Large spectral weight transfer in optical conductivity of SrTiO${}_3$ induced by intrinsic vacancies

The optical conductivity (${\sigma }_1$) of SrTiO${}_3$ for various vacancies has been systematically studied using a combination of ultraviolet - vacuum ultraviolet (UV-VUV) reflectivity and spectroscopic ellipsometry. For cation (Ti) vacancies, ${\sigma }_1$ shows large spectral weight transfer over a wide range of energy from as high as 35 eV to as low as 0.5 eV and the presence of mid-gap states, suggesting that strong correlations play an important role. While for anion (O) vacancies ${\sigma }_1$ shows changes from 7.4 eV up to 35 eV. These unexpected results can be explained in terms of orbital reconstruction.