Ab-initio Theory of Fourier-transformed Quasiparticle Interference Maps and Application to the Topological Insulator Bi2_2Te3_3

The quasiparticle interference (QPI) technique is a powerful tool that allows to uncover the structure and properties of electronic structure of a material combined with scattering properties of defects at surfaces. Recently this technique has been pivotal in proving the unique properties of the surface state of topological insulators which manifests itself in the absence of backscattering. In this work we derive a Green function based formalism for the ab initio computation of Fourier-transformed QPI images. We show the efficiency of our new implementation at the examples of QPI that forms around magnetic and non-magnetic defects at the Bi$_2$Te$_3$ surface. This method allows a deepened understanding of the scattering properties of topologically protected electrons off defects and can be a useful tool in the study of quantum materials in the future

The AiiDA-KKR plugin and its application to high-throughput impurity embedding into a topological insulator

The ever increasing availability of supercomputing resources led computer-based materials science into a new era of high-throughput calculations. Recently, Pizzi et al. [Comp. Mat. Sci. 111, 218 (2016)] introduced the AiiDA framework that provides a way to automate calculations while allowing to store the full provenance of complex workflows in a database. We present the development of the AiiDA-KKR plugin that allows to perform a large number of ab initio impurity embedding calculations based on the relativistic full-potential Korringa-Kohn-Rostoker Green function method. The capabilities of the AiiDA-KKR plugin are demonstrated with the calculation of several thousand impurities embedded into the prototypical topological insulator Sb2Te3. The results are collected in the JuDiT database which we use to investigate chemical trends as well as Fermi level and layer dependence of physical properties of impurities. This includes the study of spin moments, the impurity's tendency to form in-gap states or its effect on the charge doping of the host-crystal. These properties depend on the detailed electronic structure of the impurity embedded into the host crystal which highlights the need for ab initio calculations in order to get accurate predictions

Inter-orbital Cooper pairing at finite energies in Rashba surface states

Multi-band effects in hybrid structures provide a rich playground for unconventional superconductivity. We combine two complementary approaches based on density-functional theory (DFT) and effective low-energy model theory in order to investigate the proximity effect in a Rashba surface state in contact to an $s$-wave superconductor. We discuss these synergistic approaches and combine the effective model and DFT analysis at the example of a Au/Al heterostructure. This allows us to predict finite-energy superconducting pairing due to the interplay of the Rashba surface state of Au, and hybridization with the electronic structure of superconducting Al. We investigate the nature of the induced superconducting pairing and quantify its mixed singlet-triplet character. Our findings demonstrate general recipes to explore real material systems that exhibit inter-orbital pairing away from the Fermi energy

Strong Spin-Orbit Torque effect on magnetic defects due to topological surface state electrons in Bi2_{2}Te3_{3}

We investigate the spin-orbit torque exerted on the magnetic moments of the transition-metal impurities Cr, Mn, Fe and Co, embedded in the surface of the topological insulator Bi$_{2}$Te$_{3}$, in response to an electric field and a consequent electrical current flow in the surface. The multiple scattering problem of electrons off impurity atoms is solved by first-principles calculations within the full-potential relativistic Korringa-Kohn-Rostoker (KKR) Green function method, while the spin-orbit torque calculations are carried out by combining the KKR method with the semiclassical Boltzmann transport equation. We analyze the correlation of the spin-orbit torque to the spin accumulation and spin flux in the defects. We compare the torque on different magnetic impurities and unveil the effect of resonant scattering. In addition, we calculate the resistivity and the Joule heat as a function of the torque in these systems. We predict that the Mn/Bi$_{2}$Te$_{3}$ is optimal among the studied systems

Density-functional description of materials for topological qubits and superconducting spintronics

Interfacing superconductors with magnetic or topological materials offers a playground where novel phenomena like topological superconductivity, Majorana zero modes, or superconducting spintronics are emerging. In this work, we discuss recent developments in the Kohn-Sham Bogoliubov-de Gennes method, which allows to perform material-specific simulations of complex superconducting heterostructures on the basis of density functional theory. As a model system we study magnetically-doped Pb. In our analysis we focus on the interplay of magnetism and superconductivity. This combination leads to Yu-Shiba-Rusinov (YSR) in-gap bound states at magnetic defects and the breakdown of superconductivity at larger impurity concentrations. Moreover, the influence of spin-orbit coupling and on orbital splitting of YSR states as well as the appearance of a triplet component in the order parameter is discussed. These effects can be exploited in S/F/S-type devices (S=superconductor, F=ferromagnet) in the field of superconducting spintronics

