ABSTRACT

Fundamental electronic structure of transition-metal compounds is characterized by the Coulomb interaction U between d electrons and the charge transfer (CT) energy D from the ligand p to transition-metal d orbitals [1,2]. In case of CT type systems with D < U, the ligand p orbitals become relevant in addition to the transition-metal d orbitals. For example, YNiO3 falls in the charge-transfer regime [3], and the effect of O 2p hole plays important roles in the metal-insulator transition.

The effect of ligand holes can be investigated by means of x-ray absorption spectroscopy (XAS) and angle-resolved photoemission spectroscopy (ARPES). In the present talk, we focus on LixCoO2 [4,5] and Ta2NiSe5 [6,7]. The CT energy D of Co³⁺(d⁶) in LiCoO2 is ~ 1 eV indicating that Co⁴⁺ species in delithiated LixCoO2 have the electronic configuration of d⁶L rather than d⁵ (Here, L represents a ligand hole). The O 2p states are probed by polarization dependent O K-edge XAS [4] and ARPES [5], and are found to play effective roles to screen the Li ion charge. As for Ta2NiSe5, the negative D of Ni²⁺ is indicated by XAS and ARPES [6,7]. The attractive interaction between the Se 4p hole and the Ta 5d electron provides a unique electronic state in which the electron-hole pairs condensate in a BEC manner and induce a semimetal-insulator transition. In addition to the above two examples, we discuss other negative CT energy systems including Mn⁴⁺, Fe⁴⁺, and Au²⁺ which are expected to show interesting physical and chemical properties.

The authors would like to thank Prof. N. L. Saini (Univ. of Rome), Prof. D. I. Khomskii (Univ. of Köln), Prof. G. A. Sawatzky (Univ. of British Columbia), Prof. M. Azuma (TIT), Prof. K. Miyoshi (Shimane Univ.), and Prof. H. Takagi (MPI) for the long term collaborations and Mr.

Y. Okamoto and Mr. Mitsuoka for the contributions to the recent works.

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[2]   D. I. Khomskii, Transition Metal Compounds (Cambridge University Press, 2014).

[3]  T. Mizokawa, D. I. Khomskii, and G. A. Sawatzky, Phys. Rev. B 61, 11263 (2000).

[4]  T. Mizokawa et al., Phys. Rev. Lett. 111, 056404 (2013).

[5] Y. Okamoto et al., Phys. Rev. B 96, 125147 (2017).

[6] Y. Wakisaka et al., Phys. Rev. Lett. 103, 026402 (2009). [7] Y. Chiba et al., Phys. Rev. B 100, 245129 (2019).