The initial formation of the samarium/silicon interface is studied by combining scanning tunneling microscopy ͑STM͒, low-energy electron diffraction, and ab initio pseudopotential calculations. A Si(111)3ϫ2-Sm reconstruction is formed at a Sm coverage of 1/6 monolayer. High-resolution STM images reveal a strong bias-voltage dependence for the 3ϫ2 reconstruction. In the empty-state STM images, the rows with a ϫ2 periodicity are shown at high bias voltages, and attributed to Sm atoms. At low bias voltage, an additional double row feature with a ϫ1 periodicity appears and dominates the empty-state image as the bias voltage decreases. The paired protrusions with the ϫ1 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 3ϫ1 structure are observed and assigned to Si atoms. We propose a honeycomb chain-channel model for the 3ϫ2 phase, common to alkali metals and alkaline-earth metals on Si͑111͒. Simulated STM images based on this model are in excellent agreement with experiment.

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