用非线性薛定谔方程讨论超晶格中畴的运动

用非线性薛定谔方程讨论超晶格畴的形成和漂移,得到具有负微分电导特性的超晶格中畴的波函数是Bloch函数的一个孤子型包络函数,它在外电场作用下做漂移运动. 关键词：     

偶极相互作用对自旋轨道耦合自旋-1凝聚体中磁畴分布的影响

研究准一维简谐势阱中存在自旋轨道耦合（SOC）的自旋-1偶极玻色-爱因斯坦凝聚体,数值求解旋量Gross-Pitaevskii方程,给出磁畴的分布。计算结果表明：磁畴结构与偶极-偶极相互作用（DDIs）密切相关,随着自旋轨道耦合强度和磁化强度的增强,原来的空间对称分布被破坏,随之出现新的分布模式。     

磁畴和畴壁的实验演示

利用透射光法进行磁畴和畴壁的实验演示，在磁性材料中的薄膜中，若无外加磁场时，可以观察到很多蜿蜒曲折的条状磁畴，当垂直薄膜面上加一个磁场（磁化场）时，这些条状磁畴就会产生变化，凡与外加磁场方向一致的磁畴体积扩大，与加外磁场方向相反的磁畴体积会缩小。     

超晶格电流振荡的分岔图和Poincaré映射

基于超晶格输运的分立漂移模型,数值模拟了掺杂弱耦合GaAs AlAs超晶格在外加交流偏压下电场畴的输运行为.以黄金分割比固定交流驱动频率,模拟并分析不同交流幅度下电流的准周期、锁频及混沌现象.     

用于铁电体复杂畴结构模拟的布朗方程

铁电材料在室温下具有可以在外加电场作用下改变方向的自发极化,不同方向的极化在材料内部形成畴结构,会对其物理特性和实际应用具有显著影响.本文将最初用于微磁模拟的布朗方程引入铁电材料的大尺度模拟中,研究其中可能出现的重要畴结构.在以有效哈密顿量方法为基础推导出铁电材料中关于电偶极子的布朗方程后,以BaTiO₃,PbTiO₃块体和SrTiO₃/PbTiO₃/SrTiO₃夹心结构等钙钛矿铁电材料为研究对象,验证了布朗方程的有效性并讨论了其中的多种畴结构,如周期性条带状畴、涡旋型拓扑畴结构等,并与相关实验结果进行了对比分析.     

晶格失配应力对单晶(BiTm)_3(GaFe)_5O_(12)膜磁畴结构的影响

利用液相外延工艺在钆镓石榴石衬底上制得了单晶(BiTm)_3(GaFe)_5O_(12)膜,研究了晶格失配应力对其磁畴结构的影响.研究发现,生长速率越快,膜的晶格常数越大;晶格失配应力可以在一定范围内调整膜的垂直各向异性;随着晶格失配应力由较大张应力逐渐转变为较大压应力,磁畴形状先由磁泡畴转变成迷宫畴,然后转变为过渡态部分弯曲的条状畴,最终转变为整齐排列的条状畴;失配应力同时对畴宽也有影响,膜受到的失配应力越大,畴宽越大.这一实验研究对基于控制晶格失配应力来调控单晶膜的各向异性和磁畴结构有指导意义.     

钼酸钆晶体畴结构的同步辐射白光形貌实时研究

用同步辐射白光X射线形貌术对畴结构在温度及外加电场变化条件下的演变情况进行了研究.结果表明:在钼酸钆样品中同时存在铁电畴和铁弹畴,但这两类畴在温度高于居里温度时都消失,当施加足够外加电压时可以单畴化.     

外场下GaAs/AlGaAs超晶格中载流子的动力学行为

基于分立漂移模型研究了磁场对载流子在弱耦合GaAs/AlAs超晶格中的动力学行为的影响。研究表明磁场较小时超晶格中载流子的集聚形成电场畴,该电场畴的周期移动引起电流振荡的产生,随着磁场的增加系统中形成的电场畴变得稳定,电流振荡也逐渐消失。因此,磁场控制下系统的状态可以从动态转变到稳定态。受交流电场驱动时,由于内部电流振荡信号和外部交流信号的耦合,载流子运动展现出很多的非线性行为。     

外加电场法制备LiNbO3周期畴的反转电流特性研究

采用外加电场法制备了LiNbO3单晶周期畴结构.在对不同尺寸周期畴的反转电流进行比较研究的基础上,提出了一种确定反转畴成核时间和纵向贯穿速率的方法.根据这一方法,得到在电场强度为25.1 kV/mm,脉冲宽度为50 ms的脉冲方波作用下,LiNbO3单晶反转畴的成核时间约为80 ns,纵向贯穿速率约为O.1667 m/s.     

