Dynamics of relaxor ferroelectrics

We study a dynamic model of relaxor ferroelectrics based on the spherical random-bond---random-field model and the Langevin equations of motion. The solution to these equations is obtained in the long-time limit where the system reaches an equilibrium state in the presence of random local electric fields. The complex dynamic linear and third-order nonlinear susceptibilities $\chi_1(\omega)$ and $\chi_3(\omega)$, respectively, are calculated as functions of frequency and temperature. In analogy with the static case, the dynamic model predicts a narrow frequency dependent peak in $\chi_3(T,\omega)$, which mimics a transition into a glass-like state.Comment: 15 pages, Revtex plus 5 eps figure

THERAPY AND QUALITY OF LIFE OF PATIENTS WITH PSYCHOSIS

The symptoms and the individual experience of psychosis vary from patient to patient. Treatment, medication and cognitive psychotherapy are targeted mostly on positive and lately also on negative symptoms of psychosis. Deficits in metacognition found in patients with psychosis have a profound impact on the recovery process, their quality of life and experience of mental pain. Long term group psychotherapy helps patients to mourn their loss, improve their metacognition, and reduce the stigma and mental pain in patients with psychosis

Size effects in the thin films of order - disorder ferroelectrics subject to the depolarization field

The films of order-disorder type ferroelectrics were considered in the mean field approximation taking into account depolarization field. It was shown that size effects in this system could be described on the base of bulk system equation of state with Curie temperature dependent on the film thickness. The critical size hc and critical temperature Tc of phase transition from ferroelectric to paraelectric phase was calculated allowing for the depolarization field contribution. The comparison of the polarization dependence on the film thickness, temperature and electric field for the films of order-disorder and displacement type ferroelectrics is performed. In particular it was shown that all the dipoles become ordered at T=0 independently on the film thickness for h>hc contrary to the displacement type ferroelectrics. Critical thickness appeared larger and polarization distribution sharper for the displacement type ferroelectrics than for order-disorder type ferroelectrics.Comment: 7 pages, 4 figures, 1 tabl

Epitaxial growth and structural characterization of Pb(Fe1/2Nb1/2)O3 thin films

We have grown lead iron niobate thin films with composition Pb(Fe1/2Nb1/2)O3 (PFN) on (0 0 1) SrTiO3 substrates by pulsed laser deposition. The influence of the deposition conditions on the phase purity was studied. Due to similar thermodynamic stability spaces, a pyrochlore phase often coexists with the PFN perovskite phase. By optimizing the kinetic parameters, we succeeded in identifying a deposition window which resulted in epitaxial perovskite-phase PFN thin films with no identifiable trace of impurity phases appearing in the X-ray diffractograms. PFN films having thicknesses between 20 and 200 nm were smooth and epitaxially oriented with the substrate and as demonstrated by RHEED streaks which were aligned with the substrate axes. X-ray diffraction showed that the films were completely c-axis oriented and of excellent crystalline quality with low mosaicity (X-ray rocking curve FWHM<0.09). The surface roughness of thin films was also investigated by atomic force microscopy. The root-mean-square roughness varies between 0.9 nm for 50-nm-thick films to 16 nm for 100-nm-thick films. We also observe a correlation between grain size, surface roughness and film thickness.Comment: 13 Pages, 6 figures. To be published in J. Mag. Mag Mater. proceedings of EMRS200

Interplay between proton ordering and ferroelectric polarization in H-bonded KDP-type crystals

The origin of ferroelectricity in KH_2PO_4 (KDP) is studied by first-principles electronic structure calculations. In the low-temperature phase, the collective off-center ordering of the protons is accompanied by an electronic charge delocalization from the "near" and localization at the "far" oxygen within the O-H...O bonds. Electrostatic forces, then, push the K+ ions towards off-center positions, and induce a macroscopic polarization. The analysis of the correlation between different geometrical and electronic quantities, in connection with experimental data, supports the idea that the role of tunnelling in isotopic effects is irrelevant. Instead, geometrical quantum effects appear to play a central role.Comment: 8 pages, 2 postscript figures, submitted to the X Conference on Computational Materials Science, Villasimius, Sardinia (Italy), 200

Size and doping effects on the coercive field of ferroelectric nanoparticles

A microscopic model for describing ferroelectric nanoparticles is proposed which allows us to calculate the polarization as a function of an external electric field, the temperature, the defect concentration and the particle size. The interaction of the constituents of the material, arranged in layers, depends on both the coupling strength at the surface and that of defect shells in addition to the bulk values. The analysis is based on an Ising model in a transverse field, modified in such a manner to study the influence of size and doping effects on the hysteresis loop of the nanoparticles. Using a Green function technique in real space we find the coercive field, the remanent polarization and the critical temperature which differ significantly from the bulk behavior. Depending on the varying coupling strength due to the kind of doping ions and the surface configuration, the coercive field and the remanent polarization can either increase or decrease in comparison to the bulk behavior. The theoretical results are compared with a variety of different experimental data.Comment: 16 pages, 7 figure