Electrostatics liberating restrictions on ferroelectric by unification of polar discontinuity e^–h⁺ layers and criteria of intrinsicality

Electron e^– and hole h⁺ layers at polar discontinuities as a thermodynamically stable state previously predicted are recently shown by many experiments. We discuss that these layers including those at SrTiO₃/LaAlO₃ support our previous prediction of the substantially reduced electrostatic restrictions on ferroelectricity such as size effect and domains when the origins of these conductions are intrinsic. For this, the criteria of the intrinsicality of these layers, i.e., not due to conventional mechanisms such as defects and adsorbates, are presented. In addition, we discuss that this reduction of the electrostatic restriction is the same as recently proposed hyperferroelectricity and show that hyperferroelectricity is a universal properties of general ferroelectric.