Dielectric Response in Microscopically Heterogeneous Dielectrics: Example of KTaO_3:Nb

New experimental data on solid solutions of quantum paraelectrics with KTaO_3:Nb as an example are considered within a framework of a quantum theory of ferroelectric phase transitions. In order to describe the effect of local heterogeneities a percolation type theory together with a random field approach were employed.Comment: 4 figure

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

OFF-CENTRE IMPURITY CENTRES.Temperature dependence of the reorientation behavior of < 110 > off-center Ag+ defects in RbBr

Des mesures électro-optiques effectuées dans RbBr : Ag+ au-dessous de 4 K mettent en évidence la réorientation des défauts Ag+ ( off-center). Celle-ci s'effectue uniquement par des sauts de 90° dans un plan (100). Au-dessous de 2 K le temps de relaxation τ90 est proportionnel à T-1 et caractéristique d'un processus tunnel assisté par un phonon. La réorientation par saut de 60° est totalement congelée. Au-dessus de 4 K les réorientations par sauts de 60° et 90° sont décrites par une loi d'Arrhenius avec une énergie d'activation de (kB.161 K) et (kB.87 K) respectivement.Electro-optical measurements in RbBr : Ag+ reveal below 4 K the reorientation of off-center Ag+ defects by 90° transitions only in a (100) plane. Below 2 K a clear T-1 dependence of the relaxation time τ90 is found, characteristic for a one-phonon-assisted tunneling process while the 60° reorientation process is completely frozen in. Above 4 K, both, the 60° and the 90° reorientation process become Arrhenius-like with an activation energy of (kB.161 K) and of (kB.87 K) respectively

Percolation with constraints in the highly polarizable oxide KTaO

Experimental data on the temperature and electric field dependence of the Second Harmonic Generation Intensity in the solid solution KTaO3:Li is analyzed in the framework of a new percolation-type approach which considers the history of cluster formation averaged over a large number of samples. In order to take into account that two cluster dipole moments turn when joining clusters together, a constraint is imposed on the cluster growth. This constraint leads to cluster-size saturation just after the percolation threshold. Owing to this circumstance, the connected cluster coexists with free clusters in some range of the impurity concentration and temperature. There is qualitative agreement between the computational results and experiments on Second Harmonic Generation intensities. The algorithm was written in C++ which allowed realistic computations to be performed on a standard PC

Percolation with constraints in the highly polarizable oxide KTaO 3:Li

PACS. 71.55.Jv Disordered structures; amorphous and glassy solids - 78.20.Bh Theory, models, and numerical simulation - 42.65.Ky Harmonic generation, frequency conversion,