The Si(557)-Au surface reconstruction has been the subject of a number of experimental and theoretical studies because the surface states are known to be quasi-one dimensional and a metal to non-metal transition has been discovered on the surface upon cooling below room temperature [1-3]. A distinctive feature of the surface is extra Si adatoms that decorate the surface. These are confined to a single site within the Si(557)-Au unit cell, but only occupy ~8% of the sites that are available. Because they are restricted to specific positions along a line, and interact with one another over short distances, these adatoms represent a one-dimensional lattice gas.
We present the results of an investigation of the frozen one-dimensional lattice gas of extra adatoms. Imaging with STM provides a direct probe of the adatoms’ positions upon the lattice from which the pair correlation function can be extracted. There is a strong preference for adatoms to be spaced by even multiples of the Si(111) nearest neighbour spacing, a. The closest such spacing, 2a, is almost entirely suppressed (~1% probability). Thus, the 4a spacing sees augmented occupation from adatoms displaced by the strong 2a repulsion. Odd order spacings arise, almost entirely, from the 1a shift induced by domain boundaries. Correlation between adatoms in adjacent rows is negligible. These results closely resemble those from Si(111)-Au (5x2) which supports a denser lattice gas of Si adatoms [2]. However, our results are well described by a Hamiltonian which includes only 2a interactions; we do not see the long-range oscillatory correlations observed on Si(111)-Au(5x2).
[1] Crain, et al. PRB 69, 125401
[2] Ahn, et al. PRL 91, 196403
[3] Lipton-Duffin, et al. PRB 73, 245418
[4] Kirakosian, et al. PRB 67, 205412
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