Fingerprints of optical absorption in the perovskite LaInO3 : Insight from many-body theory and experiment

We provide a combined theoretical and experimental study of the electronic structure and the optical absorption edge of the orthorhombic perovskite $\mathrm{La}\mathrm{In}{\mathrm{O}}₃$, employing density functional theory and many-body perturbation theory. We find the lowest-energy excitation at 0.2 eV below the fundamental gap (5 eV), reflecting a sizable electron-hole attraction. Since the transition from the valence band maximum ($\mathrm{\ensuremath{\Gamma}}$ point) is, however, dipole forbidden, the onset is characterized by weak excitations from transitions around it. The first intense excitation appears about 0.32 eV above. Interestingly, this value coincides with an experimental value obtained by ellipsometry (4.80 eV) which is higher than the onset from optical absorption spectroscopy (4.35 eV). The latter discrepancy is attributed to the fact that the weak transitions that define the optical gap are not well enough resolved by the ellipsometry measurement. Through temperature-dependent measurements of the optical gap, we assess renormalization effects by electron-phonon coupling, enhancing the quantitative comparison between theoretical and experimental results.