Polaronic-type excitons in ferroelectric oxides: Microscopic calculations and experimental manifestation

We discuss the current experimental and theoretical understanding of new polaronic-type excitons in ferroelectric-oxides charge-transfer vibronic excitons (CTVE's), which are pairs of strongly correlated electronic and hole polarons. It is shown that charge-transfer-lattice distortion interactions are the driving forces for CTVE formation. Hartree-Fock-type calculations performed in the framework of the intermediate neglect of differential overlap (INDO) method as well as photoluminescence, second-harmonic generation, and UV-absorption high-temperature studies performed for ABO₃ ferroelectric oxides strongly support the CTVE existence. Both single CTVE and a phase of strongly correlated CTVE's in their ferroelectric ground and antiferroelectric excited states are analyzed.