Theory of Spin-State Selective Nonlocal Screening in Co 2p X-ray Photoemission Spectrum of LaCoO₃

Abstract

The Co 2p X-ray photoemission spectrum (XPS) of LaCoO 3 is investigated using a dp model simulating Co 3d and O 2p orbitals by means of a dynamical mean-field approach under the perovskite crystal structure. Across the spin-state transition from the low-spin to the high-spin state, the Co 2p 3=2 main-line structure is substantially changed beyond expectation of a CoO 6 cluster model calculation. In addition to the Coulombic multiplet effect, the origin of the spectral change is attributed to the nonlocal screening (NLS) from the correlated 3d band located on the top of the valence band to the core-excited Co site in the final state, where the NLS is practically active only for the high-spin state. The spinstate selectivity of the NLS is closely related to not only the spin state of the core-excited Co ion but also the spin and orbital character of the occupied Co 3d band in crystals. We emphasize that the Co 2p XPS can be an informative probe to investigate the spin state of Co ions in Co oxides, such as LaCoO 3 .The electronic property of LaCoO 3 has been extensively studied since the discovery of a spin-state transition phenomenon in the 1950s. 1) The spin-state transition is beyond a simple atomic picture and is considered a many-body problem arising from the competition among Hund's coupling, Co 3d crystal field splitting, and Co-Co interaction through the Co 3d-O 2p hybridization. In the early stage of investigations, the spin-state transition due to the temperature rise was interpreted as a gradual increase in the weight of high-spin (HS, S ¼ 2) excited states against the low-spin (LS, S ¼ 0) ground state. 2) However, after a theoretical study by the LDA+U method 3) suggested that the excited states are not the HS states but the intermediate-spin (IS, S ¼ 1) ones, the IS scenario has been widely accepted. 4,5) Nevertheless, the microscopic understanding of the spin state in LaCoO 3 seems still under debate. For example, Kyômen et al. 6) have recently pointed out the existence of a mixed spin state composed of the interacting LS and HS states. Therefore, it is important to propose a new probe to detect microscopically the spin state of Co ions.Core-level X-ray spectroscopy is a powerful and unique tool to investigate the electronic structure of strongly correlated electron systems. 7) The 2p X-ray absorption spectroscopy (XAS) and the magnetic circular dichroism have contributed to investigating the spin state of Co compounds including LaCoO 3 so far. 8) In contrast, the 2p X-ray photoemission spectroscopy (XPS) has not been utilized so much in the study of the spin state of Co 5,9) because the strong charge screening from O 2p to 3d state at the core-excited Co ion partly hinders the information on the Co spin state. Moreover, the photoemission experiment has been a surface-sensitive and low-energy-resolution technique compared with the 2p XAS before the hard X-ray photoemission spectroscopy (HAXPES) became available.Recent experimental progresses, especially in bulk sensitivity and energy resolution, have ...

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