Semiconductors with strong spin–orbit interaction as the underlying mechanism for the generation of spin-polarized electrons are showing potential for applications in spintronic devices. Unveiling the full spin texture in momentum space for such materials and its relation to the microscopic structure of the electronic wave functions is experimentally challenging and yet essential for exploiting spin–orbit effects for spin manipulation. Here we employ a state-of-the-art photoelectron momentum microscope with a multichannel spin filter to directly image the spin texture of the layered polar semiconductor BiTeI within the full two-dimensional momentum plane. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the valence and conduction band electrons in BiTeI have spin textures of opposite chirality and of pronounced orbital dependence beyond the standard Rashba model, the latter giving rise to strong optical selection-rule effects on the photoelectron spin polarization. These observations open avenues for spin-texture manipulation by atomic-layer and charge carrier control in polar semiconductors.

The ferromagnetic topological insulator V:(Bi,Sb) 2 Te 3 has been recently reported as a quantum anomalous Hall (QAH) system. Yet the microscopic origins of the QAH effect and the ferromagnetism remain unclear. One key aspect is the contribution of the V atoms to the electronic structure. Here the valence band of V:(Bi,Sb) 2 Te 3 thin films was probed in an element-specific way by resonant photoemission spectroscopy. The signature of the V 3d impurity band was extracted and exhibits a high density of states near Fermi level, in agreement with spin-polarized first-principles calculations. Our results indicate the occurrence of a ferromagnetic superexchange interaction mediated by the observed impurity band, contributing to the ferromagnetism in this system.

The surface state of a Z(2) topological insulator connects with the conduction and valence band continua of the bulk, thereby bridging the band gap of the volume. We investigate this connection of the surface and bulk electronic structure for Sb(2)Te(3)(0001) by photoemission experiments and calculations. Upon crossing the topmost valence band the topological surface state (TSS) maintains a coherent spectral signature, a two-dimensional character, and a linear dispersion relation. Surface-bulk coupling manifests itself in the spectra through (i) a characteristic kink in the TSS dispersion as it crosses the topmost valence band and (ii) the appearance of hybridization gaps between the TSS and bulk-derived surface resonance states at higher binding energies. The findings provide a natural explanation for the unexpectedly weak surface-bulk mixing indicated by recent transport experiments on Sb(2)Te(3).

Using angle-resolved photoelectron spectroscopy we investigate the surface electronic structure of the threedimensional topological insulator (TI) Sb2Te3(0001). Our data show the presence of a topological surface state in the bulk energy gap with the Dirac-point located above the Fermi level. The adsorption of Cs-atoms on Sb2Te3(0001) gives rise to a downward energy shift of the electronic valence band states which saturates at a value of ∼200 meV. For the saturation coverage the Dirac-point of the linearly dispersive surface state resides in close proximity to the Fermi level. The electronic structure of the Cs/Sb2Te3 interface therefore considerably deviates from previously studied metal-TI interfaces based on the isostructural compound Bi2Se3 which points to the importance of atomic composition in these hetero systems.Three-dimensional topological insulators (TIs) are currently generating widespread scientific interest in the condensed matter physics community as the distinct topology of their bulk band structure provokes the existence of robust metallic surface states with unique physical properties.1,2 The surface states locally span the global energy gap in the electronic excitation spectrum of the bulk material and their existence is protected by time-reversal symmetry.3,4 A salient feature of topological surface states (TSSs) lies in their characteristic spin structure introduced by spin-orbit coupling which locks the spin to the direction perpendicular to the wave vector.5 As a consequence of this spin structure the backscattering of the surface state electrons from non-magnetic impurities is strongly suppressed.6 Currently, most research on TIs is devoted to the chalcogenide semiconductors Bi 2 Se 3 7,8 and Bi 2 Te 3 8,9 , related ternary compounds 10,11 as well as to HgTe quantum wells. 12 The TSSs of these materials show a particularly simple dispersion consisting of a single spin-polarized Dirac-cone.The experimental realization of the most appealing properties of TSSs that have been predicted so far will require interface structures between TIs and metal films. This holds for example for the topological magnetoelectric effect at the interface of a TI and a ferromagnet 13,14 as well as for Majorana fermions at the interface of a TI and a superconductor. 15 It is therefore important to investigate the influence of metallic adlayers on the electronic structure of TI surfaces. Surfacesensitive spectroscopic techniques and in particular angleresolved photoelectron spectroscopy (ARPES), which also played a key role in the discovery of TIs 1 , are suitable methods to study modifications in the electronic structure during the formation of interfaces 16 . Indeed, great experimental effort on the basis of ARPES is currently directed towards an improved understanding of the influence of adsorbates on the electronic structure of TI surfaces. [17][18][19][20][21][22][23] However, most of these works have focused on the TI surface Bi 2 Se 3 (0001). It therefore appears essential to expand the present investigati...