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We have performed the photoemission and inverse photoemission experiments to elucidate the origin of Mott insulating states in A-site ordered perovskite CaCu 3 Ti 4 O 12 (CCTO). Experimental results have revealed that Cu 3 d –O 2 p hybridized bands, which are located around the Fermi level in the prediction of the local-density approximation (LDA) band calculations, are actually separated into the upper Hubbard band at ~1.5 eV and the lower Hubbard band at ~−1.7 eV with a band gap of ~1.5–1.8 eV. We also observed that Cu 3 d peak at ~−3.8 eV and Ti 3 d peak at ~3.8 eV are further away from each other than as indicated in the LDA calculations. In addition, it is found that the multiplet structure around −9 eV includes a considerable number of O 2 p states. These observations indicate that the Cu 3 d and Ti 3 d electrons hybridized with the O 2 p states are strongly correlated, which originates in the Mott-insulating states of CCTO. [ABSTRACT FROM AUTHOR]

We report angle-resolved photoemission spectroscopy results of A-site ordered perovskite CaCu3Ti4O12. We have observed the clear band dispersions, which are shifted to the higher energy by 1.7 eV and show the band narrowing around 2 eV in comparison with the local density approximation calculations. In addition, the high-energy multiplet structures of Cu 3d8 final states have been found around 8-13 eV. These results reveal that CaCu3Ti4O12 is a Mott-type insulator caused by the strong correlation effects of the Cu 3d electrons well hybridized with O 2p states. Unexpectedly, there exists a very small spectral weight at the Fermi level in the insulator phase, indicating the existence of isolated metallic states. [ABSTRACT FROM AUTHOR]