Optical spectra in (La,Y)TiO3: Variation of Mott-Hubbard gap features with change of electron correlation and band filling.

Abstract

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