First-Principles Prediction of Two-Dimensional Electron Gas Driven by Polarization Discontinuity in Nonpolar/Nonpolar AHfO3/SrTiO3 (A = Ca, Sr, and Ba) Heterostructures

The CdTiO3/BaTiO3 superlattice interface from first principles

The oxide interface has been studied extensively in the past decades and exhibits different physical properties from the constituent bulks. Using first-principles electronic structure calculations, we investigated the interface of CdTiO₃/BaTiO₃ (CTO/BTO) superlattice with ferroelectric BaTiO₃. In this case, the conduction bands of CdTiO₃ are composed of Cd-5s orbitals with low electron effective mass and nondegenerate dispersion, and thus expected to have high mobility. We predicted a controllable conductivity at the interface, and further analyzed how the polarization direction and strength affect the conductivity. We also explored the relationship between two components: thickness and polarization. Intriguingly, the total polarization in CTO/BTO might be even larger than that of ferroelectric bulk BaTiO₃. Therefore, we found a way to maximize the superlattice polarization by increasing the fraction of the CdTiO₃ layers, based on the interesting dependence of the total polarization and CTO/BTO ratio.

First-principles study of the ferroelectric properties of SrTaO2N/SrTiO3interfaces

First-principles calculations based on density-functional theory in the pseudo-potential approach have been performed for the total energy, crystal structure and cell polarization for SrTaO2N/SrTiO3heterostructures. Different heterojunctions were analyzed in terms of the termination atoms at the interface plane, and periodic or non-periodic stacking in the perpendicular direction. The calculations show that the SrTaO2N layer is compressed along theab-plane, while the SrTiO3is elongated, thus favoring the formation ofP4mmlocal environment on both sides of the interface, leading to net macroscopic polarization. The analysis of the local polarization as a function of the distance to the interface, for each individual unit cell was found to depend on the presence of a N or an O atom at the interface, and also on the asymmetric and not uniformc-axis deformation due to the induced strain in theab-plane. The resulting total polarization in the periodic array was ≈0.54 C m-2, which makes this type of arrangement suitable for microelectronic applications.

Two-dimensional polar metals in KNbO3/BaTiO3 superlattices: first-principle calculations

Polar metals, commonly defined by the coexistence of polar structure and metallicity, are thought to be scarce because free carriers eliminate internal dipoles that may arise owing to asymmetric charge distributions. By using first-principle electronic structure calculations, we explored the possibility of producing metallic states in the polar/nonpolar KNbO₃/BaTiO₃ superlattice (SL) composed of two prototypical ferroelectric materials: BaTiO₃ (BTO) and KNbO₃ (KNO). Two types of polar/nonpolar interfaces, p-type (KO)⁻/(TiO₂)⁰ and n-type (NbO₂)⁺/(BaO)⁰, which can be constituted into two symmetric NbO₂/BaO–NbO₂/BaO (NN-type) and KO/TiO₂–KO/TiO₂ (PP-type) SL, as well as one asymmetric KO/TiO₂–NbO₂/BaO (PN-type) SL. The spatial distribution of ferroelectric distortions and their conductive properties are found to be extraordinarily sensitive to the interfacial configurations. An insulator-to-metal transition is found in each unit cell of the symmetric interfacial SL models: one exhibiting quasi-two-dimensional n-type conductivity for NN-type SL, while the other being quasi-two-dimensional p-type conductivity for PP-type SL. The anisotropic coexistence of in-plane orientation of free carriers and out-of-plane orientation of ferroelectric polarization in KNO/BTO SL indicates that in-plane free carriers can not eliminate the out-of-plane dipoles. Our results provide a road map to create two-dimensional polar metals in insulating perovskite oxide SL, which is expected to promote applications of new quantum devices.

Polar metals, commonly defined by the coexistence of polar structure and metallicity, are thought to be scarce because free carriers eliminate internal dipoles that may arise owing to asymmetric charge distributions.

