Consequences of oxygen-vacancy correlations at the SrTiO3 interface.

The Kondo effect and ferromagnetism are the two many-body phenomena that emerge at the SrTiO(3) interfaces with polar materials, but do not occur in bulk SrTiO(3). By regarding the oxygen vacancy (OV) in SrTiO(3) as a magnetic impurity, we show that these two interface-specific phenomena can be attributed to the vacancies residing in the top TiO(2) plane of SrTiO(3). We identify three crucial ingredients: the local orbital mixing caused by an OV, reduced symmetry at the interface, and a strong in-plane stray electric field of the polar material. All three factors combine to result in the coupling between the impurity and conduction band at the interface, and can lead to both emergent phenomena. An OV-based Anderson impurity model is derived and solved using the numerical renormalization group method. The Kondo and Curie temperatures are estimated. Several experiments are discussed based on this interpretation.