2D Tl-Pb compounds on Ge(111) surface: atomic arrangement and electronic band structure

Structural transformations and evolution of the electron band structure in the (Tl, Pb)/Ge(1 1 1) system have been studied using low-energy electron diffraction, scanning tunneling microscopy, angle-resolved photoelectron spectroscopy and density functional theory calculations. The two 2D Tl-Pb compounds on Ge(1 1 1), root 3 x root 3-(Tl, Pb) and 3 x 3-(Tl, Pb), have been found and their composition, atomic arrangement and electron properties has been characterized. The (Tl, Pb)/Ge(1 1 1) root 3 x root 3 compound is almost identical to the alike (Tl, Pb)/Si(1 1 1) root 3 x root 3 system from the viewpoint of its atomic structure and electronic properties. They contain 1.0 ML of Tl atoms arranged into a honeycomb network of chained trimers and 1/3 ML of Pb atoms occupying the centers of the honeycomb units. The (Tl, Pb)/Ge(1 1 1) 3 x 3 compound contains six Tl atoms and seven Pb atoms per 3x3 unit cell (i. e. similar to 0.67 ML Tl and similar to 0.78 ML Pb). Its atomic structure can be visualized as consisting of Pb hexagons surrounded by Tl trimers. The (Tl, Pb)/Ge(1 1 1) root 3 x root 3 and (Tl, Pb)/Ge(1 1 1) 3 x 3 compounds are metallic and their band structures contain spin-split surface-state bands. By analogy with the (Tl, Pb)/Si(1 1 1) root 3 x root 3, these (Tl, Pb)/Ge(1 1 1) compounds are believed to be promising objects for prospective studies of superconductivity in one-atom-layer systems.