Charge transfer vibronic excitons in ferroelectric oxides: experiment and theory

Abstract

Review of the charge transfer vibronic excitons (CTVE) study on the basis of optical response investigations in ferroelectric oxides is presented. CTVE is the correlated pair of electron and hole small polarons whose stability is controlled by a pair pseudo-Jahn-Teller effect. The study is focused on the CTVE-induced critical dependence near the ferroelectric phase transition. The critical effect in absolute diffraction efficiency (ADE) formation of the laser-induced transient gratings near ferroelectric phase transition in KTa1-xNbxO3 (KTN) is considered as an example. Critical concentration dependence for slow as well as for instantaneous (fast) optical response was detected. The peak positions for these x - dependences were different. The CTVE-model can explain these experiments laking into account not only the soft TO phonon mode contribution but also that of soft tunnel quasilocal modes of clusters of the second component. (C) 1999 Published by Elsevier Science B.V. All rights reserved.