Determination of the Mott-Hubbard gap in GdTiO3

Abstract

The band gaps of rare-earth titanates are commonly reported to be 0.2–0.7 eV. These values are based on optical reflectivity measurements, from which the onset of optical absorption is derived. Here we report experimental and theoretical results on GdTiO3 (GTO) indicating that the gap is significantly larger. Photoluminescence (PL) measurements show a strong peak near 1.8 eV, consistent with an observed onset in PL excitation (PLE) at about the same energy. First-principles calculations, based either on density-functional theory (DFT) with a hybrid functional or on DFT+U, consistently show that the gap is close to 2 eV. We also propose an interpretation of the previously reported optical absorption spectra. Given the similarities in electronic structure between the rare-earth titanates, our results for GTO have repercussions for the other members of the series. The results also affect the design of complex-oxide heterostructures involving these materials.