Nontrivial dependence of dielectric stiffness and SHG on dc bias in relaxors and dipole glasses

Dielectric permittivity and Second Harmonic Generation (SHG) studies in the field-cooled mode show a linear dependence of dielectric stiffness (inverse dielectric permittivity) on dc bias in PMN-PT crystals and SHG intensity in KTaO$_{3}$:Li at small Li concentrations. We explain this unusual result in the framework of a theory of transverse, hydrodynamic-type, instability of local polarization.Comment: 5 figure

A new phase in ferroelectric oxides: The phase of charge transfer vibronic excitons

It is shown, by means of Hartree-Fock-type calculations using the intermediate neglect of the differential overlap (INDO) method, that polaronic-type charge transfer vibronic excitons (CTVE) in ferroelectric oxides could lead to the formation of a new phase. The ground-state energy of this phase of strongly correlated CTVE lies within an optical gap of pure crystal, and is characterized by a strong tetragonal lattice distortion, as well as ferroelectric and antiferromagnetic ordering. It is shown also that clusters of the CTVE phase being stabilized by oxygen vacancies could be responsible for the unusually strong optical Second Harmonic Generation (SHG) in nominally pure incipient ferroelectrics, like \chem{KTaO_{3}} (\chem{KTO}) and \chem{SrTiO_{3}} (\chem{STO}), especially in the case of SHG induced by an external electric field. Another experimental manifestation of CTVE is related to a drastic red shift with temperature of the fundamental absorption (FA) edge in ferroelectric oxides \chem{SBN} and \chem{KTO} observed at high temperatures. This effect could be explained by thermal population of a low-lying bottom part of an additional valence band induced by the CTVE phase

Quantum chemical modelling of electron polarons and 'green' luminescence in PbTiO3 perovskite crystals

In an extension of our previous study on the electron polarons and excitons in KNbO3, KTaO3 and BaTiO3 (Kotomin E A, Eglitis R I and Borstel G 2000 J. Phys.: Condens. Matter 12 L557; Eglitis R I, Kotomin E A and Borstel G 2002 J. Phys.: Condens. Matter 14 3735) by the semiempirical Hartree-Fock method we present here results for free electron polarons in the PbTiO3 perovskite crystal. We discuss the origin of the intrinsic visible band emission of PbTiO3 perovskite oxides (so-called 'green luminescence') which has remained a topic of high interest during the last quarter of a century. We present a theoretical calculation modelling this emission in the framework of a concept of charge transfer vibronic excitons, i.e. as a result of radiative recombination of correlated (bound) self-trapped electron and hole polarons in the highly polarizable PbTiO3-type matrix. The intermediate neglect of differential overlap method combined with the large unit cell periodic defect model was used for quantum chemical calculations and theoretical simulation of the green emission for a PbTiO3 perovskite. The calculated 'green' luminescence energy for PbTiO3 perovskite-type crystals agrees well with experimental measurements presented in this letter