Compositional symmetry breaking in ferroelectric bilayers

Compositional variations across ferroelectric bilayers result in broken spatial inversion symmetry that can lead to asymmetric thermodynamic potentials. For the case of insulating materials, ferroelectric multilayers will self-pole due to the electrostatic coupling between the layers. Polarization-graded ferroelectrics with smooth composition, temperature, or stress gradients are viewed as bilayer structures in the limit of the ever-increasing number of bilayer couples, thus permitting us to conclude that the unconventional hysteresis associated with “up” and “down” polarization graded structures are real phenomena, and not artifacts associated with free charge or asymmetric leakage current.