Anomalous electron emission spectra and polarization phenomena in a lead magnesium niobate single crystal

The time dependences of the intensity and the energy positions of fine-structure of the spectra of anomalous electron emission from a polarized lead magnesium niobate (PMN) single crystal irradiated with soft x-rays are investigated experimentally. It is shown that the relaxation time of the electret charge can be determined from the graphs of these dependences, and the depth distribution of the potential in the surface layer of the sample can be determined from the profile of the anomalous electron emission spectrum. Estimates of the fields in the surface layers of a polarized PMN single crystal, obtained from the characteristics of the anomalous electron emission spectra, agree with data obtained by electrophysical research techniques. Fiz. Tverd. Tela (St. Petersburg) 39, 1446–1451 (August 1997)     

Analysis and comparison of four anhysteretic polarization models for lead magnesium niobate

Four anhysteretic polarization models that have been used in the literature to evaluate data acquired from lead magnesium niobate (PMN) are analyzed and compared. Derivations of two of the models from assumed spatial distributions of dipole energy states, using first physical principles, are presented. A third model is derived from a suitable integral averaging calculation. These derivations are used as the basis for developing an integral equation for determining an energy-state distribution that produces a fourth model, which was not originally formulated in terms of an assumed distribution. A new polarization function is also presented. Excellent approximations to each of the four polarization functions of interest can be deduced from this new polarization function by adjusting the numerical value of just a single parameter. An application of two of the models to data is presented. It is shown that it can be necessary to consider a sample to be an admixture of two distinct species of poles, in the sense that two polarization functions must be added together in order to accommodate the data.     

Electroluminescence study of polarization changes in lead magnoniobate crystals in pulsed electric fields

Electroluminescence is used to study changes in the polarization of crystals of a model ferroelectric relaxor, lead magnoniobate (PMN), in pulsed electric fields. The amplitude of the electoluminescence pulses produced during polarization and depolarization of PMN crystals is found to depend on the duration of the applied electric field pulses if this duration is shorter than the most probable time for polarization buildup. These data provide evidence of rapid changes in the polarization through realignment of the domain and heterophase structure at temperatures above the temperature for destruction of the induced macrodomain ferroelectric phase and evidence of an “excited” polarization state for short-lived pulses whose decay is accompanied by an enhancement in the amplitude of the depolarizing luminescence pulse and by a reduction in the time delay of its emission following the end of the field pulse. Fiz. Tverd. Tela (St. Petersburg) 39, 341–343 (February 1997)     

Light-induced absorption of cerium-doped lead barium niobate crystals

Tetragonal lead barium niobate (Pb(1-x)Ba(x)Nb(2)O(6);1-x<0.63) crystals are promising photorefractive materials for applications such as holographic data storage. Measurements of light-induced absorption in Ce-doped lead barium niobate (Pb(0.5)Ba(0.5) Nb(2)O(6)) were performed. The induced absorption that was due to the presence of shallow traps was as large as 0.3cm(-1) at 633 nm and exhibited saturation behavior at green-pump intensity greater than 2W/cm(2) . The temporal profile of the dark decay was a double exponential, and the activation energies of the slow- and the fast-decaying components were determined to be 0.16 and 0.3 eV, respectively.     

Dielectric relaxation phenomena in lead titanate crystals

Relaxation properties of crystalline lead titanate are interpreted in terms of Maxwell-Wagner polarization.     

Effect of electric field on the percolation phase transition in lead scandotantalate single crystals

The effect of a dc electric field on the phase transition process in lead scandotantalate single crystals differing in the degree of ion ordering has been studied by small-angle light scattering (SAS). The spontaneous phase transition occurring in these crystals is shown to be accompanied by a sharp SAS intensity peak indicating the percolation nature of this transition. A phase diagram in the field-temperature coordinates has been constructed for all the PST crystals studied in the work. The electric field and the temperature variation rate have been found to affect the SAS intensity.     

Photoinduced decrease in local polarization switching fields in lithium niobate crystals

Variations in local polarization switching fields E _C in lithium niobate crystals have been investigated under laser irradiation of the z surface. It has been established that the simultaneous application of an electric field in the direction opposite to the spontaneous polarization field and laser beam leads to a decrease in the field E _C .     

