Exploring the electronic properties of novel spintronic materials by photoelectron spectroscopy

The presented thesis deals with the electronic and structural surface properties of twoimportant material classes: $\textit{Topological insulators}$ and half-metallic $\textit{Heusler alloys}$. In the ideal case these are predicted to show a full spin polarization at the Fermienergy and therefore are potential candidates for future spintronic applications. In the case of the topological insulators, the electronic band structure of thin Bi$_{2}$Te$_{3}$ and Sb$_{2}$Te$_{3}$ films was extensively studied by means of spin- and angle-resolved photoemission spectroscopy. The experimental determination of topologically protected robust and spin-polarized surface states was of particular interest. A detailed analysis of the observed results was performed within the density functional theory employing the local-density approximation. In this work, also the influence of various in situ cleaning procedures for obtaining atomically clean and well-ordered topological insulator surfaces was investigated. The Bi$_{2}$Te$_{3}$ and Sb$_{2}$Te$_{3}$ thin film surfaces studied were successfully cleaned and subsequently characterized by photomission spectroscopy. It could be shown, that the measured dispersive energy bands were insensitive to the surface modifications induced by the cleaning processes. The measured in-plane (∼ 45%) and $\textit{out-of-plane}$ (∼ 15%) spin polarization values of the Bi$_{2}$Te$_{3}$ Dirac cone surface state exhibited a very good agreement with the theoretical calculations. The degree of the spin polarization was confirmed by means of depth-resolved slab simulations. For the first time it is shown, that the delocalization of electron density over the first quintuple layer and related spin reversal effects lead to reduced spin polarization values. For the deposition of Co-based Heusler alloy thin films a new magnetron sputtering deposition system was assembled and commissioned. The material system chosen in this study was the ternary Co$_{2}$CrAl full-Heusler compound. Thin films were deposited in situ from a stoichiometric target on single crystalline MgO(001)substrates under various growth conditions. The analysis of surface and bulk structure revealed first process parameters for an epitaxial crystal growth of pure and ordered thin films. First angle-resolved photoemission measurements showed dispersive valence band features which were in good agreement to the calculated density of states