Theoretical analysis of the electronic structure of the stable and metastable c(2x2) phases of Na on Al(001): Comparison with angle-resolved ultra-violet photoemission spectra

TL;DR

This paper provides a theoretical analysis of the electronic structures of two Na on Al(001) phases, comparing calculations with experimental photoemission data, revealing how surface states and charge distributions differ between stable and metastable phases.

Contribution

It offers a detailed density functional theory analysis of the electronic structures of two Na/Al(001) phases, correlating theoretical results with experimental spectra and elucidating their surface state characteristics.

Findings

Two pronounced bands result from coupling between Na monolayer and Al surface states.

The unstable phase remains metallic, while the stable phase exhibits a band structure similar to an ionic crystal.

Calculated band structures match well with angle-resolved photoemission spectra.

Abstract

Using Kohn-Sham wave functions and their energy levels obtained by density-functional-theory total-energy calculations, the electronic structure of the two c(2x2) phases of Na on Al(001) are analysed; 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 ca. 180K 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(2x2)-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 unstable 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.