Prediction of a strong polarizing field in thin film paraelectrics

We demonstrate the existence of a polarizing field in thin films of insulators with charged ionic layers. The polarizing field derives from the same physics as the well-known depolarizing field that suppresses ferroelectric polarization in thin-film ferroelectrics, but instead drives thin films of materials that are centrosymmetric and paraelectric in their bulk form into a noncentrosymmetric, polar state. We illustrate the behavior using density-functional computations for perovskite-structure potassium tantalate, KTaO3, which is of considerable interest for its high dielectric constant, proximity to a quantum critical point, and superconductivity. We then provide a simple recipe to identify whether a particular material and film orientation will exhibit the effect and develop an electrostatic model to estimate the critical thickness of the induced polarization in terms of basic material parameters. Our results provide practical guidelines for exploiting the electrostatic properties of thin-film ionic insulators to engineer novel functionalities for nanoscale devices.