利用模拟电感改进串联谐振演示实验

针对串联谐振演示实验中常遇到的几个问题，给出了获得模拟可调电感的方法，改善了演示实验的效果。     

Dielectric properties of ferroelectric diisopropylammonium iodide

Temperature dependences of dielectric constant, amplitude of the third harmonic and heat capacity for the organic ferroelectric of diisopropylammonium iodide (C₆H₁₆NI) have been investigated. The measurements were carried out through heating and cooling cycles in the range of 300–400?K. It was found that upon the first heating, only one phase transition occurred without the presence of the ferroelectric phase. For samples preheated over 420?K, two phase transitions at 363 and 378?K appeared in the heating process, and the ferroelectric state was also observed between them. Upon cooling, the ferroelectric phase was detected in the range of lower 361?K and persisted up to room temperature.     

Correlation of acoustic and thermal anomalies in ferroelectric crystals

The Pippard-Janovec relations are derived for correlating the anomalous elastic coefficient and the anomalous specific heat near the phase transitions of ferroelectric crystals. These relations are verified in the case of ferroelectric triglycine selenate crystal.     

直流电路虚拟实验平台的研究—自组电势差计实验

以“自组电势差计实验”为对象,借助计算机图形学,虚拟现实和计算机仿真等新技术,进行了具有逼真视觉效果与精确实验结果的计算机虚拟实验平台系统的研究与尝试,它可以使学生仿真地在计算机屏幕上完成实验的全部过程,可达到学生在实验前的预习目的。     

Realization of fractional-order Liu chaotic system by circuit

In this paper, chaotic behaviours in the fractional-order Liu system are studied. Based on the approximation theory of fractional-order operator, circuits are designed to simulate the fractional- order Liu system with $q=0.1-0.9$ in a step of 0.1, and an experiment has demonstrated the 2.7-order Liu system. The simulation results prove that the chaos exists indeed in the fractional-order Liu system with an order as low as 0.3. The experimental results prove that the fractional-order chaotic system can be realized by using hardware devices, which lays the foundation for its practical applications.     

Tunneling magnetoresistance and electroresistance properties of composite‐barrier ferroelectric tunnel junctions

The multiferroic properties of ferroelectric tunnel junctions with a composite barrier comprising a fully epitaxial La_0.7Sr_0.3MnO₃/BaTiO₃/SrTiO₃/La_0.7Sr_0.3MnO₃ heterostructure are reported in this study. The patterned junctions having extended top electrodes show tunnel magnetoresistance ratios ranging from 20% to 110% at 77 K. Furthermore, tunneling electroresistance – induced by ferroelectric polarization switching and showing two‐state memory effect in the dynamic resistance – has also been observed in these junctions. Thus, with the concurrence of tunneling electroresistance and magnetoresistance, these tunnel junctions serve as memory devices with four resistance states. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Leakage currents in ferroelectric thin films

The main mechanisms of leakage currents in thin lead zirconate titanate (PZT) ferroelectric films prepared by the sol–gel method are discussed. Four specific regions are determined in I–V dependencies. At very weak fields (10–20 kV/cm), the current falls with the voltage increase as a result of depolarization. In the low fields region (about 70–100 kV/cm), the leakage current decreases with the decrease of voltage ramp speed and its components are the ohmic and displacement currents. In the high fields region (≥130 kV/cm), the leakage current increases with the decrease of step voltage ramp in contrast to the previous case. Possible conductivity mechanisms are the Poole– Frenkel emission and hopping conduction. In the transition region between above-mentioned ones (from 80–90 to ～130 kV/cm), an abrupt unstable increase of current is observed caused by breakdown of reverse bias Schottky barrier. Depolarization currents are studied for sol–gel PZT films prepared at different preparation conditions.     

