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

由化学式AM4X8描述的各种腔隙尖晶石化合物正在深入研究与强磁电耦合相关的强磁电耦合，在新型设备中具有潜在的用途。它们的磁性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-焦绿石晶体结构。到目前为止，密度泛函理论（DFT）一直是解决其电子结构的计算工具。然而，众所周知，基于波函数的最先进的从头计算对类似的化合物，例如，具有Fe4核或四面体Nb4簇的立方型铁氧蛋白，表明过渡金属四面体真正的多体物理不能被单构型DFT捕获：单粒子水平的非aufbau填充，大量电子构型的近简并，强构型混合。因此，澄清AM4X8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著磁性方面的作用是非常可取的。这定义了这个项目的主要目标。为此，我们将采用量子化学中发展起来的从头开始的多体技术，这些计算的结果将与实验研究进行仔细分析，从ESR和NMR测量到EXAFS和RIXS光谱。我们的工作将有助于对腔隙尖晶石的相关电子结构和独特的物理性质有新的认识。这些结果还将为相关簇状化合物提供有用的参考数据，在这些化合物中，相关分子轨道形式的电子态分布在小群开壳离子上，并暗示了多体效应的新风格和非常不寻常的性质，例如呼吸型焦绿石或swedenborgite型钴酸盐。

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.