Change of electronic structures with carrier doping in the highly correlated electron system ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{TiO}}_3$

A perovskitelike solid solution ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{TiO}}_3$ can be viewed as a hole-doped Mott-Hubbard insulator or strongly correlated metal that is derived from the parent insulator ${\mathrm{YTiO}}_3$ with the Mott-Hubbard gap of ∼1 eV. The Mott insulator-metal transition is observed around the hole-doping level of x∼0.35 in ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{TiO}}_{\mathit{x}}$. Change of electronic structures with hole doping has been investigated by measurements of optical spectra in a wide photon-energy range. In the insulating phase with x<0.4, the spectral weight is transferred from the Mott-Hubbard gap excitations to the inner-gap region with hole doping, giving rise to a gradual closing of the charge gap. Even in the doping-induced metallic region (x≥0.4), the midinfrared absorption still dominates the lower-lying Drude absorption, perhaps due to the effects of the impurity potential and electron correlation on the narrow 3d. band. The results were argued in comparison with the case of the high-${\mathit{T}}_{\mathit{c}}$ cuprates.