dimensional quasicrystal: a modulated thin Ag film on GaAs(110) surfaces. We demonstrate that the film exhibits a quasiperiodic super- structure with a self-similarity and analyze the defects present on the film in real space. We identify dislocations, phason defects, as well as domain walls, and we show that the stress field of dislocations give rise to phason defects. The one-dimensional quasiperiodic system investigated here is a novel system distinctively different from the one-dimensional quasicrystals known so far (11). The one-dimensional quasiperiodic structure was obtained by depositing 1.5 to 1.7 nm thick Ag layers at 135 K on freshly cleaved GaAs(110) surfaces in ultrahigh vac- uum. Following the low temperature deposition, the films were annealed at room temperature and imaged by scanning tunneling microscopy in the constant-current mode. Using this procedure we always obtained flat two- dimensional Ag films (12). The Ag films all exhibited a one-dimensional height modulation, which lead to stripes

Dislocations, Phason Defects, and Domain Walls in a One-Dimensional Quasiperiodic Superstructure of a Metallic Thin Film