Electron Correlation in Oxygen Vacancy inSrTiO3

Abstract

Oxygen vacancies are an important type of defect in transition metal oxides. In SrTiO3 they are believed to be the main donors in an otherwise intrinsic crystal. At the same time, a relatively deep gap state associated with the vacancy is widely reported. To explain this inconsistency we investigate the effect of electron correlation in an oxygen vacancy (OV) in SrTiO3. When taking correlation into account, we find that the OV-induced localized level can at most trap one electron, while the second electron occupies the conduction band. Our results offer a natural explanation of how the OV in SrTiO3 can produce a deep in-gap level (about 1 eV below the conduction band bottom) in photoemission, and at the same time be an electron donor. Our analysis implies an OV in SrTiO3 should be fundamentally regarded as a magnetic impurity, whose deep level is always partially occupied due to the strong Coulomb repulsion. An OV-based Anderson impurity model is derived, and its implications are discussed. [14,20]. The OV in SrTiO 3 is particularly intriguing. On the one hand, the OV concentration is roughly proportional to that of the carriers [14,20,21], strongly suggesting that vacancies are electron donors; on the other hand, an OV produces an in-gap signal, peaked approximately 1.0 eV below the conduction band, in angle resolved photoemission spectroscopy (ARPES), even at temperatures as low as 20 K [14,19,22]. Within the single-particle description, the ARPES spectrum implies that an OV results in a deep impurity level and therefore is not likely to be the electron donor. In this Letter, we demonstrate that, by taking the correlation effect, these two seemingly conflicting observations can be naturally reconciled. More fundamentally, our analysis suggests that an OV in SrTiO 3 should be regarded as a magnetic impurity, in the sense that a deep level is always partially occupied due to the strong Coulomb repulsion, and may account for the observed Kondo physics [10] or interface ferromagnetismThe electronic structure of an OV in SrTiO 3 has been extensively studied using density functional theory (DFT) [23][24][25][26][27][28][29][30]. Depending on the functional used, the OV induced impurity level in SrTiO 3 can lie above the conduction band edge, thus leading to a resonance [26,28,29]; be an in-gap bound state with energy level position ranging from 0.4 to 1 eV below the conduction band bottom [25,28,30]; or be a partially filled, spin-polarized gap state [27].Independent of the functional, the OV-induced state is spatially localized and is mainly composed of the next-to-OV Ti 3d 3z 2 −r 2 and 4p z orbitals [31]. To include the correlation effect in an OV, we first identify the main change in the local electronic structure that it causes. The following discussion is based on Ref. [31]. Under cubic symmetry, the proper local orbital basis of a Ti atom consists of two 3d x 2 −y 2 , 3d 3z 2 −r 2 orbitals with e g symmetry, and three 3d

Role of oxygen vacancies in room-temperature ferromagnetism in cobalt-substitutedSrTiO3

“…The formation of an oxygen vacancy near a Co atom breaks its octahedral symmetry and causes local hybridization of the Co 3d with its 4s and 4p states [37,38,39]. This results in splitting of both the Co t 2g and the e g manifolds.…”

“…A plausible source of extrinsic carriers is oxygen vacancies (OV), which are commonly occurring defects in oxides, especially thin films. Creation of an OV in STO results in two additional electrons that can potentially convert Co from a +4 to a +2 valence state [37,38,39]. Hence understanding the role of oxygen vacancies in the observed magnetism in this system (which henceforth would be denoted as SrTi 1-x Co x O 3-δ ) is crucial.…”

Quasi-two-dimensional electron gas at the epitaxial alumina/SrTiO3 interface: Control of oxygen vacancies

“…[18][19][20][21][22][23][24][25][26][27] Under certain conditions, it is energetically favorable for oxygen atoms near the interface to diffuse out of the STO during the initial stages of growth, stabilizing a confined conducting layer. [18][19][20][21][22][23][24][25][26][27]31,32 Since this metallic layer results from the formation of oxygen vacancies near the interface, it characteristically vanishes with oxygen atmospheric annealing. [23][24][25][26][27] One such heterostructure is alumina/STO.…”

Coexistence of trapped and free excess electrons inSrTiO3

“…Based on density functional theory (DFT)+U, hybrid DFT and model Hamiltonians, it was suggested that the two additional electrons donated by a single oxygen vacancy V O induce one singly-occupied in-gap state mostly localized at the Ti atoms near the V O , and one state delocalized in the conduction band [25][26][27][28][29][30] . This conclusion successfully clarifies transport and optical properties in reduced SrTiO 3 , but is inconsistent with the observed V O -induced MIT 16 , because the predicted free carriers would be mobile even at low temperature.…”