Oxygen electronic screening and hybridization determining the insulator-metal transition of Eu1−xBaxTiO3

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An insulator-metal transition (IMT) is a fascinating yet hotly debated phenomenon in condensed-matter physics. Here we report on the evolution of electronic structure across IMT as in $\mathrm{E}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{a}}_{x}\mathrm{Ti}{\mathrm{O}}_{3}$ as a function of $x$ ($x=0$ for ETO and $x=0.5$ for EBTO) and temperature comprehensively using high-resolution spectroscopic ellipsometry, soft x-ray absorption spectroscopy at Ti $L$ and O $K$ edges, and transport measurements. We observe different types of spectral weight transfers (SWTs), which are responsible for tuning optical band gaps, accompanied by dramatic changes in electrical conductivity. At room temperature, when $x$ increases, SWT occurs due to the high energy of O $2p$ to low-energy bands. Such a SWT increases Eu-O-Ti hybridization and enhances Drude response, which increases electrical conductivity in EBTO. Interestingly, upon cooling, SWT arises from low-energy to high-energy bands and opens up a gap of a low-energy oxygen band, which then suppresses electrical conductivity in EBTO. We argue that the unscreened electron-electron interaction in oxygen yields a metal-to-charge-transfer insulator-like transition. Our result reveals comprehensively the importance of oxygen screening effects and hybridizations in the IMT, and it provides insight into $\mathrm{E}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{B}{\mathrm{a}}_{x}\mathrm{Ti}{\mathrm{O}}_{3}$.