Charge transfer vibronic excitons in ferroelectric oxides: A new origin of reorienting electric dipole moments and a new charge transfer excitonic phase

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Abstract
The charge transfer vibronic excitons (CTVE) introduced are the correlated pairs of electronic and hole Jahn-Teller polarons. Their Jahn-Teller type lattice distortion is caused mainly by strengthening of the vibronic interaction in final recharged ion states of CTVE due to switching on the pair Jahn-Teller effect. Dipole-TO soft mode interaction is also effective within electron-hole “Negative-U” effect. Theoretical study of CTVE leads to the conclusion that the barrier which controls the stability of CTVE has the vibronic origin. Localized CTVE has the reoriented electric dipole moment and can be as a new type relaxator. The realization of correlated CTVE in each cell corresponds to the CTVE-phase. The latter is ferroelectric. The admixture of the CTVE-phase states to the ground state is caused by critical fluctuations (mainly connected with chain-like instability) and by defect fields. Its can play the principle role in polar microdomain and Müller phase formation. The strengthening of soft mode quantum stabilization due to central peak phenomenon on chain-like instabilities is obtained.