Ferroelectricity In Superlattices Research Articles

Emergent Ferroelectricity in Otherwise Nonferroelectric Oxides by Oxygen Vacancy Design at Heterointerfaces.

Introducing point defects in complex metal oxides is a very effective route to engineer crystal symmetry and therefore control physical properties. However, the inversion symmetry breaking, which is vital for many tantalizing properties, such as ferroelectricity and chiral spin structure, is usually hard to be induced in the bulk crystal by point defects. By designing the oxygen vacancy formation energy profile and migration path across the oxide heterostructure, our first-principles density functional theory (DFT) calculations demonstrate that the point defects can effectively break the inversion symmetry and hence create novel ferroelectricity in superlattices consisting of otherwise nonferroelectric materials SrTiO3 and SrRuO3. This induced ferroelectricity can be significantly enhanced by reducing the SrTiO3 thickness. Inspired by theory calculation, SrTiO3/SrRuO3 superlattices were experimentally fabricated and are found to exhibit surprising strong ferroelectric properties. Our finding paves a simple and effective pathway to engineer the inversion symmetry and thus properties by point defect control in oxide heterostructures.

The enhanced ferroelectricity in Sr1-δTiO3/BaTiO3 superlattices with Sr deficiency

Here we report the tunable ferroelectricity in superlattices of (Sr1-δTiO3)n/(BaTiO3)8 with δ≈ 5% and the maximally enhanced ferroelectricity is found in the n= 6 superlattice. The room-temperature remnant polarization of n =6 is enhanced by 70% and 1120% compared to those of BaTiO3 and Sr1-δTiO3 thin films, respectively. The ferroelectric transition temperature of n =6 is 40 K higher than that of the BaTiO3 film, and is independent of the measurement frequency. The dependence of remnant field 2Pr on n coincides well with the n dependence of γ/α, which is ratio between out-of-plane and in-plane octahedral rotation angles. It thus suggests a strong correlation between the octahedral rotation pattern and the ferroelectric distortion. Our study demonstrates the importance of constructing superlattices to enhance the ferroelectric properties of the defect-engineered materials to be better than the pristine materials.

Interface control of ferroelectricity in LaNiO3-BaTiO3 superlattices

LaNiO3-BaTiO3 superlattices with different types of interfaces are studied from first-principles density-functional theory. It is revealed that the ferroelectricity in the superlattice with (NiO2)−/(BaO)0 interfaces is enhanced from that of the superlattice with (LaO)+/(TiO2)0 interfaces. The origin lies at the polar discontinuity at the interface, which makes the holes localized within the (NiO2)−/(BaO)0 interface, but drives a penetration of electrons into BaTiO3 component near (LaO)+/(TiO2)0 interface. Our calculations demonstrate an effective avenue to the robust ferroelectricity in BaTiO3 ultrathin films.

Interface-Induced Modifications of Polarization in Nanoscale Ferroelectric Superlattices

A thermodynamic model to study the effect of interface intermixing on polarization of ferroelectric superlattices composed of a ferroelectric and paraelectric layers is developed. Interface intermixing forms an intermixed layer with property different from its individual layers, leading to inhomogeneous ferroelectric properties in the superlattices. Polarization induces near the interface of paraelectric layers, which extends into the layer over a distance governed by its characteristic length. Dependence of polarization on periodic thickness indicates an interface-induced modification of ferroelectricity in superlattices. Enhancement in polarization of superlattices is shown to be possible, if certain interface property and periodic thickness met.