磁畴壁拓扑结构研究进展

拓扑磁性斯格明子作为信息载体单元具备高可靠性、高集成度、低能耗等优势,有望提高数据读写精度、降低功耗,从而研发新型拓扑自旋电子学材料与原理型器件,为信息技术、5G通信和大数据等的高速发展提供材料与技术支持.但磁性斯格明子同时存在需要磁场稳定以及电流驱动下斯格明子霍尔效应引起偏转等缺点,严重阻碍了其在实际器件中的应用,因此探索新型拓扑磁畴结构和适宜应用的材料体系成为研究的关键.本文将重点介绍自2013年理论预言磁畴壁斯格明子以来,利用高分辨率洛伦兹透射电子显微镜原位实空间发现并研究磁畴壁拓扑麦纫和磁畴壁斯格明子的实验工作.首次在范德瓦耳斯Fe_5–xGeTe₂二维磁性材料中发现温度诱发的180°磁畴壁转变为拓扑麦韧链,研究了磁畴壁麦纫态在外界电场、磁场作用下的集体运动行为,揭示了基于自旋重取向、磁畴壁限域效应以及弱相互作用下生成磁畴壁拓扑态的机制.在该机制指导下,设计制备了具有自旋重取向的GdFeCo非晶亚铁磁薄膜,不仅获得了磁畴壁麦纫,验证了生成机制的普适性,还成功实现了畴壁麦韧对到畴壁斯格明子的可逆拓扑转变,开辟了基于磁畴壁等内禀限域效应开展...     

Evidence for spatial representation of object shape by echolocating bats (Eptesicus fuscus)

Big brown bats were trained in a two-choice task to locate a two-cylinder dipole object with a constant 5 cm spacing in the presence of either a one-cylinder monopole or another two-cylinder dipole with a shorter spacing. For the dipole versus monopole task, the objects were either stationary or in motion during each trial. The dipole and monopole objects varied from trial to trial in the left-right position while also roving in range (10-40 cm), cross range separation (15-40 cm), and dipole aspect angle (0 degrees -90 degrees ). These manipulations prevented any single feature of the acoustic stimuli from being a stable indicator of which object was the correct choice. After accounting for effects of masking between echoes from pairs of cylinders at similar distances, the bats discriminated the 5 cm dipole from both the monopole and dipole alternatives with performance independent of aspect angle, implying a distal, spatial object representation rather than a proximal, acoustic object representation.     

Dipole resonance muffler at the exit of a narrow pipe

Locking of sound by a dipole resonator installed at the exit of a narrow pipe is studied theoretically and experimentally. The efficiencies of locking by dipole and monopole resonators are compared, and the dipole resonator is found to be more efficient than the monopole one.     

The added masses of a monopole and a dipole in a narrow pipe

The added masses of a monopole and a dipole placed in a narrow pipe are studied experimentally. It is shown that, when a monopole passes from a free space into a narrow pipe, its added mass decreases, whereas the added mass of a dipole in a similar situation increases. Inside the pipe, for both monopole and dipole radiators, the value of the added mass depends on the radiator position with respect to the pipe wall. If the radiator is at the pipe axis, its added mass is at a minimum; when the radiator moves toward the wall, the added mass increases. This dependence is qualitatively explained using a simple example.     

The scattering cross section of a resonator in a multimode waveguide

The scattering of sound by a single monopole-dipole resonator in a multimode pipe is investigated. The resonator has the form of a Helmholtz resonator connected through a small bar to the pipe wall. In fact, this resonator is a combination of a monopole resonator and a dipole one positioned at the same point. The scattered fields of these resonators are orthogonal to each other. The scattering cross sections of the monopole and dipole resonators in a multimode pipe are calculated. The scattering cross section of the monopole-dipole resonator is determined as the sum of the scattering cross sections of the monopole and the dipole resonators. The friction in a resonator (the monopole or dipole resonator) reduces its scattering cross section by a factor of (1 + β)², where β is the ratio between the friction resistance and the radiation resistance of this resonator.     

