Critical effects in optical response due to charge transfer vibronic excitons and their structure in perovskite-like systems

A mechanism for bilinear interaction between high-frequency light-induced electronic polarization and low-frequency soft lattice polarization is proposed. It is based on the fluctuations of the charge transfer connected with charge transfer vibronic excitons (CTVE). This bilinear mechanism leads to the appearance of the critical peculiarities of the absolute diffraction efficiency of transient gratings near the ferroelectric phase transition. A semi-empirical Hartree-Fock INDO method was used for the evaluation of the energy parameters and the equilibrium displacements for the CTVE in KTaO₃. This numerical study did confirm the proposed CTVE-model. It was shown that the CTVE-phase (with correlated CTVE in each cell) is characterized by new phonon spectra with a new set of soft lattice modes. It was also shown that meta-stable oxygen vacancy states are of dipole type in the CTVE-phase.