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Monte Carlo simulations of a two-dimensional lattice gas in the grand canonical ensemble are performed to analyze the behavior of Si adatoms on Si(111)-1x1 surfaces. 2x2, c2x8 and c2x4 structures appear at 300K under the condition that only Coulomb repulsion is introduced as a pair interaction between adatoms. The adatom density increased with the temperature, and root 3 x root 3 structures appear at 550C. These results agree with our recent STM observations.
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Figure (a)Adatom configuration obtained by the MC simulation at T=300K. The small dots and solid circles indicate empty T4 sites and Si adatoms respectively. The yellow area corresponds to the 2x2 domain, and the red and blue areas correspond to the c2x8 and c2x4 domain, respectively. The gray area cannot be classified into a 2xn domain. (b) STM image of the 2x2, c2x8 and c2x4 structures on an Si(111) supercooled surface acquired at room temperature. The white lines are drawn to indicate the domain boundaries.