Correlated topological quantum matter

• Applicants: Dr. Sushmita Chandra; Professor Dr. Ronny Thomale
• Subject Area: Experimental Condensed Matter Physics ; Theoretical Condensed Matter Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 449872909

P3 will continue as a tight-knit experiment-theory collaboration that pushes the boundaries of two contemporary QUAST material platforms which grew out of the research activity of the first funding period -- kagome metals and transition-metal dichalcogenides (TMDs). Experimentally, the focus will be on expanding the range of materials in these classes and their new physical properties. Theoretically, we will concentrate on developing new methodology to meet the challenge of simulating electronically driven orders in these multi-band compounds, also accounting for mixed itinerant and localized degrees of freedom. Several families of kagome materials have been explored in recent years with a multitude of experimental techniques, demonstrating qualitatively new physics, such as giant anomalous Hall effects, emergent chirality in flux phases, an intricate interplay between superconductivity and such chiral charge orders as well as a delicate balance between electronic and phononic contributions to correlation phenomena. We expect that expanding the space of compounds will allow us to discern universal trends from the details of specific compounds. Specifically, we plan to analyze materials in which magnetic fluctuations and magnetic order will play a stronger role to see how this new understanding of itinerant Kagome systems interacts with the physics of the (theoretically) more established magnetic domains, which mostly features a Mott-insulating, local moment scenario. To that end, we explore the synthesis of chromium-based Kagome compounds, for which we expect stronger correlation effects due to the higher filling of the d shell compared to the known titanium or vanadiumm based families. A particular focus will be on the exploration of their pressure phase diagrams. Eventually, also f-orbtial Kagome metals will be in the focus of our synthetization efforts. Theoretical modelling of their physics will build up on our fluctuation-extension of the slave-boson method, developed in the first funding period of QUAST, as well as through DMFT calculations in collaboration with other QUAST projects. TMDs have driven several of the breakthrough findings in condensed matter physics in recent years (type-II Weyl semimetals, high-temperature quantum spin Hall effect, Wigner crystals, fractional Chern insulators) as a very versatile family offering both topological and correlated physics. We will focus on exploring layered bulk materials for supporting a 3D quantum Hall effect (both integer and fractional) and, once successful, attempt to exfoliate them down to monolayers. This will be accompanied by theoretical simulations similar to our past work on 1T'-WSe2 and twisted MoTe2. The exploration of 3D quantum Hall effects is a high-risk/high-gain aspect of this proposal and will be accompanied by a multi-pronged theory effort. To be able to analyze symmetry-breaking electronic instabilities in both of these families of (quasi-)two-dimensional compounds, we rely on our established methodological toolkit, where the complex multiband/multiorbital nature of the compounds of interest suggests the usage of the innovative quantix tensor train parametrization for the interaction vertex to speed up momentum integration. As a key theory method development in P3 to address the strongly correlated regime of systems with localized magnetic moment in either a Mott- or Kondo-insulating environment, we will focus on the methodological refinement of slave-boson mean-field theory and fluctuations.

• DFG Programme: Research Units
• Subproject of: FOR 5249: Quantitative Spatio-Temporal Model-Building for Correlated Electronic Matter
• International Connection: Switzerland
• Partner Organisation: Schweizerischer Nationalfonds (SNF)
• Co-Investigator: Professorin Dr. Claudia Felser
• Cooperation Partner: Professor Dr. Titus Neupert