The first-principles calculations are employed to investigate the electrical properties of polar MgO/BaTiO3(110)interfaces. Both n-type and p-type polar interfaces show a two-dimensional metallic behavior. For the n-type polar interface,the interface Ti3d electrons are the origin of the metallic and magnetic properties. Varying the thickness of Ba TiO3 may induce an insulator–metal transition, and the critical thickness is 4 unit cells. For the p-type polar interface, holes preferentially occupy the interface O 2p y state, resulting in a conducting interface. The unbalance of the spin splitting of the O 2p states in the interface Mg O layer leads to a magnetic moment of about 0.25μB per O atom at the interface.These results further demonstrate that other polar interfaces, besides LaAlO3/SrTiO3, can show a two-dimensional metallic behavior. It is helpful to fully understand the role of polar discontinuity on the properties of the interface, which widens the field of polar-nonpolar interfaces.
Density functional theory within the local density approximation is used to investigate the effect of the oxygen vacancy on the LaGaO_3/SrTiO_3(001) heterojunction. It is found that the energy favorable configuration is the oxygen vacancy located at the 3^(rd) layer of the STO substrate, and the antiferrodistortive distortion is induced by the oxygen vacancy introduced on the SrTiO_3 side. Compared with the heterojunction without introducing oxygen vacancy, the heterojunction with introducing the oxygen vacancy does not change the origin of the two-dimensional electron gas(2DEG), that is, the 2DEG still originates from the d_(xy) electrons, which are split from the t^(2g) states of Ti atom at interface; however the oxygen vacancy is not beneficial to the confinement of the 2DEG. The extra electrons caused by the oxygen vacancy dominantly occupy the 3d_(x^2-y^2) orbitals of the Ti atom nearest to the oxygen vacancy, thus the density of carrier is enhanced by one order of magnitude due to the introduction of oxygen vacancy compared with the density of the ideal structure heterojunction.
The first-principles calculations are employed to investigate the stability, magnetic, and electrical properties of the oxide heterostructure of LaAlO3/SrTiO3(110). By comparing their interface energies, it is obtained that the buckled interface is more stable than the abrupt interface. This result is consistent with experimental observation. At the interface of LaAlO3/SrTiO3(110) heterostructure, the Ti–O octahedron distortions cause the Ti t2 gorbitals to split into the twofold degenerate dxz/dyz and nondegenerate dxy orbitals. The former has higher energy than the latter. The partly filled two-fold degenerate t2 gorbitals are the origin of two-dimensional electron gas, which is confined at the interface. Lattice mismatch between LaAlO3 and SrTiO3leads to ferroelectric-like lattice distortions at the interface, and this is the origin of spin-splitting of Ti 3d electrons. Hence the magnetism appears at the interface of LaAlO3/SrTiO3(110).
First-principles calculations are performed to explore the possibility of generating the two-dimensional electron gas(2 DEG) at the interface between LaGaO_3/KTaO_3 and NdGaO_3/KTaO_3(001) heterostructures. Two different models —i.e., the superlattice model and the thin film model — are used to conduct a comprehensive investigation of the origin of charge carriers. For the symmetric superlattice model, the LaGaO_3(or NdGaO_3) film is nonpolar. The 2 DEG with carrier density on the order of 1014 cm^(-2) originates from the Ta dxy electrons contributed by both LaGaO_3(or NdGaO_3) and KTaO_3. For the thin film model, large polar distortions occur in the LaGaO_3 and NdGaO_3 layer, which entirely screens the built-in electric field and prevents electrons from transferring to the interface. Electrons of KTaO_3 are accumulated at the interface, contributing to the formation of the 2 DEG. All the heterostructures exhibit conducting properties regardless of the film thickness. Compared with the Ti dxy electrons in SrTiO_3-based heterostructures, the Ta dxy electrons have small effective mass and they are expected to move with higher mobility along the interface. These findings reveal the promising applications of 2 DEG in novel nanoelectronic devices.
