Study of the formation processes of a domain nanostructure in relaxor ferroelectrics

We present the results of an investigation into processes of formation of polar nanoregions and the relaxation dynamics of a cubic relaxor ferroelectric (PbMg_1/3Nb_2/3O₃)_0.9-(pbTiO₃)_0.1 (PMNPT10), selected to be studied as a model compound.     

Lattice-scale domain wall dynamics in ferroelectrics

Ferroelectric domain walls are atomically thin, and consequently their dynamics are sensitive to the periodic potential of the underlying lattice. Despite their central role in domain dynamics, lattice-scale effects have never been directly observed. We investigate local domain dynamics in thin film ferroelectrics using atomic-force microscopy. Upon combined dc and ac electric driving, fluctuations in the local piezoresponse are observed. Fourier analysis of the fluctuations reveals the presence of narrow band and broad band noise, and Barkhausen jumps. The narrow band noise is attributed to dynamics associated with lattice-scale pinning and is reproduced by a simple physical model.     

铁电体中新畴成核经典模型的改进

新畴成核是外加低场下铁电体中铁电畴反转的一个重要的过程.首先介绍了新畴成核的经典模型,采用该模型研究了铁电畴反转的新畴成核过程,发现理论计算的成核速率与外场关系和实验观测结果不一致.在Tagantsev模型的基础上,选取不同的成核形状对新畴成核的经典模型进行了改进,并获得了和实验观测相符的理论计算结果. 关键词： 铁电体 铁电畴 成核速率     

Surface electromagnetic waves on the domain boundaries in ferroelectrics

It is argued that low-frequency surface electromagnetic waves can propagate along the domain boundary in ferroelectrics. These waves cause the oscillatory motion of the boundary.     

On the helical domain structure of the liquid ferroelectrics

The case when a nematic liquid crystalline phase is ferroelectric is considered. Such “liquid ferroelectric” is, surely, instable and transforms into a helical cholesteric-type structure which for this case is a variety of the ferroeltric domain structure. One can think that some cholesteric phases are liquid ferroelectrics with the helical domain structure.     

On the thickness of domain walls in ferroelectrics and ferroelastics

Ferroelastic and ferroelectric domain walls are commonly described by wall profiles of the tanh(x/w)-type. We argue that this profile is still a good approximation if higher-order gradient energies are considered. Such energies are relevant for phase transitions close to structural incommensurations and also for phase transitions with dominant elastic interactions. Their effect on the wall profile is to influence the effective wall thickness. Positive gradient energies tend to widen domain walls beyond the values predicted in classic Landau-Ginzburg theory.     

Dynamics of the domain structure in ferroelectrics with an incommensurate phase

The dynamics of the domain structure of ferroelectrics with an incommensurate phase (like sodium nitrite) is considered. The polarization properties of these ferroelectrics are described using the pseudospin formalism. The equation for the polarization is numerically solved together with the equation for acoustic waves. The obtained results are analyzed for various values of the parameters of the crystal and different initial conditions. It is shown that, after a sufficiently large time, the domain structure of the crystal becomes localized.     

Investigation into the domain structure dynamics in ferroelectrics with an incommensurate phase

The domain structure dynamics in ferroelectrics with an incommensurate phase is studied. Sodium nitrite is considered to be a prominent representative of the above ferroelectrics. The polarization properties of these ferroelectrics are described using pseudospin formalism. A kinetic equation for describing polarization of ferroelectrics is derived using the Hamiltonian. This equation is numerically solved simultaneously with the sound-vibration equation. The results are analyzed for various crystal parameters and initial conditions. The crystal domain structure is shown to be localized for reasonably long times. This circumstance makes it possible to conclude that these domain-structure states are long-lived states of a soliton type. The importance of finding and describing these states is emphasized. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 66–70, August, 2006.     

Electromechanical cracking in ferroelectrics driven by large scale domain switching

Experimental results indicate three regimes for cracking in a ferroelectric double cantilever beam (DCB) under combined electromechanical loading. In the loading, the maximum amplitude of the applied electric field reaches almost twice the coercive field of ferroelectrics. Thus, the model of small scale domain switching is not applicable any more, which is dictated only by the singular term of the crack tip field. In the DCB test, a large or global scale domain switching takes place instead, which is driven...     