Hysteresis phenomena in charging of Si Mosfet in quantizing magnetic field

We report the observation of nonequilibrium processes of charging in 2D electron layer in Si MOSFET. There were investigated the hysteresis variations of a 2D-layer charge Q_S with a gate voltage V_g or with a magnetic field H, and the hysteresis of a gate potential U_g vs magnetic field at Q_S = Const. At sufficiently low temperature T < 1K the relaxation time of a nonequilibrium state is much higher than 10³ sec. The observed phenomena may be explained qualitatively in frame of a surface potential randomness conception.     

Finite-element modeling of lead magnesium niobate electrostrictive materials: dynamic analysis

A finite-element model is proposed for the time-domain analysis of electrostrictive materials. Hom's material model, developed for lead magnesium niobate (PMN) ceramics, is used. It includes the quadratic dependence of strain with polarization, the saturation of polarization, assumes constant temperature, and excludes hysteresis. The theoretical formulation is justified by the principle of virtual works. The numerical model is obtained after discretization in space and time. The validation is performed by comparing numerical results with semianalytical results for an electrostrictive spherical shell subjected to a step in voltage or in charge. From these results, a method to compute the coupling coefficient of electrostrictive materials, based on Ikeda's definition, is proposed and applied to a bar with parallel electric field.     

Pure-phase two-dimensional niobium-93 nutation spectroscopic study of lead metaniobate and the piezoelectric lead magnesium niobate

An NMR technique to measure pure-phase two-dimensional nutation NMR spectra, that yields higher resolution than traditional nutation experiments is reported. Using this technique 93Nb nutation NMR spectra of PbNb2O6 and the technologically important Pb(Mg1/3Nb2/3)O3 (PMN) have been measured and the quadrupolar coupling constant of the niobium site in PbNb2O6 (C(Q) = 19 +/- 2 MHz) determined. Estimates of the quadrupolar coupling constants for three different resonances associated with different niobium(V) sites in PMN (C(Q) < 1.2 MHz, approximately 17 MHz, and > 62 MHz) are also reported.     

Reversible polarization of ferroelectric ceramics in a rapid-growing electric field

The complex permittivity of ferroelectric PZT ceramics with different compositions was measured in a pulsed rapid-growing electric field. The time dependence of the real component exhibits three peaks. The dips between them correlate with imaginary component peaks. Domain mechanisms that contribute to the ferroelectric polarization and are responsible for the time dependences are considered.     

Raman spectra of lithium niobate crystals heavily doped with zinc and magnesium

We have examined the Raman spectra of heavily doped lithium niobate single crystals (at close-to-threshold concentrations of doping cations): LiNbO₃:Zn (4.5 mol % ZnO), LiNbO₃:Mg (5.01 mol %):Fe (0.005 mol %), LiNbO₃:Mg (5.1 mol %), and LiNbO₃:Mg (5.3 mol % MgO). Low-intensity lines with frequencies at 209, 230, 298, 694, and 880 cm^–1 have been revealed for the first time. Analysis of the data from the literature on lattice dynamics calculations from first principles (ab initio) does not make it possible to unambiguously state that these lines correspond to fundamental vibrations of the А ₂ symmetry species, which are forbidden for the С _3V ⁶ (R3c) space group. At the same time, ab initio calculations unambiguously indicate that the experimentally observed low-intensity “superfluous” lines with the frequencies at 104 and 119 cm^–1 cannot correspond to vibrations of the А ₂ symmetry species. It is most likely that they correspond to two-particle states of acoustic phonons with a total wave vector equal to zero.     

Decomposition of silver azide crystals in a constant electric field

New experimental results of investigations of the physicochemical processes initiated in filamentary crystals of silver azide by variously configured electric fields are presented and discussed. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 53–56, October, 2006.     

