Two-dimensional metallic (Tl,Au)/Si(100)c(2×2) : A Rashba-type system with C2v symmetry

A highly ordered $(\mathrm{Tl},\mathrm{Au})/\mathrm{Si}(100)\mathrm{c}(2\ifmmode\times\else\texttimes\fi{}2)$ two-dimensional compound was found to form upon room-temperature adsorption of 1.0 monolayer (ML) of Au onto a $\mathrm{Tl}/\mathrm{Si}(100)2\ifmmode\times\else\texttimes\fi{}1$ surface with 1.0 ML of Tl. The compound was determined to have a double-atomic-layer structure where Au and Tl atoms constitute the bottom and top layers, respectively. The compound is metallic due to the presence of the two dispersive surface-state bands crossing the Fermi level. The bands are spin split, and the peculiar feature of their spin texture is the occurrence of exclusively in-plane spin components that is originated from the ${\mathrm{C}}_2v$ symmetry of the system. One of the bands has a typical vortical spin texture, whereas the other exhibits an unusual nonvortical spin texture.