Optical-conductivity spectra of Sr_1-xLa_xTiO₃: Filling-dependent effect of the electron correlation

Optical-conductivity spectra have been measured on the correlated electron system Sr_1-xLa_xTiO₃ (0≤x≤1) in order to determine how the electronic structure depends on the filling of the 3d-electron conduction band. While metallic resistivity was observed in all samples with x≥0.1, the optical-conductivity spectra have revealed a critical change in the electronic structure: The effective carrier mass, which has been deduced by analysis of the optical-conductivity spectra and the energy-loss function, is critically enhanced as the x=1 filling case (Ti 3d¹ configuration) is approached, perhaps due to electron-correlation effects, in a manner reminiscent of the Mott-Hubbard transition.