George F. Tuthill

Electric-field effects of dielectric and optical properties in Pb(Mg1∕3Nb2∕3)0.65Ti0.35O3 crystal

Dielectric properties and domain structure have been measured as functions of temperature in a (001)-oriented Pb(Mg1∕3Nb2∕3)0.65Ti0.35O3 (PMNT35%) single crystal with and without a prior dc electric (E)-field poling. Without E-field poling, the dielectric loss exhibits a frequency-dependent maximum in the region of 120–180K, which can be described by a Vogel–Fulcher equation and fractal cluster model which implies structural irregularities within domains. With a prior poling a long-range monoclinic (tetragonal)→tetragonal (monoclinic) transition takes place near 212K upon heating. “Monoclinic (tetragonal)” represents that dominant monoclinic phase domains coexist with a small fraction of tetragonal phase domains. Optical transmission and birefringence were significantly enhanced by a prior E-field poling. The Cauchy equations for ordinary no and extraordinary ne refractive indices were determined between 0.45 and 1.4μm. However, the phase-matching criterion for second-harmonic generation was not found.

Conductivity across random barrier distribution as origin of large low-frequency dielectric peak in perovskite crystals and ceramics

Abstract Several perovskite crystals and ceramics show very large dielectric (ϵ′) peaks at high temperature T and low frequency ƒ . In some cases these peaks are in the cubic phase far above any ferroelectric transition. Even at the peaks, the lossy part ϵ″ is larger than the real part ϵ′. The ϵ′ vs T curves for different ƒ follow the same d.c. (low-ƒ) envelope down to some T(ƒ) below which the curve for that ƒ falls below the envelope. Similarly, the conductivity (or ϵ″) data show d.c. and a.c. (high-frequency) envelopes for which data at different ƒ overlap. As a first approximation to a crystal with random barriers impeding conductivity, a model with barriers B (in T units) every lattice constant a = 4 A and barriers B + Δ every distance d is assumed. The model is fit to permittivity and conductivity data for a strontium titanate single crystal, and a good qualitative fit is obtained.

Acoustic anomalies at phase transformation to quasi-2D proton glass state in Cs5H3(SO4)4 × xH2O crystal

The paper describes Brillouin light scattering studies of longitudinal acoustic (LA) phonons in Cs5H3(SO4)4 × xH2O (PCHS) crystals at temperatures from 100 K to 360 K. The acoustic response of the crystal at different frequencies is analysed in detail. It is shown that both the velocity and damping of sound exhibit a strong dispersion caused by relaxation processes in the region of transformation into the glass-like phase (Tg ≈ 260 K). A strong anisotropy in the acoustic response, attributable to the quasi-two dimensional (quasi-2D) structure of PCHS, is revealed. The LA phonon damping is calculated in the framework of a number of relaxation models. It is shown that, in the vicinity of Tg, anomalies in ultrasonic damping of the LA phonons propagating in the basal plane reflect the cooperative effect of freezing of acid protons. At the same time, the anomaly in damping of the LA phonon propagating perpendicular to the basal plane is described in terms of the Debye model and is due to the interaction between protons on hydrogen bonds and LA phonons. This suggests that the proton glass state realized at T< T g is of quasi-2D nature.

Director configurations of a nematic microdroplet in a ferroelectric medium

Abstract Micron-scale droplets of nematic liquid crystal, embedded in a solid polymer matrix, are the active conponents of polymer dispersed liquid crystal (PDLC) shutters. An electric field is used to orient the director fields in PDLC droplets and thereby control the transmission of light through the film. We have calculated the director configurations of PDLC droplets dispersed in a ferroelectric matrix, which could be used to stabilize the oriented state. Our approach is based on the Landau-de Gennes model free energy and determines the stable configuration by numerical solution of the Euler-Lagrange equations for the director field. Unlike earlier work on this subject, we take into explicit account the effects of spatial variations in the electric field that arise due to the liquid crystals anisotropic polarizability. We find that these inhomogeneities are significant in determining the stable director configuration, and predict a greater range of stability for the radial phase than that obtained us...

Tricritical Behavior of an Ising Antiferromagnet with Next-Nearest Neighbor Ferromagnetic Interaction

Using high temperature series expansions, we have studied an Ising model with nearest-neighbor antiferromagnetic interaction ( J 1 0), which is thought to exhibit a tricritical point (TCP). Assuming that tricritical exponents are mean-field like, we locate TCPs for several values of the exchange coupling parameter J 2 / J 1 by the method of log-Pade analysis.