We have studied the electronic structure of BaCoO3 using soft x-ray absorption spectroscopy at the Co-L2,3 and O-K edges, magnetic circular dichroism at the Co-L2,3 edges, as well as valence band hard x-ray photoelectron spectroscopy. The quantitative analysis of the spectra established that the Co ions are in the formal low-spin tetravalent 3d 5 state and that the system is a negative charge transfer Mott insulator. The spin-orbit coupling plays also an important role for the magnetism of the system. At the same time, a trigonal crystal field is present with sufficient strength to bring the 3d 5 ion away from the J ef f = 1/2 state. The sign of this crystal field is such that the a1g orbital is doubly occupied, explaining the absence of a Peierl's transition in this system which consists of chains of face-sharing CoO6 octahedra. Moreover, with one hole residing in the e π g , the presence of an orbital moment and strong magneto-crystalline anisotropy can be understood. Yet, we also infer that crystal fields with lower symmetry must be present to reproduce the measured orbital moment quantitatively, thereby suggesting the possibility for orbital ordering to occur in BaCoO3.

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Abstract. We report on a detailed experimental and theoretical study of the electronic structure of NiO. The charge-transfer nature of the band gap as well as the intricate interplay between local electronic correlations and band formation makes NiO to be a challenging case for a quantitative ab-initio modeling of its electronic structure. To reproduce the compensated-spin character of the first ionization state and the state created by hole doping requires a reliable determination of the charge transfer energy Δ relative to the Hubbard U . Furthermore, the presence of non-local screening processes makes it necessary to go beyond single-site many body approaches to explain the valence band spectrum.

We report on our investigation of the electronic structure of Ti 2 O 3 using (hard) x-ray photoelectron and soft x-ray absorption spectroscopy. From the distinct satellite structures in the spectra, we have been able to establish unambiguously that the Ti-Ti c-axis dimer in the corundum crystal structure is electronically present and forms an a 1g a 1g molecular singlet in the low-temperature insulating phase. Upon heating, we observe a considerable spectral weight transfer to lower energies with orbital reconstruction. The insulatormetal transition may be viewed as a transition from a solid of isolated Ti-Ti molecules into a solid of electronically partially broken dimers, where the Ti ions acquire additional hopping in the a-b plane via the e π g channel, the opening of which requires consideration of the multiplet structure of the on-site Coulomb interaction. The role of ion pair formation for the metal-insulator transition (MIT) in early transition metal oxides, with the octahedra sharing either a common face or a common edge, has been a matter of debate in the past several decades . Based on the presence of the c-axis V-V dimers in the corundum crystal structure of V 2 O 3 , Castellani et al. [4] proposed a molecular singlet model for the a 1g orbitals, projecting the system effectively onto a solid with S ¼ 1=2 entities, which then should carry the essential physics for the MIT and the magnetic structure in the antiferromagnetic insulating phase. However, soft x-ray absorption spectroscopy (XAS) experiments [10] showed that the two d electrons on each Vare in the high-spin S ¼ 1 state, implying that the atomic Hund's rule coupling is much stronger than the intradimer hopping integrals. Furthermore, using band structure calculations, Elfimov et al. [12] found that the intradimer hopping integral is not the most important one; rather, the hopping integrals between second, third, and fourth nearest V neighbors are at least equally important. In other words, the c-axis dimers need not be present electronically, although structurally they are present.Ti 2 O 3 shares much of the same fascination as V 2 O 3 . It also has a corundum crystal structure (see the inset in Fig. 1) and exhibits, upon lowering the temperature, a MIT [22]. The earliest models explained the low-temperature insulating phase of Ti 2 O 3 by assuming a band splitting caused by an antiferromagnetic long-range order [22]. However, in contrast to V 2 O 3 , the transition is gradual and is not accompanied by a structural transition nor magnetic ordering [23][24][25]. Goodenough and Van Zandt et al. also proposed that the short c-axis pair bond length of 2.578 Å at 300 K [26], which is much shorter than in V 2 O 3 , with 2.697 Å at 300 K [27], increases the trigonal crystal