Quasi-particle interference in surface- and bulk-doped topological insulators and Weyl semimetals

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Topological insulators are characterized by massless Dirac fermions which are topologically protected and exhibit a helical spin structure due to spin-orbit coupling. These unique electronic properties make them promising candidates for future spintronics applications. However, a detailed understanding of the correlation between the spin-resolved band structure, scattering events, and specific transport properties is still due. In this project we will continue our investigations of magnetically doped topological insulators. By spin-polarized scanning tunneling microscopy (STM) and spectroscopy (STS) we will study the mechanisms that establish long-range magnetic order between surface dopants mediated by topological states. Furthermore, we will perform XMCD and temperature-dependent scanning probe experiments on bulk-doped crystals to better understand the electronic structure of magnetically ordered TIs exhibiting the quantum anomalous Hall effect. We will pursue our investigations of hybrid organic-TI interfaces. This novel route, which we dis- covered during the first funding period, holds the promise of magnetically doping and potentially even gating TI surfaces by means of self-assembled molecular layers. We will continue our quasiparticle interference (QPI) and STS experiments to study to what extent the presence of a molecular overlayer modifies the electronic properties of the TI surface state. The potential for magnetically ordered molecular superstructures, one of the milestones towards spintronics applications, will be scrutinized by XMCD and spin-resolved STM experiments. In cooperation with collaborators from MPIs at Dresden (Yan, Felser) and Halle (Parkin) we will study the (spin-resolved) electronic properties of Weyl semimetals which exhibit pairs of Weyl points which carry topological monopoles of quantized Berry flux in momentum space. We will not only study the scattering states of theses surfaces in real space, but also investigate how local tunneling spectra and the QPI pattern change once Weyl semimetal surfaces are magnetically doped.