铁电极化子动力学理论

在铁电屏蔽理论的基础上发展铁电极化子动力学理论,用来解释铁电体的极化反转现象.理论结果与TGS单晶的实验结果符合得很好.由该理论还可进一步用来研究铁电发射的基本物理过程 关键词： 铁电极化子 极化反转 慢极化效应     

挠曲电效应对向列相液晶盒电光效应的影响

研究挠曲电效应对向列相液晶盒电光效应的影响.通过变分理论得到液晶指向矢满足的微分方程和边界条件,利用差分迭代的方法计算强锚定和弱锚定液晶盒中液晶指向矢的分布情况.借助琼斯矩阵的方法,计算两种液晶盒电光特性曲线.此外,在计算中同时考虑了挠曲电表面和体积效应.     

介电极化新概念--边界屏蔽理论

熟知的介电极化只是一种快效应,还有一种慢效应。慢极化效应使电介质的极化偏离了麦克斯韦方程组描述的规律,引入边界屏蔽电荷的概述２来描述介质的极化过程,可以固避电介质内部复杂的电位移运动,从而能用屏蔽电荷激发的量子统计理论来说明驻极体和铁电体的许多性质。     

脉冲强磁场下的电极化测量系统

多铁性材料是当前物质科学研究的热点,具有重要的科学研究意义和应用前景.低温和强磁场实验环境为研究多铁性材料提供了一种有效途径.脉冲强磁场下的电极化测量系统能实现最高磁场强度60 T、最低温度0.5 K的铁电特性测量.该系统采用热释电方法,具有磁场强度高、控温范围广、转角测量等特点,可用于强磁场下的磁电特性研究.本文介绍了该系统的测量装置和实验原理,并展示了其在多铁性材料研究中的一系列应用,揭示了脉冲强磁场电极化测量系统在磁电特性探索中的重要作用.     

铁电材料光催化活性的研究进展

光催化技术被认为是最有前景的环境污染处理技术,这就使得光催化剂材料备受瞩目.近年来,铁电材料作为新型光催化剂材料受到人们越来越多的关注,其原因在于铁电材料特有的自发极化有望解决催化反应过程中的电子-空穴对复合问题,进而提高光催化活性.本文从两个方面对铁电极化如何影响光催化进行综述:一方面,从铁电极化入手归纳总结其对电子-空穴对分离的影响,进而更深入地从极化引发的退极化场和能带弯曲两个部分来阐述具体的影响机理;另一方面,为了消除静电屏蔽,分别从温度、应力(应变)、电场三个外场因素调控极化入手,归纳总结外场调控极化对电子-空穴对分离的影响,进而影响光催化活性.最后对该领域今后的发展前景进行了展望.     

铁电屏蔽理论

利用电极化的边界屏蔽近似,可计算出一些典型铁电单晶的自发极化强度．铁电屏蔽电荷处于表面束缚极化子偶状态;其产生和湮没要跨越位垒．由统计方法给出的一些常见晶体和陶瓷的四种典型电滞回线形状,和实验观察到的结果一致 关键词：     

液晶聚合物/柔性链聚合物共混体系相分离形态

利用元胞动力学方法在二维情况下对浓度、取向序参量的含时Ginzberg Landau方程进行数值求解 ,研究了液晶聚合物 /柔性链聚合物共混体系的相分离动力学 ,考察了浓度、取向有序过程的耦合对相分离形态的影响 .结果表明 ,此耦合作用对相分离的时间进程以及相分离图样的空间排布都有影响 .液晶聚合物的取向有序相当于增加了两组分间的不相容性而促进两相分离 ;两个序参量在热力学方面的耦合使液晶聚合物趋向于沿着界面方向取向 ,而动力学方面的耦合使液晶聚合物分子沿着其取向方向扩散 ,相分离图样的空间排布由这两种效应共同决定 .通过极化率张量的定义用数值方法模拟得到了相分离体系的小角光散射图样 ,结果表明 ,散射强度分布具有方位角依赖性 ,它是由浓度、取向序参量的空间变化共同决定的 .     

表面极化对弱锚定向列液晶盒饱和特性的影响

以修正后的Rapini-Papoular锚定能公式为基础, 用理论分析和数值计算相结合的方法, 详细研究表面极化对液晶盒饱和特性的影响. 得出了求解指向矢分布的数学方程, 推导出了计算正常二级转变饱和电压的解析式, 同时结合最新报道, 给出了异常一级转变饱和电压的数值计算方法. 此外, 本文引入了反映指向矢分布特征的参量, 讨论了表面极化对此参量的影响. 结果表明, 指向矢最大倾角的位置随表面极化的增大将远离中央平面向基板移动. 一级转变饱和电压随弱锚定基板表面极化的增大而减小, 随强锚定基板表面极化的增大而增大. 而对二级转变, 饱和电压随表面极化的变化与一级转变恰恰相反. 本文所得结论对液晶表面物理的理解及液晶实际应用都具有重要的指导意义.     

