Low-energy orbital excitations in strained LaCoO3 films

We present 90 meV resolved Co $2p3d$ resonant inelastic x-ray scattering linear dichroism spectra of strained ${\mathrm{LaCoO}}₃$ films and a ${\mathrm{LaCoO}}₃$ single crystal. A polarization-dependent low-energy excitation is observed at $\ensuremath{\sim}0.2\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ on the tensile-strained ${\mathrm{LaCoO}}₃/{\mathrm{SrTiO}}₃$ film, while it is not observed in either bulk ${\mathrm{LaCoO}}₃$ or the compressive-strained ${\mathrm{LaCoO}}₃/{\mathrm{LaAlO}}₃$ film. Guided by cluster calculations, we are able to distinguish the spin-state manifolds close to their transition point of ${\mathrm{Co}}³⁺$ ions in ${\mathrm{LaCoO}}₃$ systems. Through a polarization analysis, we show that the spin state can easily flip from a low-spin $¹\mathrm{A}_1g$ state in an octahedral symmetry to the high-spin $⁵\mathrm{B}_2g$ or $⁵\mathrm{E}_g$ states with a small tetragonal distortion. A mixture of spin states suggests that the high-spin ${\mathrm{Co}}³⁺$ plays an important role in long-range ferromagnetic order on both tensile- and compressive-strained ${\mathrm{LaCoO}}₃$ films.