SFB 1170


Symmetry and correlation in epitaxial topological insulators


One of the central goals of this SFB is to understand and control the interplay of spin-orbit interaction and electronic correlations at surfaces, interfaces and in thin films. In our project we shall continue our research activities of the first funding period on the investigation of the spin-resolved electronic structure in epitaxial topological materials by means of advanced electron spectroscopies.

The first part of the project will be devoted to studying the influence of magnetic order on the electronic (band) structure of topological insulators (TI), particularly in different epitaxial thinfilm systems. Besides well-established magnetic topologial insulators, such as magnetically doped V,Cr:(BiSb)2Te3 and Mn:HgTe, our investigations will focus on novel materials that intrinsically combine magnetism and non-trivial topological properties, namely antiferromagnetic MnBi2Te4 and ferromagnetic MnBi4Te7. Furthermore, we will study magnetic proximity effects between ultrathin topological-insulator films, like Bi2Te3, and the magnetic semiconductors Eu(S,Te). The overarching goal is to gain a profound understanding of how the interplay of local magnetic moments and strong spin-orbit interaction in these materials shapes their electronic structure and gives rise to emerging topological states, such as the quantum anomalous Hall (QAH) state and possibly an antiferromagnetic TI.

In the second part of the project we will investigate the electronic structure of different 2D and 3D topological materials and particularly its tunability by use of structural, compositional or external parameters. One central material platform is HgTe, for which Weyl fermions in compressively strained films and Kane fermions in HgCdTe can be induced near the Fermi level. In addition to investigations of as-grown films we shall explore the possibility of spectroscopy on back-gated films in order to externally control the Fermi level in direct comparison to the result of transport measurements performed elsewhere in our SFB. A complementary research direction in this project part will be the topological electronic properties in epitaxial monolayer systems for which the interaction with the substrate can play a decisive role. We plan to study different heavy-adatom monolayers on metallic substrates, such as AgTe/Ag(111) and related systems with a potentially inverted band structure, as well as Ge/MoS2, predicted to realize a compensated Dirac semimetal.

The experimental techniques for these investigations shall be spin- and angle-resolved photoelectron spectroscopy (SR-ARPES) with a broad range of excitation energies, in combination with X-ray magnetic dichroism (XMCD/XMLD) and scanning tunneling microscopy (STM) as local and complementary probes of magnetic, structural and electronic properties. The planned activities will extend ongoing collaborations with other projects within the SFB, dedicated to the growth of advanced thinfilm systems, complementary spectroscopic approaches, and to ab initio calculations of the bulk and surface electronic structure.