Because the permittivity for the depolarization field Ed depends on the definition of the spontaneous polarization Ps, improper permittivity is often used in the literature. Therefore, we define proper combinations of the permittivity and Psfor Ed that are consistent with experimental conditions. We show that the permittivity for Ed should be unity in most cases, especially for nonideal capacitor structures, which are often discussed on the basis of the Mehta formula. This choice of permittivity is shown to be consistent with the experimental linear permittivity in response to the external field. This permittivity indicates the problem with assuming a ferroelectric as an insulator under Ed: A strain-free two-dimensional single-domain ferroelectric phase with a Ps ⊥surface cannot exist regardless of the thickness of the ferroelectric, when at least one of the surfaces is unscreened. Alternatively, the agreement between experimental and theoretical results obtained at a permittivity «1 for Ed suggests the existence of nondielectric screening mechanisms. Simple criteria for the stability of the insulating ferroelectric phase in capacitor structures are presented. copy; 2010 The Physical Society of Japan.
All Science Journal Classification (ASJC) codes
- Physics and Astronomy(all)