Correlated electronic structure, spin interactions, and electron-lattice couplings in mixed-valence lacunar spinels

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Various lacunar-spinel compounds, described by the chemical formula AM4X8, are being intensively investigated in relation to strong magneto-electric couplings, of potential use in novel devices. Their magnetically active structural units are tetrahedral clusters of d-metal ions, arranged in a manner resembling the breathing-pyrochlore crystal structure.So far, density-functional theory (DFT) has been the computational tool to address their electronic structures. It is known and well documented however that state-of-the-art wavefunction-based ab initio computations on similar compounds, e.g., cubane-type ferrodoxin proteins with a Fe4 kernel or tetrahedral Nb4 clusters, indicate for the transition-metal tetrahedra genuine many-body physics that can not be captured by single-configuration DFT: non-Aufbau filling of the single-particle levels, near-degeneracy of a large number of electron configurations, strong configurational mixing. It is then highly desirable to clarify the extent of such many-body effects in AM4X8 lacunar spinels and their role when it comes to the remarkable magnetic properties of these systems. This defines the main goal of this project.We shall employ to this end ab initio many-body techniques as developed in quantum chemistry and the outcome of such computations will be carefully analyzed vis-a-vis experimental investigations, ranging from ESR and NMR measurements to EXAFS and RIXS spectroscopy. Our work will contribute to new insights into the correlated electronic structures and the unique physical properties of the lacunar spinels. The results will also provide useful reference data in the context of related cluster compounds where electronic states in the form of correlated molecular orbitals are spread out over small groups of open-d-shell ions and imply new flavors of many-body effects and very unusual properties, e.g., the breathing pyrochlores or the swedenborgite-type cobaltites.