Spin-resolved electronic response to the phase transition in MoTe2_2

The semimetal MoTe$_2$ is studied by spin- and angle- resolved photoemission spectroscopy to probe the detailed electronic structure underlying its broad range of response behavior. A novel spin-texture is uncovered in the bulk Fermi surface of the non-centrosymmetric structural phase that is consistent with first-principles calculations. The spin-texture is three-dimensional, both in terms of momentum dependence and spin-orientation, and is not completely suppressed above the centrosymmetry-breaking transition temperature. Two types of surface Fermi arc are found to persist well above the transition temperature. The appearance of a large Fermi arc depends strongly on thermal history, and the electron quasiparticle lifetimes are greatly enhanced in the initial cooling. The results indicate that polar instability with strong electron-lattice interactions exists near the surface when the bulk is largely in a centrosymmetric phase

Author Correction: Non-local effect of impurity states on the exchange coupling mechanism in magnetic topological insulators

A Correction to this paper has been published: https://doi.org/10.1038/s41535-021-00314-

Spin scattering of topologically protected electrons at defects

This thesis provides a detailed microscopic understanding of the impurity scattering of topologically protected electrons, which are studied within the example of strong three- dimensional topological insulators (TIs) and type-II Weyl semimetals. The immense research interest in the recent past in topological materials is to a large extend due to the fact that their unconventional electronic surface states are robust against perturbations, such as surface structural relaxations or defects. One of the most intriguing physical properties of topological surface states in TIs is the forbidden backscattering off time-reversal invariant defects, which makes TI materials very promising candidates for future low-power electronics or quantum information technology. In a theoretical and computational study, we were able to verify the predicted protection against backscattering off non-magnetic defects and reveal the strong suppression of, on the one hand, formally allowed near-backscattering processes, and, on the other hand, backscattering off small time-reversal-breaking defects, e.g. single Mn atoms embedded in the surface of Bi2Te3. Furthermore, we demonstrated that suitable band structure and defect engineering can lead to a focusing effect in charge density oscillations in form of standing waves around impurities. These oscillations, commonly named quasiparticle interference (QPI), allow to extract scattering information of defects at the surface, and form a very important tool in the research concerning topological materials. We investigated the effect that scattering of topologically protected electrons has on distance-dependent oscillatory exchange interactions between magnetic impurities, and explained experimentally observed trends in the collective magnetization at different transition metal coverages on Bi2Te3. Finally, we explored the scattering signatures of Fermi arcs in the type-II Weyl semimetal candidates WTe2 and MoTe2, where, moreover, a universal response to impurities across the topological phase transition was uncovered. For the very first time the scattering properties of topologically protected electrons were investigated on the basis of ab-initio calculations, employing the relativistic full-potential Korringa-Kohn-Rostoker Green function method. The simulations were made possible by improving the available methods in terms of an efficient parallelization of existing computer codes and by developing new tools to compute QPI images from first principles. The results of this thesis underline the importance of a realistic description of the impurities embedded into the clean topological surfaces and thus the need for ab-initio calculations. The very good agreement between our results and several different experiments such as scanning tunneling microscopy, X-ray magnetic circular dichroism, or angular-resolved photo-emission spectroscopy highlight the extremely high accuracy we were able to achieve in our calculations and provides confidence in the predictive power of our method. Throughout our investigation the importance of resonances in the scattering potential became evident, giving a unified view on QPI, quantum coherence, and magnetic interactions in both TIs as well as Weyl semimetals. These fundamental new insights set the foundations of tuning transport properties and controlling the magnetization in the fascinating class of topological materials in the future. Our results are thus of importance in overcoming the material science challenges on the way to topological-materials-based technology

Investigation of Rotor Blade Adjustment on a Counter-Rotating Shrouded Fan withblades made of CFRP

Die Erforschung von innovativen Konzepten für Flugzeugtriebwerke zur besseren Umweltverträglichkeit spielt heutzutage durch die ehrgeizigen ACARE-Ziele eine wichtige Rolle. In der vorliegenden Arbeit wird der gegenläufige Fan CRISP2, der vom DLR ausgelegt wurde, im Hinblick auf die Verstellung der Fanschaufeln untersucht. Dafür wird eine neue Strategie entwickelt, mit der es möglich ist, verdrehte und deformierte Fanschaufeln, die aus einer FE-Rechnung stammen, für die CFDRechnung zu vernetzen. Mit Hilfe dieser Strategie wird anschließend eine Untersuchung des Spalts zwischen den Fanschaufeln und dem Gehäuse durchgeführt. Abschließend werden verschiedene Kombinationen von Rotorblattverstellungen berechnet und analysiert.The investigation of innovative concepts for aircraft engines with better environmental performance nowadays is of great importance due to the ambitious objectives of ACARE. In the present work the counter-rotating integrated shrouded propfan 2 (CRISP2), which was designed by the DLR, is investigated with regard to the variable pitch of the fan blades. Therefore, a new strategy is being developed in order to create a CFD-mesh from deformed fan blades with variable pitch, which originate from a FE calculation. Using this strategy, a study of the gap between the fan blades and the casing is performed subsequently. Finally various combinations of rotor blade adjustments are calculated and analyzed