Electrostatic coupling with interfacial free charge in ferroelectric-paraelectric bilayers and superlattices

Based on a thermodynamic model, we studied the interfacial free charge effect on the polarization, spatial inversion symmetry, and dielectric response in ferroelectric-paraelectric bilayer. The broken spatial inversion symmetry and imprint due to free charge are not found. Effect of free charge on the dielectric constant is not monotonic. Contradiction between the equilibrium polarization and non-linear field-induced dielectric polarization in paraelectric layer is found. Our study reveals that treating coupling strength as an “input” parameter taken dependent of the polarization is inappropriate to describe electrostatic coupling with interfacial free charge.