Electronic and Optical Properties of Rare Earth Titanates
The rare-earth titanates (RTiO3, where R is a rare-earth atom) have become the focus of great interest because of their use in complex-oxide heterostructures that display two-dimensional electron gases (2DEGs) with unprecedented high densities. These compounds are Mott insulators, with a Mott-Hubbard gap opening up within the Ti 3d states. This gap is commonly reported to be 0.2-0.7 eV across the series. The values are based on optical reflectivity measurements, from which the onset of optical absorption is derived. We demonstrate that the Mott-Hubbard gap of GTO is actually much larger, close to 2 eV. This conclusion is based on both first-principles calculations (using hybrid density functional theory) and photoluminescence (PL) measurements, which display a strong peak near 1.8 eV and an onset in PL excitation at about the same energy. The experimentally observed onset in absorption is not related to the gap, but is associated with small polarons. We find an excellent match between first-principles calculations of small-polaron related optical transitions and optical conductivity measurements on Sr-doped GTO, where the Sr doping introduces small hole polarons. 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.