Strong correlation effects in the $A$-site ordered perovskite CaCu${}_3$Ti${}_4$O${}_{12}$ revealed by angle-resolved photoemission spectroscopy

We report angle-resolved photoemission spectroscopy results of $A$-site ordered perovskite CaCu${}_3$Ti${}_4$O${}_{12}$. 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 3$d^8$ final states have been found around 8–13 eV. These results reveal that CaCu${}_3$Ti${}_4$O${}_{12}$ is a Mott-type insulator caused by the strong correlation effects of the Cu 3$d$ electrons well hybridized with O 2$p$ states. Unexpectedly, there exists a very small spectral weight at the Fermi level in the insulator phase, indicating the existence of isolated metallic states.

Figure 1

(a) The EDCs along the $\Gamma H\left(\Delta \right)$ direction in the normal emission to the (100) plane, observed by using the photon energies from 30 to 93 eV at intervals of 3 eV. The blue dashed lines are guides for the eyes. (b) Body-centered cubic structure (Ref. 10) and (c) Brillouin zone of $A$-site ordered peroviskite CaCu${}_3$Ti${}_4$O${}_{12}$.

Figure 2

(a) AIPES spectrum of CCTO obtained at $h\nu =90$ eV and $T=300$ K in the valence band region. The inset shows the AIPES spectrum in the wide valence band region. For comparison, the partial DOS of Cu 3$d$ is inserted below the AIPES spectrum. (b) Total DOS and the partial DOS for the valence bands of Cu 3$d$, O 2$p$, and Ti 3$d$ in the LDA calculations.

Figure 3

(a) ARPES spectra of CCTO along the $\Delta$ direction in the parallel to the cleaved surface (100) plane at $h\nu =90$ eV and $T=300$ K. (b) ARPES image obtained from the second derivatives of the EDCs. The theoretical band dispersions (white dashed line), shifted by 1.7 eV, are plotted on the ARPES image. (c) Theoretical band dispersions obtained from the LDA calculations.

Figure 4

(a) The enlarged ARPES spectra at the $\Gamma$ and H points near $E_F$ in Fig. 3a. (b) The bulk sensitive AIPES spectra near $E_F$ at $h\nu =7$ eV. The solid circle is the AIPES sepctrum and the open circle is the spectral weight near $E_F$ obtained from dividing the AIPES spectrum by the convoluted Fermi-Dirac function ($\Delta E\sim 15$ meV and $T=300$ K).