液晶盒中挠曲电效应的计算

负性液晶MBBA在液晶盒中垂面排列,沿面外加电场后产生挠曲电效应.基于液晶弹性理论研究这一效应,基于变分理论得到了液晶指向矢满足的微分方程及边界条件.应用差分迭代方法进行了数值求解.在小角度近似下,得到了指向矢分布的解析解,与数值解和专著结果进行了比较.     

弹性形变对手征液晶螺旋光轴及电光响应时间的影响

手征向列相液晶螺旋轴即为光轴,此轴的空间取向直接影响着液晶中光传播的特性。采用理论分析和数值模拟相结合的方法,研究了液晶展曲与弯曲形变存在的差异和对手征向列相液晶挠曲电螺旋光轴倾角及动力学响应特性的影响。假设在静电平衡及动力学响应两种状态下,系统均具有统一的挠曲电螺旋光轴,忽略介电各向异性,分别计算了两种不同状态下系统的平均自由能密度。利用欧拉方程及转矩平衡方程得到了螺旋光轴倾角满足的平衡方程及动力学方程。通过数值计算,讨论了两种形变的差异对挠曲电螺旋光轴倾角及动力学响应特性的影响。结果表明两种形变差异的存在,均使螺旋光轴扭曲角及特性响应时间变化,差异越大变化越快,这种影响是不可忽略的,这为液晶电光快速响应提供了依据。     

Influences of surface and flexoelectric polarization on the effective anchoring energy in nematic liquid crystal

The physical effects on surface and flexoelectric polarization in a weak anchoring nematic liquid crystal cell are investigated systematically. We derive the analytic expressions of two effective anchoring energies for lower and upper substrates respectively as well as their effective anchoring strengths and corresponding tilt angles of effective easy direction.All of these quantities are relevant to the magnitudes of both two polarizations and the applied voltage U. Based on these expressions, the variations of effective anchoring strength and the tilt angle with the applied voltage are calculated for the fixed values of two polarizations. For an original weak anchoring hybrid aligned nematic cell, it may be equivalent to a planar cell for a small value of U and has a threshold voltage. The variation of reduced threshold voltage with reduced surface polarization strength is also calculated. The role of surface polarization is important without the adsorptive ions considered.     

Flexoelectric polarization and second order elasticity for nematic liquid crystals

The flexoelectric properties of nematic liquid crystals are analysed. It is shown that in the frame of the usual elasticity two coefficients characterize completely the flexoelectric properties of the liquid crystal. These coefficients, in the limit of small scalar order parameter, i.e. near the clearing point, are approximately equal. More precisely they have the same linear term in the scalar order parameter, and differ for terms quadratic in this parameter. Their difference behaves, hence, as the usual nematic liquid crystal elastic constants, whereas their sum depends on the temperature, like the mixed splay-bend elastic constant. It is shown furthermore that in the frame of a second order elastic theory in the flexoelectric polarization there are no terms from the second order spatial derivatives of the nematic director, or the nematic tensor order parameter. Consequently also in the frame of the second order elasticity the flexoelectric polarization is given by the usual expression. This conclusion is important in connection with the surface polarization recently discovered in pretilted nematic liquid crystal samples.     

超声多波聚焦及声偏振方向控制方法*

在超声多波聚焦思想的基础上,通过数值模拟计算,分析了时间反转法的多波聚焦特性以及对声场偏振方向进行控制的可行性。结果表明,在待测目标的不同位置处,时间反转法都能够实现多波聚焦的效果,使具有不同传播速度、不同偏振特性的多种声波自适应聚焦。但是,在介质的近表面处,由于受到表面波的影响,多波聚焦声场仍然具有椭圆偏振特性,无法实现声场偏振方向精确控制的目的;而位于介质内部的多波聚焦点受到表面波影响很小,数值计算结果表明此时多波聚焦声场具有线偏振特性,通过改变声源前后两个脉冲的激发幅度和相位,可以控制声场的偏振方向,达到偏振方向扫描的目的。该文的研究为精确检测裂纹方向或界面性质提供一种可能的途径。     

Inhomogeneous threshold director reorientation in a planar nematic flexoelectric cell with finite anchoring energy

The dependence of the threshold parameters and the period of the electric-field-induced spatially periodic reorientation of the director in a flexoelectric nematic liquid crystal (NLC) on the anchoring conditions at the surface of a planar NLC cell has been studied. The threshold electric field and the corresponding wave-number of the periodic structure of the director field have been numerically calculated for arbitrary values of the anchoring energy. In the case of strong anchoring, the corresponding analytical expressions are obtained in a single-constant approximation. A decrease in the azimuthal anchoring energy leads to an increase in the intervals of possible values of the flexoelectric parameter ν and the ratio K₂/K₁ of the Frank elastic constants. A decrease in the polar anchoring energy leads to narrowing of these intervals as compared to the case of infinitely strong anchoring at the NLC cell surface.     

