Magnetically doped topological insulators: From the Ground State to Dynamics

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The project investigates primarily the impact of magnetic doping on three-dimensional topological insulator (TI) systems. Detailed information on their electronic structure is obtained by means of spin and angle-resolved photo emission spectroscopy (SARPES). Corresponding pump-probe experiments allow to characterise the dynamical behaviour of the investigated systems in a direct and reliable way. In addition the transport properties of the investigated systems will be determined, studying apart from the temperature dependence of the longitudinal and transversal magneto-transport its spatial distribution by varying the sample thickness. The experiments are accompanied by theoretical work done within the framework of density functional theory combined with dynamical mean field theory. Fully relativistic calculations account reliably all spin-orbit induced characteristic features in the electronic structure. This also applies for dichroic effects in SARPES, that are described by means of the spin density matrix formulation of the one-step model of photo emission. In a analogous way the impact of spin-orbit coupling on transport properties is accounted for, when applying Kubo's linear response formalism. While the project work was restricted so far to ground state properties, emphasis will be put on the influence of finite temperatures and on dynamical phenomena. One of the central issues will be the identification of magnetically doped TI systems for which the surface band gap opening unambiguously originates from magnetic ordering in the system. The largest magnetically induced gap, obtained so far, is only 10 meV wide with a Curie temperature around 10 K. It is expected that switching from (Bi1-xMnx)2(Se,Te)3 to (Sb1-xVx)2Te3 will enhance the effect by an order of magnitude. The corresponding Curie temperatures for the surface and bulk regime, that are expected to differ, will be determined in a addition as a function of the composition. Concerning magneto-transport, the spatial distribution of the conductivity as well as its dependency on the magnetic doping and temperature will be in the focus of our investigations. To determine the spin texture and its dependency on magnetic ordering SARPES measurements will be performed above and below the Curie temperature of magnetically ordered TIs. Pump-probe experiments, that allow spin manipulation, will be supported by simulations to determine the relevant channels for spin and charge relaxation together with their characteristic time scales. The static transport measurements will be augmented by the generation and observation of laser-induced photo currents, with their dependency on the polarisation, the photon energy and laser fluence followed across the ferromagnetic phase transition.