Correlation of geometric and electronic structure at surfaces and interfaces of adsorbate covered topological insulators

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It is the aim of the project to study the correlation between geometric and electronic structure of pristine and adsorbate covered topological insulators (TI). In this context, we apply surface x-ray diffraction (SXRD), x-ray absorption fine structure (XAFS), scanning tunnelling microscopy (spectroscopy) (STM/STS) and angular and spin resolved photo electron spectroscopy (ARPES). During the previous period all proposed experiments could be carried out successfully, in addition some further studies were completed leading to a substantial gain in knowledge regarding the structure of the pristine and adsorbate covered Bi2Se3(0001) surface together with its relation with the topological surface state. Up to now five publications have been published and several more are in preparation. Adsorption of magnetic (Fe, Cr) and non-magnetic (Cs, Au, Ag, H) adsorbates induces a strong interface reaction even at a temperature at about 160 K, i.e. well below room temperature. In general, we find that substitution processes take place. For instance Bi atoms within the top quintuple layer are replaced by Fe, Au, Ag, involving the formation of a local metal selenide structure. Our results are important in the context of possible applications in novel spin electronic architectures and magnetic storage devices. For instance the Quantum Anomalous Hall Effect (QAHA) is exploited for dissipation less transport of spin-polarized carriers, where a gap in the topological surface state is induced by a magnetic exchange field, induced by adsorption (doping) with magnetic species. Our results have shown that the adsorption of foreign species in general induces an uncontrolled reaction at the interface which is disadvantageous for several reasons (e.g. formation of interface states acting as scattering centers or as alternative conducting channels). Consequently, for the continuation of the project alternative routes are proposed to deposit lattice matched anti-ferromagnetic or ferromagnetic insulating materials like EuS, MnSe, and MnBi, and to exploit the magnetic proximity effect to create an exchange field in the TI by simultaneously preserving the integrity of the adsorbate/TI interface. Secondly, deposition of an ultra thin insulating barrier (e.g. Al2O3 or CaF2) is proposed to serve as a diffusion barrier upon which a ferromagnetic species such as Fe will be deposited. As a third route we propose to prepare and to characterize a combination of a Rashba system (Te Bi I) with a TI (Pb Sb2 Te4), which is theoretically predicted to form a new helic state, which could serve for spin polarized transport.