A termination-insensitive and robust electron gas at the heterointerface of two complex oxides

The single-crystal SrTiO₃ (001) has two different surface terminations, TiO₂ and SrO. One most remarkable observation in previous studies is that only the heterointerfaces with TiO₂-terminated SrTiO₃, which usually combines with polar oxides such as LaAlO₃, host an electron gas. Here we show that a robust electron gas can be generated between a non-polar oxide, CaHfO₃, and SrTiO₃ (001) with either termination. Unlike the well-known electron gas of LaAlO₃/SrTiO₃, the present one of CaHfO₃/SrTiO₃ essentially has no critical thickness of CaHfO₃, can survive a long-time oxygen annealing at high temperature, and its transport properties are stable under exposure to water and other polar solvents. By electrostatic gating through CaHfO₃, field-effect devices are demonstrated using CaHfO₃/SrTiO₃ heterointerfaces with both terminations. These results show that the electron gas reported in the present study is unique and promising for applications in oxide electronics.

Two dimensional electron gases (EG) at the heterointerface of complex oxides show fascinating properties. The authors report on an EG formed at the CaHfO₃/SrTiO₃ interface independent of interface termination and robust against environmental conditions most likely originating from oxygen non-stoichiometry.

First-principles studies of a two-dimensional electron gas at the interface of polar/polar LaAlO3/KNbO3 superlattices

We explored the possibility of producing a two-dimensional electron gas (2DEG) in polar/polar (LaAlO₃)_m/(KNbO₃)_n perovskite superlattices that have N type and P type interfaces using the first-principles electronic structure calculations.

Extreme Reconfigurable Nanoelectronics at the CaZrO3 /SrTiO3 Interface

Complex oxide heterostructures have fascinating emergent properties that originate from the properties of the bulk constituents as well as from dimensional confinement. The conductive behavior of the polar/nonpolar LaAlO3 /SrTiO3 interface can be reversibly switched using conductive atomic force microscopy (c-AFM) lithography, enabling a wide range of devices and physics to be explored. Here, extreme nanoscale control over the CaZrO3 /SrTiO3 (CZO/STO) interface, which is formed from two materials that are both nonpolar, is reported. Nanowires with measured widths as narrow as 1.2 nm are realized at the CZO/STO interface at room temperature by c-AFM lithography. These ultrathin nanostructures have spatial dimensions at room temperature that are comparable to single-walled carbon nanotubes, and hold great promise for alternative oxide-based nanoelectronics, as well as offer new opportunities to investigate the electronic structure of the complex oxide interfaces. The cryogenic properties of devices constructed from quasi-1D channels, tunnel barriers, and planar gates exhibit gate-tunable superconductivity, quantum oscillations, electron pairing outside of the superconducting regime, and quasi-ballistic transport. This newly demonstrated ability to control the metal-insulator transition at nonpolar oxide interface greatly expands the class of materials whose behavior can be patterned and reconfigured at extreme nanoscale dimensions.

Tuning Interfacial Magnetic Ordering via Polarization Control in Ferroelectric SrTiO3/PbTiO3 Heterostructure

The electromagnetic properties at the interface of heterostructure are sensitive to the interfacial crystal structure and external field. For example, the two-dimensional magnetic states at the interface of LaAlO₃/SrTiO₃ are discovered and can further be controlled by electric field. Here, we study two types of heterostructures, TiO₂/PbTiO₃ and SrTiO₃/PbTiO₃, using first-principle electronic structure calculations. We find that the ferroelectric polarization discontinuity at the interface leads to partially occupied Ti 3d states and the magnetic moments. The magnitude of the magnetic moments and the ground-state magnetic coupling are sensitive to the polarization intensity of PbTiO₃. As the ferroelectric polarization of PbTiO₃ increases, the two heterostructures show different magnetic ordering that strongly depends on the electron occupation of the Ti t_2g orbitals. For the TiO₂/PbTiO₃ interface, the magnetic moments are mostly contributed by degenerated d _yz/d _xz orbitals of interfacial Ti atoms and the neighboring interfacial Ti atoms form ferromagnetic coupling. For SrTiO₃/PbTiO₃ interface, the interfacial magnetic moments are mainly contributed by occupied d _xy orbital because of the increased polarization intensity, and as the electron occupation increases, there exists a transition of the magnetic coupling between neighboring Ti atoms from ferromagnetism to antiferromagnetism via the superexchange interaction. Our study suggests that manipulating the polarization intensity is one effective way to control interfacial magnetic ordering in the perovskite oxide heterostructures.

