Optical spectra in (La,Y)${\mathrm{TiO}}_3$: Variation of Mott-Hubbard gap features with change of electron correlation and band filling

Optical spectra have been investigated for the prototypical Mott-Hubbard (MH) insulators ${\mathrm{LaTiO}}_3$ and ${\mathrm{YTiO}}_3$, and their solid solutions ${\mathrm{La}}_{\mathit{x}}$${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{TiO}}_3$. The MH gap shows a critical variation with x or equivalently with the change of the one-electron bandwidth W. By an extrapolation of the rapidly varying MH gap with the electron correlation strength (Ũ=U/W), we could determine the relative distance of each compound from the hypothetical Mott transition point (U${\mathrm{̃}}_{\mathit{c}}$). The band-filling dependence of the gap spectra has been also investigated for ${\mathrm{LaTiO}}_{3+\mathrm{\delta }/2\mathrm{}}$ with the nominal hole concentration δ. The systematic collapse of the MH gap is argued, together with similar results on filling-controlled compounds ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Ca}}_{\mathit{x}}$${\mathrm{TiO}}_3$, in terms of doping-induced spectral weight transfer.