By use of spin- and angle-resolved photoelectron spectroscopy (ARPES) we aim in this project to study two different material classes in which strong spin-orbit coupling lifts the spin degeneracy of the electronic surface state bands close to the Fermi level and gives rise to characteristic spin textures in momentum space. The investigations shall provide fundamental insight into the properties of topologically protected and Rashba-split states, in the presence of a possibly highly complex combination of aspects like exchange interaction, strongly correlated magnetic impurities, the entanglement of spin and orbital degrees of freedom, and finally the coupling to the lattice symmetry and to the bulk electronic structure. The resulting interplay of magnetism and spin-orbit interaction is of direct relevance for new topological phenomena, such as the quantum anomalous Hall effect.
The planned experiments will be performed on epitaxially grown thin-film systems which allows for controlled variation of several parameters, such as stoichiometry, film thickness, or doping concentration, which have an immediate influence on topology, spin-orbit coupling, time-reversal symmetry, and magnetic interactions. The first approach in our proposal is to investigate the influence of spin-orbit coupling on the spin- and orbital-dependent symmetry of electronic (surface) states. This shall be addressed by a systematic measurement of the angle-resolved photoelectron spin-texture in dependence of light polarization and photon energy. Starting with experiments on surface alloys with tunable Rashba splitting these studies will eventually be targeting on topological insulators (TI) and strongly correlated materials incorporating heavy elements. In the second approach we will address the effect of magnetic impurities on the electronic structure of two- and three-dimensional (2D and 3D) topological insulators, namely in particular V2VI3 materials and HgTe. Besides material related questions – concerning primarily the incorporation of the magnetic dopants in the TI host – the central goal here is to understand how exchange interaction and correlations lead to a magnetic spin splitting in the bulk and at the surface or, potentially, to non-magnetic ground states, resulting e.g. from Kondo screening. In this context not only modifications of the electronic band dispersion will be a major concern but also the combined influence of spin-orbit and exchange interaction on the spin-polarization of the electronic states.
[A01.3] H. Bentmann, S. Abdelouahed, M. Mulazzi, J. Henk, and F. Reinert, Direct observation of interband spin-orbit coupling in a two-dimensional electron system, Phys. Rev. Lett. 108, 196801 (2012)