TRR 80: From Electronic Correlations to Functionality

`Materials featuring strong electronic correlations are prone to the emergence of unexpected new properties offering novel functionalities for applications. Many decades of research have led to a deep and well-developed understanding of the effects of strong electronic correlations based on concepts such as generalized rigidities and symmetry breaking. Over the past decade the basic notions of topology, describing generic geometrical properties that remain unchanged under gradual (elastic) deformations, have become key issues in condensed-matter physics. The underlying gradual variations of the physical properties of interest may be driven by the effects of spin-orbit coupling and/or geometric frustration. Prominent examples for topological excitations in real space include the formation of skyrmions in chiral magnets, which are classified by an integral winding number, or fractionalized excitations such as monopoles in spin ice. Similarly, topological invariants in reciprocal space are the key characteristic of the celebrated discovery of topological insulators and Weyl semimetals, but are also important for the characterization of topological superconductors.`

Pressing challenges in research on the implications of strong electronic correlations for functionalities concern the effects of non-trivial topological winding in real and reciprocal space. In bulk materials these pertain to new forms of topological phases with coupled spin, orbital, charge, and lattice degrees of freedom. Work focusing on the identification of such phenomena will ultimately permit to address the topological properties of coupled degrees of freedom and/or coupling of non-trivial topologies in real and reciprocal space. Project Area E (Topological Aspects) bundles these activities in terms of five experimental and two theoretical projects.

The choice of activities in Project Area E defines a platform comprising the preparation of bulk materials and thin films, and the characterization of real space topology (projects E1, E4, and E6), specific techniques to track the topology in reciprocal space (projects E2, E3, and E4) and, perhaps most importantly, the theoretical framework in terms of general notions and specific experimental signatures (projects E5 and E7). Further questions on topological aspects of correlated matter will also be addressed in the projects of areas F and G, such as Raman, neutron, dielectric, and ferromagnetic resonance spectroscopy of topological materials, as well as nuclear magnetic resonance. Likewise the evolution of topological properties under extreme conditions, such as high pressures, large magnetic fields, or reduced dimensionality in tailored systems are specifically addressed in the other areas.