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  Takegami, D.; Tanaka, A.; Agrestini, S.; Hu, Z.; Weinen, J.; Rotter, M.; Schüßler-Langeheine, C.; Willers, T.; Koethe, T.C.; Lorenz, T.; Liao, Y.F.; Tsuei, K.D.; Lin, H.J.; Chen, C.T.; Tjeng, L.H.: Paramagnetic LaCoO₃: A Highly Inhomogeneous Mixed Spin-State System. Physical Review X 13 (2023), p. 011037/1-10

10.1103/physrevx.13.011037
Open Access Version

Abstract:
We investigate the electronic structure of LaCoO₃ across the gradual spin-state and insulator-to-metal transitions using bulk-sensitive hard x-ray photoelectron and soft x-ray absorption spectroscopies. The spectra exhibit strong variations with temperature. The energy gap is reduced by about 0.6 eV in going from 80 to 650 K but the near Fermi level intensity remains small, classifying LaCoO₃ as a bad metal even in the metallic phase. We are able to explain the spectra in terms of incoherent sums of low-spin and high-spin Co³⁺ spectra. We also find that the energy parameters for the two Co sites are very different, revealing that paramagnetic LaCoO₃ is a highly inhomogeneous system with local lattice relaxations that are spin-state-specific. This, in turn, provides a natural explanation for the much-debated temperature dependence of the activation energy for the transitions.