Topological phases in heavy-fermion insulators and semimetals, and the importance of time-reversal or inversion symmetry
The aim of this project is to realize and investigate novel topological phases in heavy fermion sys- tems, whose exotic electronic and magnetic properties are driven by the interaction of the local- ized electrons and the conduction electrons under the influence of crystalline symmetry and band topology. The characteristic spectral key features shall be investigated experimentally by different complementary spectroscopy methods, both in the laboratory and at synchrotron radiation facili- ties. Based particularly on the first-period results on SmB6 , we will extend our research from topo- logical Kondo insulators to semimetallic Kondo systems with non-centrosymmetric structure such as Ce3Bi4Pd3−xPtx , which are predicted to show a topologically nontrivial surface state with 4f - character, and semimetallic mixed valence systems with antiferromagnetic ordering (TmSe1−xTex ), expected to form a semimetallic topological phase. Furthermore, we aim to approach the semimetal Dirac (Weyl) phase by tuning the Te content x, and to study here the interaction with the antiferro- magnetically ordered phase at low temperatures. The experimental methods include molecular beam epitaxy (MBE), high-resolution angle-resolved photoemission (ARPES), spin-resolved ARPES and non-resonant inelastic X-ray scattering (NIXS) to study the surface and bulk electronic structure, fo- cusing on near gap states and the ground state symmetries, and X-ray magnetic circular and linear dichroism (XMCD/XMLD) and neutron scattering to obtain information about the magnetically ordered phase properties. The investigated samples are both single crystals and epitaxial thin films of Ce and Tm systems which offer the option to systematically vary the composition. For the analysis of the com- plex spectroscopic data we will actively bring forward the development of appropriate cluster-model calculations considering in particular the multiplet structure of the 4f core levels.