Estimation of SEM imaging resolution of the domain structure of pyroprobe-heated ferroelectrics

The specific contrast of scanning electron microscope (SEM) images of ferroelectric domains observed in the pyrocurrent (pyroprobe) mode is analyzed. Calculations taking into account both the nonuniform heating of domains and the heat diffusion via the domain walls and the crystal boundaries are performed. It is established that the heat diffusion smears the domain images of small domains. Along with the probe diameter, the scan rate is shown to be an important factor determining the character of the SEM image contrast. A decrease in the scan rate may appreciably lower the resolution even in case of a fairly thin probe.     

Zurek-Kibble domain structures: the dynamics of spontaneous vortex formation in annular Josephson tunnel junctions

Phase transitions create a domain structure with defects, which has been argued by Zurek and Kibble (ZK) to depend in a characteristic way on the quench rate. We present an experiment to measure the ZK scaling exponent sigma. Using long symmetric Josephson tunnel junctions, for which the predicted index is sigma=0.25, we find sigma=0.27+/-0.05. Further, we agree with the ZK prediction for the overall normalization.     

Simulation on dielectric susceptibility and domain evolution of relaxor ferroelectrics

Dielectric susceptibility and domain evolution of the relaxor ferroelectrics have been simulated using the Monte Carlo method upon the Potts-Ising model. The grain size effect and the applied ac field frequency effect on the dielectric susceptibility were theoretically investigated. We found that the dielectric susceptibility increases and the T_m (the temperature at which the dielectric susceptibility reaches the maximum) shifts to lower temperature with increasing average grain size or decreasing frequency. In addition, we obtained the value of the relaxation parameter γ estimated from the linear fit of the modified Curie-Weiss law; its changing trend with increasing average grain size or increasing frequency was well consistent with the experimental observation. From the results of the domain pattern evolution process, we observed the differences between relaxor ferroelectrics and normal ferroelectrics subjected to an applied ac field.     

Unusual domain evolution in semiconducting ferroelectrics: A phase field study

The effect of electrical conductivity on the domain evolution of semiconducting ferroelectrics is investigated using a phase field model which includes the drift of space charges. Phase field simulations show that the tail-to-tail 90° charged domain wall appears during the domain formation in the semiconducting ferroelectrics at zero field, which is prohibited in common insulating ferroelectrics. Due to the screening of polarization charges, the domain switching takes place through the motion of head-to-head 180° charged domain wall in the semiconducting single-domain ferroelectrics subjected to an electric field. Comparing to the insulating ferroelectrics, the semiconducting ferroelectrics have a lower speed of domain evolution due to the decrease of mobility of charged domain walls. The response of semiconducting ferroelectrics to a mechanical load is also found different from that of insulating ferroelectrics.     

Anti-phase domain structuring in oxide ferroelectrics: Physical properties and application

The results from studies of domain borders by the optical and acoustic topography methods in LiNbO₃:Fe samples are presented. The formation of Fe ion concentration gradients, a change in the refractive index, and a gradient of the photoinduced field and parameters of acoustic waves near the domain borders were observed. It is shown that main reason for the distortion of crystalline structure parameters is the formation of photoinductive field gradients of spatially separated Fe₂₊ and Fe₃₊ ions.     

Double random-phase encoding in the Fresnel domain

A lensless optical security system based on double random-phase encoding in the Fresnel domain is proposed. This technique can encrypt a primary image to random noise by use of two statistically independent random-phase masks in the input and transform planes, respectively. In this system the positions of the significant planes and the operation wavelength, as well as the phase codes, are used as keys to encrypt and recover the primary image. Therefore higher security is achieved. The sensitivity of the decrypted image to shifting along the propagation direction and to the wavelength are also investigated.     

在空间-时间域测量飞秒脉冲

依据飞秒脉冲测量中的空间-时间耦合理论,通过采用空间分辨和光谱分辨在时间和空间域对飞秒脉冲强度和相位进行测量.测量装置主要由两个光学元件组成,实现了空间和光谱干涉.待测脉冲和参考脉冲通过一个绕中心轴转过某一角度的二维衍射光栅,产生倾斜的二维阵列全息图;一个带通滤波片对全息图进行分离,从而每个全息图含有一个唯一的频率.用照相机记录下所有的全息图,可以在时间和空间域再现出待测脉冲,并给出了测量结果. 关键词： 飞秒脉冲 时间-空间耦合 二维衍射光栅 带通滤光片     

Effect of impurity iron ions on the formation of photoinduced lattices and domain structures in lithium niobate

Processes and mechanisms of the formation of lattices and periodic domain structures in iron-doped lithium niobate monocrystals under the effect of laser beams are examined. The role of Jahn-Teller Fe²⁺ ions in the formation of photoinduced periodic structures is demonstrated.