Unoccupied electronic structure ofY1−xCaxTiO3investigated by inverse …

We have studied the unoccupied electronic structure of the band insulator $\mathrm{Ca}\mathrm{Ti}{\mathrm{O}}_{3}$, the Mott insulator $\mathrm{Y}\mathrm{Ti}{\mathrm{O}}_{3}$, and ${\mathrm{Y}}_{0.61}{\mathrm{Ca}}_{0.39}\mathrm{Ti}{\mathrm{O}}_{3}$ with a metal-insulator transition (MIT) around $150\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ by means of $\mathrm{Ti}\phantom{\rule{0.2em}{0ex}}3p\text{\ensuremath{-}}3d$ resonant inverse photoemission spectroscopy (RIPES). The $\mathrm{Ti}\phantom{\rule{0.2em}{0ex}}3d$ partial density of states of $\mathrm{Ca}\mathrm{Ti}{\mathrm{O}}_{3}$ deduced from the on- and off-resonance RIPES spectra is in good agreement with results of a band-structure calculation, while that of $\mathrm{Y}\mathrm{Ti}{\mathrm{O}}_{3}$ is explained based on a calculation with the dynamical mean-field theory taking into account an electron correlation. In the case of ${\mathrm{Y}}_{0.61}{\mathrm{Ca}}_{0.39}\mathrm{Ti}{\mathrm{O}}_{3}$, we have successfully observed the temperature dependence of RIPES spectra across MIT; the intensity of the incoherent part (a remnant of the upper Hubbard bands) relative to the coherent part (quasiparticle bands) is reduced with decreasing temperature from the insulator to metal phases.