B. E. Vugmeister

Coexistence of the critical slowing down and glassy freezing in relaxor ferroelectrics

We have developed a dynamical model for the dielectric response in relaxor ferroelectrics which explicitly takes into account the coexistence of the critical slowing down and glassy freezing. The application of the model to the experiment in PMN allowed for the reconstruction of the nonequilibrium spin glass state order parameter and its comparison with the results of recent NMR experiment (Blinc et al., Phys. Rev. Lett. 83, No. 2 (1999)). It is shown that the degree of the local freezing is rather small even at temperatures where the field-cooled permittivity exceeds the frequency dependent permittivity by an order of magnitude. This observation indicates the significant role of the critical slowing down (accompanying the glass freezing) in the system dynamics. Also the theory predicts an important interrelationship between the frequency dependent permittivity and the zero-field-cooled permittivity, which proved to be consistent with the experiment in PMN (A. Levstik et. al., Phys. Rev. B 57, 11204 (1998)).

Coexistence of long range order with glassy low frequency kinetics in random site electric dipole systems

Abstract We show that low frequency dispersion of dielectric constant and hysteresis loop above Tc can be explained by the existence of metastable states formed by clusters of close spaced dipoles. Interactions between clusters are responsible for ferroelectric ordering if dipole-dipole interaction possesses ferroelectric component like that in KTaO3:Li, Nb.

Raman study of the transition dynamics in KLT and KTN

Abstract A Raman scattering study of the hard TO2 optic mode in KTaO3:Li (KLT) and KTaO3:Nb (KTN) has revealed the existence of dynamical polar nanoregions. Associated with these precursor effects, the TO2 lineshape exhibits noticeable changes. In KLT two such changes have been observed, the first one at about 200K and the second one through the transition. The behavior in KTN is different in that only one change in the linewidth (which occurs at the transition) can be resolved. A model explaining this behavior is proposed. In KLT, the high temperature change in linewidth can be attributed to the increase in the lifetime of the polar nanoregions, such that the dynamical or homogeneous broadening becomes less than the static or inhomogeneous broadening. The second (lower temperature) decrease in the linewidth therefore results from the rapid increase in the correlation length at the transition. In KLT, below the transition, the width continues to slowly decrease which is indicative of a more progressively ...

Polarization switching and long-time relaxation effects in KTa1-xNbxO3 and K1-yLiyTaO3

Abstract A most striking anomaly is observed in the behavior of KTa1-x Nb x O3 (KTN) and K1-y Li y TaO3 (KLT) in the form of a remanent polarization well above Tc. We have measured hysteresis loops at different frequencies and for several concentrations of KTN and KLT single crystals. The concentrations that have been investigated are 1.2%, 3.5%, 15.7% Nb, and 3.5% Li. The shape of the loop changes significantly below a certain temperature signaling the onset of collective effects. Further changes as a function of frequency are shown to be due to the slow relaxation of randomly distributed off-center Nb or Li ions interacting in clusters. We have also developed a model of the dynamical polarization and calculate hysteresis loops which are in good agreement with experiment.

Comparative Raman study of the precursor order in KLT and KTN

A recent Raman study of the hard optic modes TO2 and TO3 in K1-xLixTaO3 (KLT) and KTa1-xNbxO3 (KTN) has confirmed the existence, above the transition, of precursor order in the form of polar nanoregions. These dynamical regions give rise to otherwise forbidden first-order Raman scattering. The primary difference between KLT and KTN is found in the temperature dependence of this impurity induced scattering. With a higher relaxation frequency of the bare dipoles in KTN, the nanoregions appear much more dynamic in KTN than they do in KLT. Consequently the TO2 impurity induced scattering appears at lower temperature and increases more rapidly. The difference between KTN and KLT is also revealed by a detail analysis of the TO2 lineshape above the transition, which is seen to be composed of a dynamic and a quasi-static component. The relative contributions of these two components are compared in the two systems. A microscopic theory of the lineshape is also presented.