Hole-induced insulator-to-metal transition inLa1−xSrxCrO3epitaxial films

We have investigated the evolution of the electronic properties of $\mathrm{L}{\mathrm{a}}_{1\ensuremath{-}x}\mathrm{S}{\mathrm{r}}_{x}\mathrm{Cr}{\mathrm{O}}_{3}\phantom{\rule{0.16em}{0ex}}(0\ensuremath{\le}x\ensuremath{\le}1)$ epitaxial films deposited by molecular beam epitaxy (MBE) using x-ray diffraction, x-ray photoemission spectroscopy, Rutherford backscattering spectrometry, x-ray absorption spectroscopy, electrical transport, and ab initio modeling. $\mathrm{LaCr}{\mathrm{O}}_{3}$ is an antiferromagnetic insulator, whereas $\mathrm{SrCr}{\mathrm{O}}_{3}$ is a metal. Substituting $\mathrm{S}{\mathrm{r}}^{2+}$ for $\mathrm{L}{\mathrm{a}}^{3+}$ in $\mathrm{LaCr}{\mathrm{O}}_{3}$ effectively dopes holes into the top of valence band, leading to $\mathrm{C}{\mathrm{r}}^{4+}$ (${3d}^{2}$) local electron configurations. Core-level and valence-band features monotonically shift to lower binding energy with increasing $x$, indicating downward movement of the Fermi level toward the valence band maximum. The material becomes a $p$-type semiconductor at lower doping levels and an insulator-to-metal transition is observed at $x\ensuremath{\ge}0.65$, but only when the films are deposited with in-plane compression via lattice-mismatched heteroepitaxy. Valence-band x-ray photoemission spectroscopy reveals diminution of electronic state density at the $\mathrm{Cr}\phantom{\rule{0.16em}{0ex}}d\phantom{\rule{0.16em}{0ex}}{t}_{2g}$-derived top of the valence band, while O K-edge x-ray absorption spectroscopy shows the development of a new unoccupied state above the Fermi level as holes are doped into $\mathrm{LaCr}{\mathrm{O}}_{3}$. The evolution of these bands with Sr concentration is accurately captured using density functional theory (DFT) with a Hubbard U correction of 3.0 eV $(\mathrm{DFT}+U)$. Resistivity data in the semiconducting regime $(x\ensuremath{\le}0.50)$ do not fit perfectly well to either a polaron hopping or band conduction model but are best interpreted in terms of a hybrid model. The activation energies extracted from these fits are well reproduced by $\mathrm{DFT}+U$.