Electronic structure of Mott-insulator CaCu3Ti4O12: Photoemission and inverse photoemission study

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 $\sim$ 1.5 eV and the lower Hubbard band at $\sim$ $-$1.7 eV with a band gap of $\sim$ 1.5-1.8 eV. We also observed that Cu 3$d$ peak at $\sim$ $-$3.8 eV and Ti 3$d$ peak at $\sim$ 3.8 eV are further away from each other than as indicated in the LDA calculations. In addition, it is found that the multiplet strucutre 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.