Atomic and electronic structure of Sr/Si(001)-(2 x 2)
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The adsorption of Sr on the Si(001) surface with the semiantiphase dimer (2 x 2) reconstruction is studied, based upon the ab initio pseudopotential calculations. It is calculated that the sermantiphase dimer (2 x 2) reconstruction (2 dimers per unit cell) is more favorable than the (2 x 1) phase (I dimer per unit cell) by an energy of about 0.24 eV/dimer. Considering the energetically more stable reconstruction, we have assumed four possible locations for 1/4 monolayer (ML) Sr adsorption on this surface: (i) bridge, (ii) cave, (iii) pedestal, and (iv) valley-bridge. We find that Sr adsorption on the valley-bridge site is energetically more favorable than all other cases studied here. Interestingly, one of the dimers becomes symmetric, but the other one is still asymmetric with the buckling angle reduced from 18 degrees to 14 degrees, when compared with the clean Si(001)-(2 x 2) surface. The calculated bond length between Sr and Si in the case of valley-bridge adsorption site is 3.05 angstrom, and in good agreement with other theoretical calculations. We also present and compare the electronic band structures for the clean and covered surfaces as well as the corresponding charge density plots.