Thickness Control of the Spin-Polarized Two-Dimensional Electron Gas in LaAlO3/BaTiO3 Superlattices

We explored the possibility of increasing the interfacial carrier quantum confinement, mobility and conductivity in the (LaAlO₃)_n/(BaTiO₃)_n superlattices by thickness regulation using the first-principles electronic structure calculations. Through constructing two different interfacial types of LaAlO₃/BaTiO₃ superlattices, we discovered that the LaO/TiO₂ interface is preferred from cleavage energy consideration. We then studied the electronic characteristics of two-dimensional electron gas (2DEG) produced at the LaO/TiO₂ interface in the LaAlO₃/BaTiO₃ superlattices via spin-polarized density functional theory calculations. The charge carrier density of 2DEG has a magnitude of 10¹⁴ cm⁻² (larger than the traditional system LaAlO₃/SrTiO₃), which is mainly provided by the interfacial Ti 3d_xy orbitals when the thicknesses of LaAlO₃ and BaTiO₃ layers are over 4.5 unit cells. We have also revealed the interfacial electronic characteristics of the LaAlO₃/BaTiO₃ system, by showing the completely spin-polarized 2DEG mostly confined at the superlattice interface. The interfacial charge carrier mobility and conductivity are found to be converged beyond the critical thickness. Therefore, we can regulate the interfacial confinement for the spin-polarized 2DEG and quantum transport properties in LaAlO₃/BaTiO₃ superlattice via controlling the thicknesses of the LaAlO₃ and BaTiO₃ layers.

First-principles studies of polar perovskite KTaO3 surfaces: structural reconstruction, charge compensation, and stability diagram

First-principles calculations predict a surface phase stability diagram for the polar perovskite KTaO₃.

Comparison Studies of Interfacial Electronic and Energetic Properties of LaAlO3/TiO2 and TiO2/LaAlO3 Heterostructures from First-Principles Calculations

By using first-principles electronic structure calculations, we studied electronic and energetic properties of perovskite oxide heterostructures with different epitaxial growth order between anatase TiO₂ and LaAlO₃. Two types of heterostructures, i.e., TiO₂ film grown on LaAlO₃ substrate (TiO₂/LaAlO₃) and LaAlO₃ film grown on TiO₂ substrate (LaAlO₃/TiO₂), were modeled. The TiO₂/LaAlO₃ model is intrinsically metallic and thus does not exhibit an insulator-to-metal transition as TiO₂ film thickness increases; in contrast, the LaAlO₃/TiO₂ model shows an insulator-to-metal transition as the LaAlO₃ film thickness increases up to 4 unit cells. The former model has a larger interfacial charge carrier density (n ∼ 10¹⁴ cm^-2) and smaller electron effective mass (0.47m_e) than the later one (n ∼ 10¹³ cm^-2, and 0.70m_e). The interfacial energetics calculations indicate that the TiO₂/LaAlO₃ model is energetically more favorable than the LaAlO₃/TiO₂ model, and the former has a stronger interface cohesion than the later model. This research provides fundamental insights into the different interfacial electronic and energetic properties of TiO₂/LaAlO₃ and LaAlO₃/TiO₂ heterostructures.