3×2reconstruction of the Sm/Si(111) interface

The initial formation of the samarium/silicon interface is studied by combining scanning tunneling microscopy (STM), low-energy electron diffraction, and ab initio pseudopotential calculations. A $\mathrm{Si}(111)3\ifmmode\times\else\texttimes\fi{}2$-Sm reconstruction is formed at a Sm coverage of 1/6 monolayer. High-resolution STM images reveal a strong bias-voltage dependence for the $3\ifmmode\times\else\texttimes\fi{}2$ reconstruction. In the empty-state STM images, the rows with a $\ifmmode\times\else\texttimes\fi{}2$ periodicity are shown at high bias voltages, and attributed to Sm atoms. At low bias voltage, an additional double row feature with a $\ifmmode\times\else\texttimes\fi{}1$ periodicity appears and dominates the empty-state image as the bias voltage decreases. The paired protrusions with the $\ifmmode\times\else\texttimes\fi{}1$ periodicity in the double rows in the empty-state image are attributed to Si atoms. In the filled-state STM images, double rows of protrusions forming zigzag chains with a $3\ifmmode\times\else\texttimes\fi{}1$ structure are observed and assigned to Si atoms. We propose a honeycomb chain-channel model for the $3\ifmmode\times\else\texttimes\fi{}2$ phase, common to alkali metals and alkaline-earth metals on Si(111). Simulated STM images based on this model are in excellent agreement with experiment.