Scholz, Markus Reiner: Spin Polarization, Circular Dichroism, and Robustness of Topological Surface States: A Photoemission Study = Spinpolarisation, Zirkulardichroismus und Robustheit Topologischer Oberflächenzustände. , 2012
Univ. Potsdam
Open Accesn Version

This thesis is focussed on the electronic properties of the new material class named topological insulators. Spin and angle resolved photoelectron spectroscopy have been applied to reveal several unique properties of the surface state of these materials. The rst part of this thesis introduces the methodical background of these quite established experimental techniques. In the following chapter, the theoretical concept of topological insulators is introduced. Starting from the prominent example of the quantum Hall e ect, the application of topological invariants to classify material systems is illuminated. It is explained how, in presence of time reversal symmetry, which is broken in the quantum Hall phase, strong spin orbit coupling can drive a system into a topologically non trivial phase. The prediction of the spin quantum Hall e ect in two dimensional insulators an the generalization to the three dimensional case of topological insulators is reviewed together with the rst experimental realization of a three dimensional topological insulator in the Bi1��xSbx alloys given in the literature. The experimental part starts with the introduction of the Bi2X3 (X=Se, Te) family of materials. Recent theoretical predictions and experimental ndings on the bulk and surface electronic structure of these materials are introduced in close discussion to our own experimental results. Furthermore, it is revealed, that the topological surface state of Bi2Te3 shares its orbital symmetry with the bulk valence band and the observation of a temperature induced shift of the chemical potential is to a high probability unmasked as a doping e ect due to residual gas adsorption. The surface state of Bi2Te3 is found to be highly spin polarized with a polarization value of about 70% in a macroscopic area, while in Bi2Se3 the polarization appears reduced, not exceeding 50%. We, however, argue that the polarization is most likely only extrinsically limited in terms of the nite angular resolution and the lacking detectability of the out of plane component of the electron spin. A further argument is based on the reduced surface quality of the single crystals after cleavage and, for Bi2Se3 a sensitivity of the electronic structure to photon exposure. We probe the robustness of the topological