Correlated visible-light absorption and intrinsic magnetism of SrTiO₃ due to oxygen deficiency: Bulk or surface effect?

The visible-light absorption and luminescence of wide band gap (3.25 eV) strontium titanate (SrTiO₃) are well-known, in many cases, to originate from the existence of natural oxygen deficiency in the material. In this study based on density functional theory (DFT) calculations, we provide, to the best of our knowledge, the first report indicating that oxygen vacancies in the bulk and on the surfaces of SrTiO₃ (STO) play different roles in the optical and magnetic properties. We found that the doubly charged state of oxygen vacancy (VO2+) is dominant in bulk SrTiO₃ and does not contribute to the sub-band gap photoexcitation or intrinsic magnetism of STO. Neutral oxygen vacancies (VO0) on (001) surfaces terminated with both TiO₂ and SrO layers induce magnetic moments, which are dependent on the charged state of V_O. The calculated absorption spectra for the (001) surfaces exhibit mid-infrared absorption (<0.5 eV) and sub-band gap absorption (2.5-3.1 eV) due to oxygen vacancies. In particular, VO0 on the TiO₂-terminated surface has a relatively low formation energy and magnetic moments, which can explain the recently observed spin-dependent photon absorptions of STO in a magnetic circular dichroism measurement [ Rice, W. D.; et al. Nat. Mater. 13, 481, 2014 ].