Photoemission spectral weight distribution in Y1-xCaxTiO3

We have studied Y1-xCaxTiO3 by photoemission and inverse-photoemission spectroscopy. Valence-band photoemission spectra show a d-band peak similar to 1.4 eV below the Fermi level (E(F)), which evolves into the lower Hubbard band in the x=0 (d(1)) limit. The spectra show quasiparticle emission at E(F) with an extremely small spectral weight, z similar to 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 La1-xSrxTiO3 is attributed to the larger U/W, where U is the an-site d-d Coulomb energy and W is the d-band-width. The presence of the similar to 1.4-eV peak for a marry empty d band (x similar to 1) end the small spectral weight at E(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.