Consequences of Oxygen-Vacancy Correlations at theSrTiO3Interface

Abstract

The Kondo effect and ferromagnetism are the two many-body phenomena that emerge at the SrTiO3 interfaces with polar materials, but do not occur in bulk SrTiO3. By regarding the oxygen vacancy (OV) in SrTiO3 as a magnetic impurity, we show that these two interface specific phenomena can be attributed to the vacancies residing in the top TiO2 plane of SrTiO3. We identify three crucial ingredients the local orbital mixing caused by an OV, reduced symmetry at the interface, and 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 based on this interpretation are discussed.PACS numbers: 79.60. Dp, 71.27.+a Perovskite SrTiO 3 (STO) is a non-magnetic band insulator with a band gap of 3.2 eV [1, 2]. Its versatile response to different dopants makes STO an ideal host for building functional materials and has drawn significant attention.For example, an insulating ferromagnetic (FM) order at room temperature is observed in STO when replacing some Ti atoms by Co [3,4]. The band gap of STO can be engineered by Al doping [5]. When a small number of conduction electrons are introduced, even more fascinating phenomena such as superconductivity [6][7][8], long-lived magnetic moments [9], metallic ferromagnetism or super-paramagnetism (SPM, several separate macroscopic FM domains) [10][11][12][13][14][15], Kondo resistance minimum [16][17][18], and two-dimensional electron gas [19][20][21][22] emerge. Among these emergent phenomena, the ferromagnetism and Kondo effect are special in that they, unlike the superconductivity, only occur at the STO/LaAlO 3 (LAO) (for FM) [10][11][12][13]23], and STO/ionic liquid interfaces (for Kondo effect) [17,18, 24], but do not occur in the n-doped bulk STO [25][26][27][28][29][30]. In this letter, we investigate the role of oxygen vacancies (OV) in these two phenomena. We find the OV, residing in the top TiO 2 plane, plays an important role in distinguishing the interface behavior from the bulk. By regarding OVs in STO as magnetic impurities [31], we identify three crucial ingredients necessary for FM and Kondo effect to occur -(i) local orbital mixing at the adjacent Ti sites caused by an OV, (ii) symmetry reduction near the interface, and (iii) strong in-plane stray electric field induced by the polar interface. A combination of these three factors results in an impurity-conduction band coupling near the interface and can lead to the emergent FM phase and Kondo effect.We begin by summarizing the key features of an OV in STO from the density functional theory (DFT) [32][33][34][35][36][37][38].The conduction bands of bulk STO are mainly derived from Ti 3d orbitals, with triple-degenerate t 2g bands about 2.5 eV below the double-degenerate e g bands in energy. Whe...

Spin Hall magnetoresistance in Nb/Y3Fe5O12 hybrids

“…1c. The magnitude of ΔR is 10 -2 Ω with a magnetoresistance ratio of 0.003-0.007%, consistent with the absolute value of spin Hall angle of Nb (0.87%) taking the SMR ratio (0.02%) and the spin Hall angle (2.1%) of the Pt/YIG system into account [15,[20][21][22]. The data strongly suggest the existence of SMR in the Nb layer, an illustration of which is shown in Fig.…”

Localized states induced by an oxygen vacancy in rutile TiO2

“…By considering only three orbitals, an exact diagonalization can be performed to investigate the effect of correlation, which is crucial for the net magnetic property. 48 A similar procedure has been applied to CaO 49 and SrTiO 3 . 50 We would like to point out that these e g -based orbitals include some 4s and 4p components.…”

Influence of oxygen vacancies on two-dimensional electron systems at SrTiO3-based interfaces and surfaces

“…The positively charged oxygen vacancies provide a confinement potential at the surface resulting in a downward bending of the Ti 3d t 2g conduction bands. At the same time the oxygen vacancies donate a certain fraction of electrons to these bands forming two-dimensional quantum well states while the remaining fraction stays localized at Ti sites adjacent to the oxygen vacancies [59] as discussed in Section 2.1.…”

“…From the Ti 3+ /Ti 4+ ratio one can infer the sheet carrier density of the itinerant Ti 3d electrons averaged over the whole sampled surface area, n avg (assuming a 1 : 1 disproportionation between localized and itinerant electrons due to the strong onsite Coulomb repulsion which prevents two electrons from being trapped at the same Ti site [59]). For a homogeneous distribution of charge carriers n avg and n F S would equal each other (blue line in Fig.…”