Surface states at close-packed surfaces of simple metals

Angle-resolved photoemission spectra from close-packed surfaces of thin epitaxial films of the earth-alkali metals Mg, Ca, and Sr on W(110) are reported. While normal emission spectra of Mg are dominated by huge emissions from a Shockley surface state, respective emissions are lacking for Ca and Sr. It is shown by means of LDA band-structure calculations that the band gap responsible for the surface state of Mg is Shockley inverted, while admixtures of d states to the valence bands lead in case of Ca and Sr to the formation of ordinary gaps that are not suited for Shockley surface states. Consequences of this finding for the surface related electron structure of noble metals, Zn and Al group elements and rare-earth metals are discussed.