When One Plus One is More than Two Simulations show that the interface between two perovskite oxides can be a much stronger ferroelectric than either oxide on its own.

Sixty years ago, scientists discovered that they could stack ordinary doped semiconductors together to make diodes and transistors. The heart of such devices is an interface whose electronic transport properties are unlike either material from which it formed. Today researchers are applying the same strategy to build “functional interfaces” with perovskite oxides, materials in which the structural, magnetic, and electronic properties are strongly coupled. The properties of an interface between two perovskites are rarely a simple mixture of the parent materials [1]. For example, the interface between two insulating perovskites can be conducting [2]. Now, based on simulations, a team led by Xifan Wu at Temple University in Pennsylvania has made the similarly surprising finding that the interface between two model perovskites, calcium titanate and barium titanate, has a structure whose spontaneous electric polarization is greater than that of either material in its bulk form [3]. Materials with a spontaneous electric polarization are known as ferroelectrics, and because their polarity can be switched with an applied electric field, they can be used to store bits of information. The researchers’ approach for enhancing electric polarization should apply to many perovskites and has the potential to greatly expand the number of available ferroelectric materials. Perovskite oxides have a generic formula ABO3, where A is typically an alkaline-earth or rare-earth element and B is a transition metal. There are many ways to combine the A and B cations, and the properties of the resulting materials are diverse. A fraction of the electrons from the A and B cations transfer to the oxygen anions. Those electrons that remain on the B cations tend to occupy the d orbitals, which are localized. These electronic states can be spin-polarized (the root of magnetism); be strongly correlated; or produce charge and orbital ordering. Moreover, these degrees of freedom are strongly coupled, and a small change to one of them—say, from applying strain to the material [4] or changing the local chemical environment (as at an interface)—can greatly affect