The observation of a universal response of electromagnetic, acoustic, and mechanical influences shows the existence of some transfer processes in a medium, which are not described by a usual diffusion equation. It is shown that these processes can be explained by making use of the diffusion equation taking into account the ‘non-Marcovian’ character of the excitation transfer process. In accordance with the experimental data a memory function is found which in one limiting case describes pure diffusion (absence of memory) and in another case turns into the wave equation (full memory). A local field frequency distribution function is suggested which confirms the suppositions made. The conditions of the realisation of the transfer process with ‘remnant’ memory are found and described by a generalized linear transfer equations in fractional derivations.

To the Theoretical Explanation of the ``Universal Response''

Polarization dynamics and formation of polar nanoregions (PNR) in relaxor ferroelectrics is considered within a model of interacting short range polar clusters formed by off-center ions in highly polarizable materials. The model is applicable on the mesoscopic level and takes explicitly into account the distribution of cluster relaxation times and the existence of quenched random fields which control the size of PNR. Using self-consistent random field theory and continuous time random walk approximation, a relationship is established between the average polarization dynamics contributing to the low frequency dielectric response, and the local polarization dynamics determining NMR spin lattice relaxation time [Blinc et al., Phys. Rev. B 63, 024104 (2001); Cordero et al., ibid. 71, 094112 (2005)]. The lengthscale of PNR estimated from the obtained universal relationship between the parameters of the soft mode dispersion curve and the static dielectric permittivity is in agreement with the experiment [Vakhrushev et al., Physica B 156-157, 90 (1989)]. The predicted proportionality, between the intensity of diffuse neutron scattering from dynamic PNR and square of PNR correlation length, which has been recently a subject of controversy, is also in agreement with experiment by Vakhrushev et al. Therefore, we conclude that the model captures essential features of the static and dynamic behavior of relaxor ferroelectrics, and could be used for the characterization of new relaxor materials.

Polarization dynamics and formation of polar nanoregions in relaxor ferroelectrics

Energy transfer kinetics in disordered systems

Quantum yields of selective and non-selective luminescence in solid solutions

The self-consistent method of partial summation of the Green function density series for disordered systems, proposed by Gochanour. Andersen and Fayer

What are the GAF and LAF approaches in essence

The character of spatial and spectral spin diffusion Processes in disordered spin systems is studied. The analysis is based on averaging of the rate equations, valid for fixed spin positions, over different particle configurations. New Expressions for the spatial and spectral diffusion constants are derived and compared with earlier theories.



[Russian Text Ignored].

Spatial and Spectral Spin Diffusion in Dilute Spin Systems

A theory of the spatial and spectral spin diffusion in magnetically dilute crystals is developed. New expressions for the Spatial and spectral spin diffusion constants are obtained and the spin diffusion effect on the spin-lattice relaxation is investigated.

Spin diffusion and spin–lattice relaxation in paramagnetic crystals

The concentration dependence of niobium-induced ferroelectric phase transition temperature Tc(f) in KTa1−fNbfO3 is explained assuming an eight-well niobium off-center potential. Single-ion model and mean-field theory is used. The best agreement between the theory and experiment is reached with the following values of parameters: barrier height U = 60 K, off-center displacement x0 = 0.008 nm which are in good agreement with previous experimental results.



[Russian Text Ignored].

B. E. Vugmeister

Papers

Spatial and Spectral Spin Diffusion in Dilute Spin Systems

Spin diffusion and spin–lattice relaxation in paramagnetic crystals

Simple Model for the Ferroelectric Phase Transition in KTa1-fNbfO3

Nuclear Spin-Lattice Relaxation Due to Quadrupole Interaction with Current Carriers in Quantizing Magnetic Fields