Intrinsic free electrons/holes at polarization discontinuities and their implications for basics of ferroelectricity and its origin

This article reviews the theories and experiments on the macroscopic/nanoscopic scales, which indicate that “nearly free” electrons/holes appear at polarization discontinuities as a result of not only extrinsic mechanisms such as imperfections but also intrinsic mechanisms. We examine the consistency of these theories and experiments with conventional ones. Such electrons/holes lead to various novel properties of ferroelectrics and provide new insight into ferroelectricity, including fundamental issues such as the origin of ferroelectricity. This origin appears naturally compatible with the existence of multiferroicity.