Theoretical analysis of the electronic structure of the stable and metastablec(2×2)phases of Na on Al(001): Comparison with angle-resolved ultraviolet photoemission spectra

Using Kohn-Sham wave functions and their energy levels obtained by density-functional-theory total-energy calculations, the electronic structure of the two $c(2\mathrm{}\times 2)$ phases of Na on Al(001) are analyzed, namely, the metastable hollow-site structure formed when adsorption takes place at low temperature, and the stable substitutional structure appearing when the substrate is heated thereafter above 180 K or when adsorption takes place at room temperature from the beginning. The experimentally obtained two-dimensional band structures of the surface states or resonances are well reproduced by the calculations. With the help of charge-density maps, it is found that, in both phases, two pronounced bands appear as the result of a characteristic coupling between the valence-state band of a free $c(2\mathrm{}\times 2)$-Na monolayer and the surface-state/resonance band of the Al surfaces; that is, the clean (001) surface for the metastable phase and the unstable, reconstructed “vacancy” structure for the stable phase. The higher-lying band, being Na derived, remains metallic for the metastable phase, whereas it lies completely above the Fermi level for the stable phase, leading to the formation of a surface-state/resonance band structure resembling the bulk band structure of an ionic crystal.