多分量方位远探测声波测井的理论与应用

基于线性拟合的利用岩石动态模量预测静态模量方法存在经验性强且精度不准的问题,利用实验测量的岩石纵波速度、孔隙度和体密度作为输入参数,基于多基因遗传规划方法,不断进化模型得到静态模量关于三个参数的解析表达式,对储层砂岩的静态模量进行了预测。结果表明,多基因遗传规划方法得到的模量表达式更为精确,此外,由于多基因遗传规划方法中横波速度不必要作为计算过程中的输入参数,因此避免了横波速度测量不准确或缺失而带来的静态模量计算误差。     

电磁学与电动力学中的磁单极-Ⅱ

本系列文章一共4篇,在电磁学和电动力学框架内用尽量科普的方式分别介绍磁单极的若干奇特性质.本篇文章主要介绍狄拉克磁单极是如何展示矢量势的规范变换的.我们首先简要介绍规范变换与规范对称性及狄拉克磁单极与狄拉克弦,然后讨论狄拉克磁单极与规范变换的联系.我们显式演示狄拉克弦摆动产生的规范变换,弦摆动区域对场点所张的立体角正比于规范变换的变换函数.磁偶极子则可以由两个无穷靠近的正反狄拉克磁单极构成.相应两条狄拉克弦位置的变化都对应磁偶极子矢量势的规范变换,特别当两条弦重合时弦效应相互抵消,只剩下纯的磁偶极子.传统的由磁偶极子产生的矢量势的规范变换则可以图像化为组成磁偶极子的正反狄拉克磁单极的狄拉克弦的摆动.我们显式地计算了位于坐标原点弦为直线的狄拉克磁单极,并进一步构造了没有奇异的吴大峻-杨振宁磁单极.     

Perturbative QCD analysis of B to π and B to ρ transitions

We calculate the form factors and of and V, , and of transition matrix elements by using the factorization formalism of perturbative QCD in the region . In the limit of , , and , the results show that and are of monopole type, V, and of dipole type, and of a combination of monopole and dipole types with dipole type dominating. Received: 20 April 1998 / Published online: 5 October 1998     

Isoscalar monopole and dipole transition matrix elements from inelastic alpha scattering

Using an implicit folding procedure, isoscalar monopole and dipole transition matrix elements are expressed in terms of deformation parameters and moments of the optical potential forα-scattering. Application to monopole states in light nuclei gives good agreement with data determined electromagnetically. For isoscalar dipole states, the method gives information on 〈r ³〉 matrix elements which cannot be obtained otherwise.     

The interaction of transverse domain walls

The interaction between transverse domain walls is calculated analytically using a multipole expansion up to third order. Starting from an analytical expression for the magnetization in the wall, the monopole, dipole, and quadrupole moments are derived and their impact on the interaction is investigated using the surface and volume charges. The surface charges are important for the dipole moment while the volume charges constitute the monopole and quadrupole moments. For domain walls that are situated in different wires it is found that there is a strong deviation from the interaction of two monopoles. This deviation is caused by the interaction of the monopole of the wall in the first wire with the dipole of the wall in the second wire and vice versa. The dipole-dipole and the quadrupole-monopole interactions are found to be also of considerable size and non-negligible. A comparison with micromagnetic simulations shows a good agreement.     

Empirical transverse charge densities in the nucleon and the nucleon-to-delta transition

Using only the current empirical information on the nucleon electromagnetic form factors we map out the transverse charge density in proton and neutron as viewed from a light front moving towards a transversely polarized nucleon. These charge densities are characterized by a dipole pattern, in addition to the monopole field corresponding with the unpolarized density. Furthermore, we use the latest empirical information on the N-->Delta transition form factors to map out the transition charge density which induces the N-->Delta excitation. This transition charge density in a transversely polarized N and Delta contains both monopole, dipole and quadrupole patterns, the latter corresponding with a deformation of the N and Delta charge distribution.