层状磁电复合材料谐振频率下的巨磁电容效应

对三明治复合结构Tb_xDy_1-xFe_2-y/Pb(Zr, Ti)O₃/Tb_xDy_1-xFe_2-y的电容与频率及磁场的函数关系进行了实验和理论研究. 实验发现,该复合材料样品的电容随频率的增加而出现多个谐振峰,并且其谐振点随磁场的增加而发生频移. 在谐振点附近,观察到样品的阻抗随磁场的增加由容抗性转变为感抗性,从而同时观察到巨大的正磁电容效应和负磁电容效应. 由复合材料的弹性力学本构方程出发,对该类样品的电容随频率及磁场的变化进行了理论模拟. 结果显示,模拟曲线与实验结果符合得很好. 理论表明该磁致伸缩/压电复合材料的磁电容效应源于磁场诱变的铁磁相柔顺系数. 关键词： 层状复合材料 界面弹性耦合 磁电容效应     

金属柱平板慢波系统高频特性研究

对于离散的金属柱结构构建的周期性平板慢波系统,本文利用3维FDTD方法结合HFSS仿真软件深入分析了该慢波系统的高频特性.研究了金属柱高度、周期长度对色散特性的影响,计算了耦合阻抗,并与传统光栅慢波系统特性进行了对比分析.分析表明金属柱慢波系统既有与传统光栅慢波系统相似的高频特性,又具有独自的特点,位于离散的金属柱周期间隙中的电子注互作用耦合阻抗具有对称性;金属柱结构用作真空电子器件的高频系统可增加发生有效互作用的电子注厚度,降低起振电流密度,提高器件效率.本文的分析结果为设计低电流密度工作的多电子注短毫 关键词： 金属柱平板慢波系统 3维FDTD算法 色散特性 耦合阻抗     

在群论框架下电子三重态与声子耦合的理论研究

本文基于绝热近似和群论导出了电声耦合系统的哈密顿量的一般形式,讨论了电声耦合系统中的电子算符和活跃的声子模式. 应用幺正平移变换和能量最小化方法,进一步计算了正四面体群下T*(e+t₂)杨-泰勒系统中的激发态能量,从对称性的角度分析了T₁电子态的能级分裂以及晶格体系的对称性破缺,得出了对称性的破缺方式和电声耦合系统密切相关的结论. 结果表明:通过群论与对称性分析完全可以定性地解释由于电声耦合所造成的简并电子 关键词： 电声耦合 杨-泰勒畸变 活跃的杨-泰勒声子模式 电声耦合哈密顿量     

自旋轨道耦合作用对碳纳米管电子能带结构的影响

本文考虑自旋轨道耦合作用的情况下,采用紧束缚近似螺旋对称模型计算了单壁碳纳米管的电子能带结构.研究发现:对于Armchair型单壁碳纳米管,自旋轨道耦合作用和弯曲效应共同导致了费米面Dirac点附近电子能带结构的能隙;对于Zigzag型和手性单壁碳纳米管,自旋轨道耦合作用使得电子最高占据态和最低未占据态产生能级劈裂,能级劈裂的大小不但与碳纳米管的直径和手性角密切相关,而且相对于费米面是不对称的;根据指数(n,m)可以将Zigzag型和手性单壁碳纳米管分为金属性碳纳米管(ν=0) 关键词： 单壁碳纳米管 自旋轨道耦合 紧束缚近似螺旋对称模型     

Multiferroic magnetoelectric coupling and relaxor ferroelectric behavior in 0.7BiFeO3-0.3BaTiO3 nanocrystals

The structural, microstructural, polarization, magnetization, dielectric constant, and relaxor characteristics of 0.7BiFeO₃-0.3BaTiO₃ (BF-BT) nanocrystals have been studied. BF-BT nanocrystals were prepared by a chemical route using polyvinyl alcohol as surfactant. The phase structure is confirmed by X-ray diffraction and average particle size by transmission and scanning electron microscopy. The magnetoelectric coupling is studied by polarization hysteresis loops under the influence of applied magnetic field and the phase transition anomaly. The diffuse phase transition is studied by modified Curie-Weiss law and relaxor characteristics by Vogel-Fulcher relation.     

Desynchronization transitions in nonlinearly coupled phase oscillators

We consider the nonlinear extension of the Kuramoto model of globally coupled phase oscillators where the phase shift in the coupling function depends on the order parameter. A bifurcation analysis of the transition from fully synchronous state to partial synchrony is performed. We demonstrate that for small ensembles it is typically mediated by stable cluster states, that disappear with creation of heteroclinic cycles, while for a larger number of oscillators a direct transition from full synchrony to a periodic or a quasiperiodic regime occurs.     

Magnetic and electrical properties of bulk BaTiO3+MgFe2O4 composite

BaTiO₃+MgFe₂O₄ material system was synthesized by hybrid chemical process using chlorides and nitrates of barium, titanium, iron, and magnesium. Magnetic properties of the composite samples measured as a function of annealing conditions indicated soft magnetic behavior. Saturation specific magnetization from 8 21 emu/g was observed for samples annealed at temperature between 950 and 1150 °C. Variation of specific saturation magnetization with respect to annealing temperature was related with the distribution of Fe cations in the tetrahedral and octahedral sites of MgFe₂O₄. Electrical properties of the samples annealed at different temperatures were measured to analyze the coexistence of ferroelectric phase. Dielectric constant varying from 15 to 200 with respect to frequency was observed for samples annealed from 950 to 1150 °C.