We explored the possibility of enhancing interfacial conductivity and spatial charge confinement of LaAlO3/SrTiO3 (LAO/STO) heterostructure (HS) via strain engineering using first-principles electronic structure calculations. We found that applying a tensile strain on the STO substrate along the ab-plane can significantly enhance the interfacial conductivity, magnetic moments, and the spatial charge confinement of the HS system. In contrast, a compressive strain can dilute the interfacial charge carrier density, make the mobile charges transfer to deep STO substrate, and weaken the spatial charge confinement along the c-axis. Hence, we propose that applying a tensile strain can be an effective way to enhance the interfacial conductivity and magnetism of STO-based HS systems.
The two-dimensional electron gas (2DEG) formed at the interface between two insulating oxides such as LaAlO3 and SrTiO3 (STO) is of fundamental and practical interest because of its novel interfacial conductivity and its promising applications in next-generation nanoelectronic devices. Here we show that a group of combinatorial descriptors that characterize the polar character, lattice mismatch, band gap, and the band alignment between the perovskite-oxide-based band insulators and the STO substrate, can be introduced to realize a high-throughput (HT) design of SrTiO3-based 2DEG systems from perovskite oxide quantum database. Equipped with these combinatorial descriptors, we have carried out a HT screening of all the polar perovskite compounds, uncovering 42 compounds of potential interests. Of these, Al-, Ga-, Sc-, and Ta-based compounds can form a 2DEG with STO, while In-based compounds exhibit a strain-induced strong polarization when deposited on STO substrate. In particular, the Ta-based compounds can form 2DEG with potentially high electron mobility at (TaO2)+/(SrO)0 interface. Our approach, by defining materials descriptors solely based on the bulk materials properties, and by relying on the perovskite-oriented quantum materials repository, opens new avenues for the discovery of perovskite-oxide-based functional interface materials in a HT fashion.
The emerging two-dimensional electron gas (2DEG) at the interface between polar LaAlO3 (LAO) and nonpolar SrTiO3 (STO) provides potential applications in low-dimensional nanoelectronic devices because of its exceptional electron transport property. To form 2DEG in the LAO/STO heterostructure (HS), a minimum thickness of approximately 4 unit cells of LAO is necessary. Herein, we modeled the n-type (TiO2)(0)/(LaO)(+1) HS by depositing (LAO)n (n = 4, 5, and 6) thin films on the STO substrate and explored strain effects on the critical thickness for forming 2DEG in the LAO/STO HS-based slab systems using first-principles electronic structure calculations. A vacuum layer was added along the [001] direction on the LAO film to resemble the actual epitaxial growth process of the materials. An insulator-to-metal transition is predicted in unstrained (LAO)n/STO systems when n ≥ 5. Our calculations indicate that O 2px/py states give rise to the surface conductivity, while Ti 3dxy states are responsible for the interfacial conductivity. For the tensilely strained HS system, an increased film thickness of LAO (n ≥ 6) is required to form the 2DEG, and a remarkable shift of O 2p orbitals toward higher energy in LAO layers is found, which is caused by the strain-induced change of the electrostatic potential. For the compressively strained HS system, the critical thickness of LAO film for forming 2DEG is between 5 and 6 unit cells of LAO. In addition, our calculations suggest that the interfacial charge carrier density and magnetic moment can be optimized when a moderate tensile strain is applied on the STO substrate in the ab-plane.
Two years' follow-up investigation of a hepatitis E virus (HEV) outbreak in West Kalimantan, Indonesia in 1991 was carried out to investigate the epidemiology of epidemic HEV transmission and the persistence of the immunoglobulin G (IgG) antibody response. Sixty cases identified as anti-HEV IgG positive during the outbreak in 1991 were matched with 67 controls and examined, together with 318 members of their families. Overall, the prevalence of anti-HEV IgG among the 445 subjects (representing 127 households) was 59%. There was no significant difference in anti-HEV prevalence between cases (72%) and controls (61%). Loss of detectable anti-HEV IgG after 2 years was demonstrated in 17 of 60 subjects (28%) who were originally positive for anti-HEV in 1991. The mean number of anti-HEV positive subjects per household was 2.04. Cross-sectional prevalence of anti-HEV IgG increased significantly with age (P = 0.01). When communities were grouped into areas of low (< 40%), medium (40-59%) and high (> or = 60%) anti-HEV prevalence, use of river water for drinking and cooking (P < 0.001), personal washing (P < 0.0001), and human excreta disposal (P < 0.001) were associated with high prevalence communities. Conversely, boiling drinking water was negatively associated with increased prevalence (P = 0.02). Subnormal rainfall during the month (August) leading up to the 1991 outbreak (19 cm compared to the monthly mean of 209 cm in 1985-1993) may have contributed to favourable epidemic conditions.
The highly mobile two-dimensional electron gas (2DEG) formed at the polar/nonpolar LaAlO3/SrTiO3 (LAO/STO) heterostructure (HS) is a matter of great interest because of its potential applications in nanoscale solid-state devices. To realize practical implementation of the 2DEG in device design, desired physical properties such as tuned charge carrier density and mobility are necessary. In this regard, polar perovskite-based transition metal oxides can act as doping layers at the interface and are expected to tune the electronic properties of 2DEG of STO-based HS systems dramatically. Herein, we investigated the doping effects of LaTiO3(LTO) layers on the electronic properties of 2DEG at n-type (LaO)(+1)/(TiO2)(0) interface in the LAO/STO HS using spin-polarized density functional theory calculations. Our results indicate an enhancement of orbital occupation near the Fermi energy, which increases with respect to the number of LTO unit cells, resulting in a higher charge carrier density of 2DEG than that of undoped system. The enhanced charge carrier density is attributed to an extra electron introduced by the Ti 3d(1) orbitals from the LTO dopant unit cells. This conclusion is consistent with the recent experimental findings (Appl. Phys. Lett. 2013, 102, 091601). Detailed charge density and partial density of states analysis suggests that the 2DEG in the LTO-doped HS systems primarily comes from partially occupied dyz and dxz orbitals.
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