Topological states in a two-dimensional metal alloy in Si surface: BiAg/Si(111)- 4×4 surface

A bridging topological state with a conventional semiconductor platform offers an attractive route towards future spintronics and quantum device applications. Here, based on first-principles and tight-binding calculations, we demonstrate the existence of topological states hosted by a two-dimensional (2D) metal alloy in a Si surface, the BiAg/Si(111)-$4\ifmmode\times\else\texttimes\fi{}4$ surface, which has already been synthesized experimentally. It exhibits a topological insulating state with an energy gap of 71 meV $(\ensuremath{\sim}819\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ above the Fermi level and a topological metallic state with quasiquantized conductance below the Fermi level. The underlying mechanism leading to the formation of such nontrivial states is revealed by analysis of the ``charge-transfer'' and ``orbital-filtering'' effect of the Si substrate. A minimal effective tight-binding model is employed to reveal the formation mechanism of the topological states. Our finding opens opportunities to detect topological states and measure its quantized conductance in a large family of 2D surface metal alloys, which have been or are to be grown on semiconductor substrates.