Photoemission spectral weight distribution in ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{TiO}}_3$

We have studied ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{TiO}}_3$ by photoemission and inverse-photoemission spectroscopy. Valence-band photoemission spectra show a d-band peak ∼1.4 eV below the Fermi level (${\mathit{E}}_{\mathit{F}}$), which evolves into the lower Hubbard band in the x= 0 (${\mathit{d}}^1$) limit. The spectra show quasiparticle emission at ${\mathit{E}}_{\mathit{F}}$ 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 ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{TiO}}_3$ 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 ${\mathit{E}}_{\mathit{F}}$ 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. © 1996 The American Physical Society.