Photoferroelectric Phenomena and Photostimulated Phase Transitions

Abstract

Photoferroelectric phenomena encompass a large class of effects with a wide variety of mechanisms. Nevertheless, they can be divided into two main groups. The first group includes phenomena associated with the excitation of nonequilibrium carriers and the optical recharging of levels in the neutral volume of the ferroelectric with no internal field present [3.6]. This group of phenomena is described by the thermodynamic relations of Sect. 2.3 and by the mechanisms considered in Chap. 3, in particular, the pseudo-Jahn-Teller effect. As shown in Chap. 4, however, a real ferroelectric always has an internal field, whose value and distribution depend on the screening of the spontaneous polarization in the volume and on the surface of the crystal. Excitation of nonequilibrium carriers, changing the screening conditions, simultaneously changes the value and distribution of the internal field in the ferroelectric. This may result in a shift of the Curie point and a change in the whole set of properties of the ferroelectric, including its domain structure. This second group of phenomena due to the influence of the spontaneous polarization will be conventionally called screening phenomena. Thus, the ferroelectric phenomena that are actually observed may be associated with two completely different mechanisms, whose experimental differentiation usually involves difficulties. In most cases the influence of intrinsic illumination on the physical properties of a ferroelectric is due to the effects of both mechanisms.