Effects of orbital composition in a pair of spin-orbit-split surface bands at Tl/Ge(111)

The spin texture of the unoccupied surface electronic structure of the metal-semiconductor hybrid system Tl/Ge(111)-(1 x 1) is investigated by spin- and angle-resolved inverse photoemission as well as quasiparticle band-structure calculations. Spin-polarized surface bands with rotating spin and giant energy splitting are found along (Gamma) over bar (K) over bar ((K) over bar'), forming valleys with alternating out-of-plane spin polarization at (K) over bar and (K) over bar'. This behavior is known from the equivalent hybrid system on Si(111). Along (Gamma) over bar (M) over bar, a pair of surface bands appears within a projected bulk band gap, whose equivalent on Tl/Si(111) is a surface resonance because, there, it overlaps with bulk states. Surprisingly, the spin splitting of these bands on Tl/Ge(111) is much smaller than on Tl/ Si(111) despite the stronger surface localization and the heavier substrate. Our detailed analysis of the band structure and a tight-binding model including all relevant interactions show that a remarkable interplay between spin- orbit coupling and hybridization is responsible for this unexpected result. The comparison between the two similar hybrid systems demonstrates that the strength of the spin- orbit coupling alone, based on the atomic number of the respective elements, is not sufficient to estimate spin splittings of spin-orbit-influenced surface states.