Validity of Nonlinear Thermodynamic Models in Ferroelectric-Paraelectric Bilayers and Superlattices

We examine the validity of nonlinear thermodynamic models which incorporate the contribution of electrostatic coupling and interfacial space charge in ferroelectric-paraelectric bilayers. Inconsistency of the directions between build-in polarization and field is found. Evolution of ferroelectricity does not follow the variation in coupling strength between different layers. In addition, we observe that the field-induced polarization in the paraelectric layer deviates from the polarization derived by thermodynamic theory. These findings indicate that the current versions of thermodynamic models need to be modified.