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

本项目的目的是研究原始和吸附复盖拓扑绝缘子(TI)的几何结构和电子结构之间的相关性。在此背景下，我们应用了表面X射线衍射(SXRD)、X射线吸收精细结构(XAFS)、扫描隧道显微镜(STM/STS)以及角分辨和自旋分辨光电子能谱(ARPES)。在此期间，我们成功地进行了所有的实验，并完成了一些进一步的研究，使人们对Bi2Se3(0001)原始表面和吸附表面的结构及其与拓扑表面态的关系有了实质性的了解。到目前为止，已经出版了五份出版物，还有几份正在筹备中。磁性(Fe，Cr)和非磁性(Cs，Au，Ag，H)的吸附即使在160K左右，即远低于室温的温度下，也会引起强烈的界面反应。一般说来，我们发现发生了替代过程。例如，顶层五元层中的铋原子被Fe、Au、Ag取代，这涉及到局部金属硒结构的形成。我们的结果在可能应用于新型自旋电子结构和磁存储设备的背景下是重要的。例如，量子反常霍尔效应(QAHA)被用于自旋极化载流子的耗散较少的输运，其中拓扑表面态的间隙是由与磁性物种的吸附(掺杂)诱导的磁场引起的。我们的结果表明，外来物种的吸附一般会在界面上引起不受控制的反应，这是不利的，原因有几个(例如，界面态的形成充当散射中心或替代导电通道)。因此，为了项目的继续，提出了另一种方法来沉积晶格匹配的反铁磁或铁磁绝缘材料，如EUS、MnSe和MnBi，并利用磁邻近效应在TI中创建交换场，同时保持吸附/TI界面的完整性。其次，建议沉积超薄的绝缘层(如Al_2O_3或CaF_2)作为扩散势垒，在其上沉积铁等铁磁性物种。作为第三条路线，我们建议制备和表征Rashba系统(Te BiI)和Ti(PbSb2Te4)的组合，理论上预测它将形成一个新的螺旋态，可以用于自旋极化输运。

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.