铁电体中偶极子的滞后对剩余极化的影响

铁电体的剩余极化强度随温度降低而下降的特性引起了人们对铁电体存储数据失效的担心.运用铁电体的唯象理论和偶极子对交变电场的响应,提出了在电滞回线测量中偶极子的滞后冷冻效应模型,对极化的低温退化现象做了合理解释:温度下降导致吉布斯自由能势垒增大,致使偶极子对交变电场的响应时间延长.引入响应的滞后因子发现,极化强度随温度降低会出现峰值,在低温下降直至为零,可用偶极子的滞后与冻结效应描述.详细研究结果表明:因材料组份变化导致热力学参量的变化是重要因素:铁电-顺电相变中软模系数的增大会导致剩余极化峰移向高温;铁电性的增强,温度极化系数的增大和耐压强度或饱和电场的增强均会抑制滞后效应,从而使低温滞后效应移向低温.运用导出的公式数值模拟Ba Ti O_3/Bi Sc O_3复合陶瓷剩余极化强度的实验结果发现,Bi Sc O3含量的增加,使居里温度略有减小,但导致了软模系数较大幅度的增加,其结果是使偶极子的滞后效应发生在较高的温度.软模系数与铁电体的极化特性、铁电性、介电性和力学性均密切相关.研究结论表明:在低温下铁电体的铁电性没有失效,偶极子的低温冻结效应更有利于铁电体长久地保存数据.     

Surface and flexoelectric polarization in a nematic liquid crystal 5CB

The temperature dependence of the surface polarization has been measured for both the planar and homeotropic orientation of a nematic liquid crystal at a solid substrate. A conventional liquid crystal 5CB, pure and doped with an azo-dye, was used in cells with controlled asymmetry for light absorption. The measurements have been made by a pyroelectric technique using short pulses of a YAG laser to create a temperature increment. The latter, in turn, was measured independently by a novel time-resolved “optical thermometer” technique monitoring temperature-dependent birefringence by a He- Ne laser beam. In accordance with the symmetry of the order parameter, the surface polarization has different sign for the two orientations, its magnitude ranges from -4 to +2pC/m. The same technique has been used for the measurement of the flexoelectric polarization in hybrid cells. The sum of the flexoelectric coefficients is e ₁ + e ₃ = - 13pC/m at 25°C. Received 28 February 2000 and Received in final form 5 September 2000     

Thermoelectric response of spin polarization in Rashba spintronic systems

Motivated by the recent discovery of a strongly spin–orbit-coupled two-dimensional (2D) electron gas near the surface of Rashba semiconductors BiTeX (X= Cl, Br, I), we calculate the thermoelectric responses of spin polarization in a 2D Rashba model. By self-consistently determining the energyand band-dependent transport time, we present an exact solution of the linearized Boltzmann equation for elastic scattering. Using this solution, we find a non-Edelstein electric-field-induced spin polarization that is linear in the Fermi energy E_F when E_F lies below the band crossing point. The spin polarization efficiency, which is the electric-field-induced spin polarization divided by the driven electric current, increases for smaller E_F .We show that, as a function of E_F, the temperaturegradient-induced spin polarization increases continuously to a saturation value when EF decreases below the band crossing point. As the temperature tends to zero, the temperature-gradient-induced spin polarization vanishes.     

Nematic ordering in a cell with modulated surface anchoring: effects of flexoelectricity

We have analyzed molecular ordering in a nematic sample sandwiched between two parallel substrates, characterized by a periodically varying anchoring easy axis. If the periodicity lambda is smaller than the Debye screening length l(D) and the nematic material possesses flexoelectric properties, it is necessary to take into account also the electrostatic and flexoelectric contributions in the thermodynamical potential when the actual director field is determined. In this framework, for small deviations from the homeotropic alignment we have derived analytical expressions for the tilt angle (theta) and the electrical potential. To establish a connection with experimentally observable quantities, we have related the theta profile to the average and investigated its behavior for different values of lambda, the flexoelectric coefficient, and the anchoring strength w. Our results indicate that in a nematic with pronounced flexoelectric properties for small enough lambda, a kind of subsurface deformation appears, which substantially decreases . Therefore, effects of flexoelectricity cannot be neglected in treating nematic cells with modulated anchoring which allows bistable ordering.     

多功能太赫兹超表面偏振控制器

本文提出了一种“金属栅-开口环/硅环-金属栅”结构的透射式超表面偏振控制器, 研究了入射角度和抽运光对该器件传输及偏振态控制性能的影响. 研究结果表明, 当线偏振太赫兹波垂直入射时, 可对0.39-1.11 THz频段的太赫兹波实现偏振方向90°旋转, 偏振旋转效率为99%, 损耗为1 dB. 对于斜入射的情况, 偏振转换性能在0-60°范围内基本保持不变, 且透过率达到90%以上. 同时, 通过调控抽运光强度的方式, 该器件能够实现对透射与反射太赫兹光束的强度调制, 调制深度均达到90%, 且可以实现太赫兹波偏振分束功能. 该器件可以作为未来太赫兹空间光通信和信息处理的宽带、角度不敏感、可调谐的偏振转换器和分束器.