Kondo insulators (KI), in which the typically narrow band gap is formed by heavy quasi-particles, can produce three-dimensional topological insulating (TI) phases of a different physical nature than for the “classical” TI systems . It is the interplay of strong electronic correlations and spin-orbit coupling that results in gap-less, spin-polarized topologically protected surface states (TSS) which attracted a lot of interest recently. However, only a few strongly correlated TI systems have been investigated experimentally in the present context so far and even for these the surface electronic structure is not yet fully understood. In addition to questions concerning material related aspects – e. g. the importance of defects and effective doping, or deviating surface and bulk properties – it is the complex entanglement of correlations, spin- and charge-fluctuations, topology, and spin-orbit coupling which makes a comprehensive understanding of these systems challenging.
In the proposed project we shall investigate rare-earth compounds targeting in particular at the emergence of new topological phases based on the hybridization of the conduction-band states with significantly renormalized 4f electrons in the presence of strong spin-orbit coupling. The project is based on the work in the Research Unit FOR1162 where we have been investigating the electronic structure of epitaxial surface systems with a particular focus on the influence of spin-orbit interaction by means of high-resolution angle-resolved photoelectron spectroscopy (ARPES), and on first experimental studies on single-crystalline 4f hexaborides, both by ARPES and other techniques. These studies shall be extended and transferred to the investigation of other strongly correlated 4f materials, in particular SmB6, YbB6, YbB12, and related compounds, for which a strong topological phase with the possible emergence of TSS has been proposed or already observed. The influence of doping – or more precisely: the substitution of the rare-earth element by another – on the electronic structure plays a crucial role here, because it changes certain parameters like the rare-earth valence, the coupling between 4f electrons and conduction band states, or the formation of coherent bands in the system. Furthermore, we explore the relation to systems like Ce-based Kondo insulators, although they are most likely only weakly topological, i.e. the electronic states are rather unstable to disorder.
The mentioned 4f-based KI and charge-fluctuation systems shall be investigated in detail by high-resolution ARPES, spin-resolved ARPES, and hard X-ray photoemission spectroscopy (HAXPES) in order to understand the temperature-dependent bulk band gap evolution, the occurrence of TSS, and the complex interplay of charge- and spin-fluctuations, both at the surface and in the bulk. The analysis and interpretation of the photoemission data requires theoretical calculations of the electronic band structure and temperature dependent many-body effects, as being addressed within the framework of the planned Collaborative Research Center. These collaborations are essential to describe i) the specific single-particle properties (e. g. spin-orbit splitting, topological band structure), ii) the effect of strong many-body interactions leading to heavy quasi-particles near the Fermi level and a characteristic temperature dependence of the physical properties, and therewith iii) the formation of novel and peculiar phases in the ground state.
[C06.1] C.-H. Min, P. Lutz, S. Fiedler, B.Y. Kang, B. K. Cho, H.-D. Kim, H. Bentmann, and F. Reinert, Importance of charge fluctuations for the topological phase in SmB6, Phys. Rev. Lett. 112, 226402 (2014).
 M. Dzero, K. Sun, V. Galitski, and P. Coleman, Phys. Rev. Lett. 104, 106408 (2010).