Electronic structure ofLaNiO3−x: Anin situsoft x-ray photoemission and absorption study

We study the electronic structure of $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_{3\ensuremath{-}x}$ thin films using in situ soft x-ray photoemission and absorption spectroscopy. The in situ high-resolution measurements reveal that states at and near the Fermi level $({E}_{F})$ in the occupied and unoccupied densities of states are very sensitive to the oxygen content and are directly related to a metal-insulator transition. The highest quality epitaxial films of $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_{3}$ show a temperature-dependent sharp peak at ${E}_{F}$. A detailed analysis of its electrical resistivity confirms a ${T}^{1.5}$ behavior over a large temperature range, which has been observed in earlier studies. Local density approximation band structure calculations indicate that the narrowing of the $\mathrm{Ni}\phantom{\rule{0.2em}{0ex}}d$ ${e}_{g}$ electron derived peak at ${E}_{F}$ cannot be reproduced by a strained crystal structure, suggesting a renormalization of electronic states at ${E}_{F}$ in $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_{3}$. The $T$-dependent spectral changes at ${E}_{F}$, coupled with the resistivity behavior and proximity to a metal-insulator transition, suggest the role of electron correlations in $\mathrm{La}\mathrm{Ni}{\mathrm{O}}_{3}$.