四方铁电体PbFe0.5Nb0.5O3精细结构的第一性原理研究

采用全电势线性缀加平面波法(FPLAPW),计算了PbFe0.5Nb0.5O3的总能量,从而确定了它的四方铁电稳态结构,即B位离子(Fe和Nb)在四方铁电相的平衡构型.计算结果表明,Fe相对于氧八面体沿[001]方向的位移为0.022nm,而Nb的偏心位移为0.0020nm,其中铁电非稳性主要归因于前者 关键词： 铁电体 密度泛函理论 平面波法     

Realization of fractional-order Liu chaotic system by a new circuit unit

A new circuit unit for the analysis and the synthesis of the chaotic behaviours in a fractional-order Liu system is proposed in this paper. Based on the approximation theory of fractional-order operator, an electronic circuit is designed to describe the dynamic behaviours of the fractional-order Liu system with α = 0.9. The results between simulation and experiment are in good agreement with each other, thereby proving that the chaos exists indeed in the fractional-order Liu system.     

Size and interface effects on Curie temperature of perovskite ferroelectric nanosolids

A simple and unified model, without any adjustable parameter, is established for size effect on Curie temperature of low-dimensional ferroelectrics (thin films, nanowires and nanoparticles), T _c(D), where D denotes size of low-dimensional ferroelectrics. T _c(D) function is based on consideration on the size dependence of spontaneous polarization of low-dimensional ferroelectrics P _s(D), which is determined by the misfit strain at the ferroelectrics/substrate interface. It is shown that P _s(D) and T _c(D) functions decrease or increase when the misfit strain is tensile or compressive. The numerically predicted results are in agreement with the available experimental results of BaTiO₃ and PbTiO₃ nanoparticles and thin films.     

Asymmetric reversible diode-like resistive switching behaviors in ferroelectric BaTiO3 thin films

In this work, the resistive switching behaviors of ferroelectrictric BaTiO3/La0.67Sr0.33MnO3 .heterostructures de- posited by pulsed laser deposition are investigated. The BaTiO3 films show both well-established P-E hysteresis loops, and asymmetric reversible diode-like resistive switching behaviors, involving no forming process. It is found that both the ON/OFF ratio and the stability of resistive switching are substantially dependent on operation voltage （Vmax）. At a Vmax of 15 V, a large ON/OFF resistance ratio above 1000 is obtained at a Vmax of 15 V, which is able to maintain stability up to 70-switching cycles. The above resistive switching behaviors can be understood by modulating interface Schottky barriers as demonstrated by I-V curve fitting.     

Glassy properties of the relaxor ferroelectric strontium barium niobate at low temperatures

Relaxor ferroelectrics are known to show a low-temperature behaviour that quantitatively resembles the features of canonical glasses. In order to get a comprehensive characterisation of the responsible low-energy excitations, single crystals of strontium barium niobate were investigated below 10 K. The density of localised states was obtained from heat capacity, and their coupling to phonons from sound velocity and thermal conductivity. The presence of slowly relaxing systems could be estimated from heat release data, respectively.

In contrast with structural glasses, however, SBN and further relaxor ferroelectrics show an energy dependent density of states and an upper limit of the relaxation time spectrum. Considering these properties, which appear to be typical also for other glassy crystals, a modification of the standard tunneling model is proposed.     

A fractional order hyperchaotic system derived from Liu system and its circuit realization

In this paper we propose a novel four-dimensional fractional order hyperchaotic system derived from Liu system. Electronics workbench (EWB) and Matlab simulations show the dynamical behavior of the proposed four-dimensional fractional order hyperchaotic system. Finally, after separately using EWB and Matlab, an electronic circuit is designed to realize the novel four-dimensional fractional order hyperchaotic system and the experimental circuit results are obtained which are identical to software simulations.     

Diffuse ferroelectric phase transitions in cubically sabilized perovskites

Typical examples of ferroelectrics with diffuse phase transitions (relaxor ferroelectrics), like Pb(Mg_1/3Nb_2/3)O₃ are actually non transforming. The paraelectric phase is fully stablized against a ferroelectric phase transition in this case. A phase transition can be induced, however, by an electric field with appropriate orientation below the temperature of the dielectric constant maximum. The analogy with stress-induced martensitic phase transitions in metallic alloys is pointed out. Pecularities of the properties and of the polarization reversal of such systems are demonstrated. Actual diffuse ferroelectric phase transitions in disordered solid solutions and mixed compounds with a partially stabilized parent phase are compared with athermal martensitic transformations. With particular regard to technical ceramics based on PZT, the influence of interfaces between transformed regions and remnants of the parent phase which have to distinguished from domain walls, and of the reduced stability of the ferroelectric phase on the properties of these systems is discussed. Some effects usually explained solely by domain processes may be understood also from this point of view.