Integrated optical electric field sensor based on a Bragg grating in lithium niobate

We demonstrate a new sensor concept for the measurement of oscillating electric fields that is based on Bragg gratings in LiNbO₃:Ti channel waveguides. This miniaturized sensor that works in a retroreflective scheme does not require metallic electrodes and can be directly immersed in an oscillating electric field. The electric field induces a shift of the Bragg wavelength of the reflection grating that is due to the electro-optic effect. The operating point of the sensor is chosen by adjusting the laser wavelength to the slope of the spectral reflectivity function of the grating. In this way the magnitude of an external electric field is measured precisely as the amplitude of modulated reflected light intensity by using a lock-in amplifier. The sensor principle is demonstrated by detecting low-frequency electric fields ranging from 50 V/cm to 5 kV/cm without any conducting parts of the sensor head. Furthermore, the ability of the sensor to determine the three-dimensional orientation of an external electric field by a single rotation along the waveguide direction is demonstrated. PACS 42.40.Eq; 42.82.-m; 42.82.Et     

Electrooptic properties of lithium niobate crystals for extremely high external electric fields

The electrooptic effect in lithium niobate crystals (LiNbO₃) for extremely high externally applied electric fields of up to 65 kV/mm is investigated. Homogeneous electrooptic refractive-index changes of up to 4.8×10^-3 are found for ordinarily polarized light. No quadratic electrooptic effect is observed. An upper bound for the quadratic electrooptic coefficient of |s₁₃|≤2.3×10^-21 m²/V² is determined. Electrooptic, angular, and wavelength tuning of the Bragg condition of a thermally fixed hologram are demonstrated. Received: 29 October 2002 / Revised version: 14 January 2003 / Published online: 26 March 2003 RID="*" ID="*"Corresponding author. E-mail: ml@uni-bonn.de     

Incommensurate phases in lead potassium niobate and lead sodium niobate,homologues of barium sodium niobate.

Electron diffraction and high resolution electron microscopy reveal the presence of incommensurate structures in lead potassium niobate, lead sodium niobate and in mixed crystals. The incommensurability can be described either as being due to the presence of quasi-periodic “diffuse” anti-phase boundaries or as being due to the presence of deformation waves, the deformation mode being shearing of the NbO₆ octahedra. The incommensurability depends on composition as well as on temperature.     

Hysteresis phenomena of the parametric oscillations

Parametric amplifiers have been described which are based on the magnetoelastic interaction of the oscillation of a torsional pendulum and an electrical circuit. When the inverting amplifier is driven overcritically, selfexcitation of the pendulum occurs. Because of the nonlinear magnetoelastic damping, it may happen that the pendulum oscillates only in a certain range of amplitudes. Varying the intensity of the magnetic pumping field, hysteresis effects of the oscillation appear which can be explained by damping measurements. Oscillation measurements of other authors can now be understood.The author thanks Prof. Dr. G.Dietz for many valuable remarks to this paper and Prof. Dr. H.Happ in whose institute the experiments were performed.     

Excitons in ZnP2 crystals in the electric field of a Schottky barrier

A study is made of the effect of electric fields on the exciton states of β-ZnP₂ crystals (T=77 K) in structures with Schottky barriers formed by depositing semitransparent electrically-conducting InSnO₂ films on the crystal surface. The observed changes in the exciton optical reflection spectra when an electrical potential is applied to a barrier are explained by the shift and broadening of the exciton level caused by the Stark effect. The experimental data are compared with calculations based on a theory of exciton optical reflection from planar spatially nonuniform structures. Fiz. Tverd. Tela (St. Petersburg) 40, 884–886 (May 1998)     

Investigation of thermal evolution of nanodomain structures in nonlinear barium sodium niobate crystals

By the 90°elastic light scattering investigation and far field observation in the range of 20-800℃,the relation between behavior of light scattering anomalies and evolution of nanodomain structures in lattice of barium sodium niobate(Ba2NaNb5O15,BSN)crystal was clarified.The correlation between anomalies on the temperature curves of the elastic light scattering intensity and temperature transformations of nanodomains was studied by X-ray and electron microscope methods.Phase transition near 500℃ and movement in field of scattering light could be explained by appearance of a new incommensurate phase.