Electron compound nature in a surface atomic layer of a two-dimensional hexagonal lattice

Abstract

The two-dimensional (2D) ordered phase of monovalent metal alloy, $\sqrt{21}\ifmmode\times\else\texttimes\fi{}\sqrt{21}$, is formed on the Si(111) surface with the constant electron/atom ratio, indicating electron compound nature. Two conventional theories of the Hume-Rothery compounds, Jones model (nearly-free-electron model), and pseudopotential model (interionic interaction model), were applied to examine stability of the 2D phase. We found breakdown of the former and confirmation of the latter approaches with importance of medium-range interatomic interaction, mediated by the 2D surface-state electrons, in the latter approach.