Experimental evidence for two-dimensional states localized in subsurface region of Ge(1Â 1Â 1)

The present study gives experimental evidence of two-dimensional electronic states localized in subsurface layers of a Ge(1 1 1) substrate, in which a monolayer of Pb adsorbed on a Ge(1 1 1) surface is used as a template. We observed three pairs of the subsurface states by angle-resolved photoelectron spectroscopy, which are closely related to bulk heavy-hole, light-hole and spin-orbit split-off bands of Ge(1 1 1). The heavy-hole- and light-hole-derived bands show the Rashba-type band crossing at the Γ¯ point. A density functional theory calculation suggests that the branches of these bands exhibit the Rashba-type spin splitting. On the other hand, we observed peculiar spin polarization for the subsurface states attributed to the bulk spin-orbit split-off band by spin- and angle-resolved photoelectron spectroscopy, where spin polarization of each branch is inverted with respect to Γ¯ and the degeneracy, expected for a normal Rashba spin-split surface state is lifted at Γ¯.