Optical spectra have been investigated for the prototypical Mott-Hubbard (MH) insulators LaTiO3 and YTiO3, and their solid solutions LaxY1-xTiO3. 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 (U=U/W), we could determine the relative distance of each compound from the hypothetical Mott transition point (Uc). The band-filling dependence of the gap spectra has been also investigated for LaTiO3+δ/2 with the nominal hole concentration δ. The systematic collapse of the MH gap is argued, together with similar results on filling-controlled compounds Y1-xCaxTiO3, in terms of doping-induced spectral weight transfer.
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