Photoemission spectral weight distribution

We have studied (Formula presented)(Formula presented)(Formula presented) by photoemission and inverse-photoemission spectroscopy. Valence-band photoemission spectra show a d-band peak ∼1.4 eV below the Fermi level ((Formula presented)), which evolves into the lower Hubbard band in the x= 0 ((Formula presented)) limit. The spectra show quasiparticle emission at (Formula presented) with an extremely small spectral weight, z∼0.01, which vanishes as the system approaches either the Mott insulator limit (x=0) or the band insulator limit (x=1). Correspondingly, inverse-photoemission spectra show the upper Hubbard band and a quasiparticle feature in the unoccupied state. The fact that the observed quasiparticle spectral weight is smaller than that of (Formula presented)(Formula presented)(Formula presented) is attributed to the larger U/W, where U is the on-site d-d Coulomb energy and W is the d-band-width. The presence of the ∼1.4-eV peak for a nearly empty d band (x∼ 1) and the small spectral weight at (Formula presented) cannot be explained within the Hubbard model, indicating the importance of interactions which are not included in the model, such as the long-range Coulomb interaction and the electron-phonon interaction.