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

化学式AM4 X8描述的各种空隙尖晶石化合物正在深入研究与强磁电耦合的关系，其在新型器件中具有潜在的用途。它们的磁活性结构单元是四面体d-金属离子簇，排列方式类似于呼吸烧绿石晶体结构，密度泛函理论（DFT）是研究其电子结构的计算工具。然而，已知的并且有充分的文献记载的是，对类似化合物的基于波函数的从头计算，例如，立方烷型铁氧还蛋白与Fe 4内核或四面体Nb 4簇，表明过渡金属四面体真正的多体物理，不能被捕获的单配置DFT：非Aufbau填充的单粒子水平，近简并的大量电子配置，强的组态混合。因此，非常需要澄清AM4 X8腔隙尖晶石中这种多体效应的程度以及它们在这些系统的显着磁性方面的作用。我们将采用量子化学中发展起来的从头算多体技术，并将这些计算结果与实验研究（从ESR和NMR测量到EXAFS和RIXS光谱）进行仔细分析。我们的工作将有助于对相关的电子结构和独特的物理性质的腔隙尖晶石的新的见解。这些结果还将在相关簇合物的背景下提供有用的参考数据，其中相关分子轨道形式的电子态分布在开d壳层离子的小基团上，并暗示了多体效应和非常不寻常的性质的新风味，例如，呼吸辉石岩或瑞典硼铁矿型钴矿。

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.