Quantitative LEED I-V and ab initio study of the Si(111)-3×2-Sm surface structure and the missing half-order spots in the 3×1 diffraction pattern

We have used low-energy electron diffraction (LEED) I-V analysis and ab initio calculations to quantitatively determine the honeycomb chain model structure for the Si(111)-3×2-Sm surface. This structure and a similar 3×1 recontruction have been observed for many alkali-earth and rare-earth metals on the Si(111) surface. Our ab initio calculations show that there are two almost degenerate sites for the Sm atom in the unit cell, and the LEED I-V analysis reveals that an admixture of the two in a ratio that slightly favors the site with the lower energy is the best match to experiment. We show that the I-V curves are insensitive to the presence of the Sm atom and that this results in a very low intensity for the half-order spots, which might explain the appearance of a 3×1 LEED pattern produced by all of the structures with a 3×2 unit cell.