Atomic and electronic structure of Tl/Ge(111)-(1x1): LEED and ARPES measurements and first-principles calculations

We have investigated the atomic and electronic structure of the Tl/Ge(111)-(1x1) surface. The Tl adsorption site was determined to be the T-4 site by dynamical low-energy electron diffraction I-V analysis. The surface electronic structure was measured using angle-resolved photoelectron spectroscopy. By comparing the observed band structure with the first-principles calculation, we revealed that only the Tl 6p electrons are involved in the Tl-Ge bonding and that hybridization of Tl 6s(2)6p valence electrons is negligible. This indicates that the inert-pair effect plays a decisive role in the formation of the (1x1) surface. On the other hand, a surface band is not fully occupied but forms a small hole pocket around (Gamma) over bar. We propose that it is caused by a small amount of remaining defects due to the small energy gap. Our calculation including spin-orbit interaction indicates the existence of a large spin-orbit splitting in the lowest unoccupied surface band of Tl 6p(x)+6p(y) character.