Stabilization of a parent phase against an order-disorder-type phase transition is supposed to be caused by glass-like freezing caused by inelastic cooperative interactions between disordered molecular groups.     

Size dependence of phase transition temperatures of ferromagnetic ,ferroelectric and superconductive nanocrystals

With the miniaturization of devices, size and interface effects become increasingly important for the properties and performances of nanomaterials. Here, we present a thermodynamic approach to the mechanism behind size-induced unusual behavior in the phase stabilities of ferromagnetic (FM), antiferromagnetic (AFM), ferroelectric (FE), and superconductive (SC) nanocrystals, which are different dramatically from their bulk counterparts. This method is based on the Lindemann criterion for melting, Mott’s expression for the vibrational melting entropy, and the Shi model for the size-dependent melting temperature. Simple and unified functions, without any adjustable parameter, are established for the size and interface dependences of thermal and phase stabilities of FM, AFM, FE and SC nanocrystals. According to these analytic functions, as the size of nanocrystals is reduced, the thermal and phase stabilities may strengthen or weaken, depending on the confluence of the surface/volume ratio of nanocrystals and the FM(AFM, FE or SC)/substrate interface situations. The validity of this model is confirmed by a large number of experimental results. This theory will be significant for the choice of materials and the design of devices for practical application.      

A fractional order hyperchaotic system derived from a Liu system and its circuit realization

In this paper we propose a novel four-dimensional fractional order hyperchaotic system derived from a Liu system.Electronics workbench(EWB) and Matlab simulations show the dynamical behavior of the proposed four-dimensional fractional order hyperchaotic system.Finally,after separately using EWB and Matlab,an electronic circuit is designed to realize the novel four-dimensional fractional order hyperchaotic system and the experimental circuit results are obtained which are identical to software simulations.     

Stochastic resonance in chaotic systems

The phenomenon of stochastic resonance (SR) is investigated for chaotic systems perturbed by white noise and a harmonic force. The bistable discrete map and the Lorenz system are considered as models. It is shown that SR in chaotic systems can be realized via both parameter variation (in the absence of noise) and by variation of the noise intensity with fixed values of the other parameters.     

Linear and nonlinear dielectric properties of nanocomposites based on the organic ferroelectric of diisopropylammonium bromide

ABSTRACT

The present work aims at investigating linear and nonlinear dielectric properties of nanocomposites based on diisopropylammonium bromide (C₆H₁₆NBr, DIPAB) embedded into Al₂O₃ films having honeycomb structure with pores of 100?nm in diameter, and into opal matrices with three-dimensional structure containing pores of 100 and 60?nm. The obtained results indicated the shift of Curie point toward lower temperatures for DIPAB in porous aluminum oxide and the appearance of two phase transitions, detected upon heating and cooling for DIPAB in opal matrices. In addition, a ferroelectric phase was found to form between these two phase transitions without significant change of Curie temperature.     

Time domain response and the data analysis of ferroelectric discharge current

The time domain method is more reliable for the study of nonlinear dielectric response compared with frequency domain analysis. A Tikhonov regularization method, which is widely adopted for ill-posed problem, is described for derivation of the relaxation time distribution function, g(τ), from the ferroelectric discharge current in time domain. The new method allows study of the structure variation and the relaxation behavior of ferroelectrics at different temperatures. For barium stannate titanate ceramics (BTS20), g(τ) has been successfully derived; the relaxation peaks move to shorter times with increasing temperature in the range 20–60°C, which may indicate a space charge thermal activation process. However, g(τ) could not be derived from the discharge current by the regularization method for BTS20 at temperatures above 60°C or for lanthanum-doped lead zirconium titanate transparent ceramic (PLZT), since the data do not satisfy the discrete Picard condition, which is a valid criterion for regularization method.