| Summary: | The π -bond Seiwatz chain (SC) is one of the well-known reconstruction induced by alkaline or rare earth metals on Si(111) surface. Here we identify by ab initio calculations a new reconstruction of La induced quasi-two-dimensional Si _3 trimer monolayer on Si(111)- $(\sqrt{3}\times \sqrt{3})R30{}^{\circ}$ surface. Its surface unit cell has one La atom and one Si _3 trimer with the same La coverage (1/3 monolayer) as SC structure and the Si _3 trimer satisfies the electron counting rule with a transfer of valence electrons from La atom, formally as ${\mathrm{L}\mathrm{a}}^{3+}{[{\mathrm{S}\mathrm{i}}^{-}]}_{3}$ , in correspondence to the milkstool model for Bi trimers on Si(111) surface. Band structure calculations show a semiconducting character with an indirect surface band gap of 0.76 eV. Moreover, a two-stage conversion process between the Si _3 trimer and SC structure is verified by the climbing-image nudged elastic band method. These findings pave the way for further exploration of the new surface structure and its outstanding properties.
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