高温超导体磁通动力学和混合态相图(II)

(上接2006年第1期第16页)5涡旋态相图的发展随着理论和实验工作的不断进展,高温超导体混合态相图变得丰富多彩.首先在涡旋玻璃态理论出现后,相图就较常规超导体有了很大的变化.在常规超导体中,相图上就只有两根标志相边界的线,即上临界场Hc2(T)和下临界场Hc1(T).在Hc2(T)和Hc1(T)之间就是混合态.由于涨落较弱,Hc2(T)和磁通运动的不可逆线Hirr(T)很靠近.而高温超导体中由于涨落较强,Hc2(T)以下较大的区域内形成了涡旋液态,而Hirr(T)以下才能形成涡旋固态(可以是涡旋玻璃或磁通格子有序态).在下临界场Hc1(T)附近,如果结构无序较少,则…     

超导电性的交流磁化率研究

本文综述了交流磁化率在超导电性物理研究方面的进展.主要包括如何用交流磁化率确认超导电性,测定各种超导电参量(临界温度,临界磁场等),研究涡旋物质的强度(临界电流密度,不可逆场等),涡旋动力学(磁弛豫,钉扎能等).     

拉盖尔高斯涡旋光束的传输

针对拉盖尔高斯涡旋光束,推导了其传输后目标平面上光电场的解析表达式,理论研究表明,传输一段距离后, 对于拉盖尔高斯光束的光斑大小的描述,高斯光斑尺寸已经不再适用.如果采用光强最亮处的半径来表示目标平面上的光斑大小则比较方便. 除了传输中的衍射导致光束展宽以外, 横截面上光束的相位分布也发生了独特的变化. 等相位线由原来的射线转化为弧线,拓扑电荷数为正时,弧线朝顺时针方向弯曲,拓扑电荷数为负时,弧线朝逆时针方向弯曲. 关键词： 涡旋光束 传输 光斑尺寸 相位分布     

基于射流冲击横流原理产生涡旋及其测量

采用射流冲击横流的方法产生涡旋,模拟水岸和流水域的涡旋生成.通过CFD软件模拟真实的水流,研究射流和横流不同流速比以及射水口的位置对涡旋产生的影响,确定涡旋生成的最佳参量.实验中在水槽里产生稳定的涡旋,加入红色泡沫颗粒实现涡旋可视化,并采用双摄像头和3D-PIV技术对涡旋的三维流场特征和结构研究分析,计算得到涡旋的速度环量场、线速度场、涡量场以及黏度.     

两束部分偏振光的干涉场分析

从波的叠加与干涉的基础理论出发,给出了当两束部分偏振光的一个光强极值方向平行时,其干涉场的光强分布和干涉条纹衬比度表达式.该表达式可应用于自然光、线偏振光和由二者组成的部分偏振光参与的干涉过程.在此基础上,对线偏振光与线偏振光、自然光与自然光、线偏振光与自然光等不同偏振态光叠加时的干涉场进行了具体分析,给出了其相应干涉场的光强分布及其干涉条纹衬比度的具体表达式,得到的结果对识别前述各种偏振态光形成的干涉场以及提高干涉仪中相应干涉场的干涉条纹衬比度具有重要意义.     

散斑场相位涡旋及其传播特性的实验研究

用CCD记录散斑场和参考光的干涉图样, 利用傅里叶变换法实现了散斑场复振幅以及相位分布的实验提取, 在复振幅实部零值线和虚部零值线切点处出现一种新的相位奇点, 与两条零值线交点处的传统相位奇点周围单调螺旋变化的相位分布不同, 绕切点一周的相位具有先增大后减小几乎呈对称性分布的特性, 为了解释这种新的相位奇点,提出了准孪生相位涡旋的概念. 在理论研究散斑场强度的纵向自相关函数的基础上, 实验观察散斑场相位涡旋的传播, 发现在光强纵向相关长度范围内的不同传播距离处的观察面上, 复振幅实部和虚部随机变化而涡旋的 关键词： 相位涡旋 散斑 干涉     

超导环电流的研究

以修正的含时金兹堡-朗道理论去研究超导环的超流特性；计算了涡旋运动方程，理论上得到超导环电流是超流量子的整数倍，并且具有多涡旋跳跃；从理论上得到超导环的磁场是量子化的.一定条件下，超流的符号可以发生反转，并且理论上得到了它的表达式.认为超导环是具有对势的对称. 关键词： 超导环 电流 超导电性     

YBa2Cu3O7-δ外延薄膜的不可逆线与磁通玻璃线

在0—10T磁场范围内,系统地测量了YBa₂Cu₃O_7-δ外延薄膜处于磁场平行和垂直膜面两种情况下的R-T曲线和I-V曲线, 并对该样品的不可逆线和磁通玻璃线作了直接的比较.结果表明, 由不同约化电阻率判据给出的不可逆线和磁通玻璃线遵守相同的H∝(T(0)-T(H))ⁿ 关系.对于不可逆线, n=3/2,对于磁通玻璃线,n=4/3.不可逆线的位置不仅与判据有关而且与测量电流密度也有关,磁通玻璃线位于不可逆线的下方.探讨 关键词：     

s+id波高温超导体的单根涡旋线结构

用松弛叠代法数值求解了在s+id混合波对称性考虑下的Ginzburg-Landau方程，得到s+id高温超导体单根涡旋线的结构，并研究了其随温度磁场变化的特性.同时，也计算了在该考虑下的上临界场H_c2的曲线，相应的数值结果与实验结果定性一致. 关键词：     

NiFe/FeMn双层膜的交换耦合

采用平面霍尔效应测量方法，对NiFe/FeMn双层膜的交换耦合进行了研究. 结果表明，在NiFe/FeMn体系中不存在spin-flop模型给出的单轴各向异性场. 而导致交换耦合场可逆与不可逆测量结果之间较大差异的原因是反铁磁颗粒的不稳定性或铁磁层的分畴现象. 关键词： 反铁磁/铁磁双层膜 交换偏置场 可逆与不可逆测量     

Flux vortices and transport currents in type II superconductors

This article is concerned with the mechanisms by which type II superconductors can carry currents. The equilibrium properties of the vortex lattice are described and the generalized driving force in gradients of temperature and field is derived using irreversible thermodynamics. This leads to expressions for thermal cross effects which can include pinning forces. The field distributions which occur in a range of situations are derived and a number of useful solutions of the critical state given. In particular, the distribution in a longitudinal field is obtained, and the conditions under which force-free configurations can break down by the cutting of vortices discussed. The effects of lattice rigidity on the summation of pinning forces is considered and it is shown that a summation based on statistical arguments uses the same approximations and leads to the same results as a dissipation argument. Theoretical expressions are derived for the vortex pinning interaction to a number of different metallurgical defects. The theoretical models are compared critically with experimental measurements of pinning forces and other related phenomena, such as flux creep, low amplitude vortex oscillations and vortex lattice defect effects. Finally, the implications for technological materials are assessed.     

由N-E-V分布及赝势法研究弱磁场中弱相互作用费米子气体的热力学性质

基于低温下量子系统的相关实验多是在体积、能量和粒子数都可变的外场束缚下进行的事实, 由体积、能量和粒子数可变的完全开放系统的统计分布(N-E-V分布)研究了弱磁场中弱相互作用费米系统的热力学性质. 首先求出了一般情况下由费米积分表示的内能和热容的解析表达式. 在此基础上, 又给出了在低温极限条件下内能与热容的解析表达式和数值计算结果, 并将N-E-V分布(粒子数密度变化)的结果与赝势法(粒子数密度不变)的结果进行了比较. 结果表明: N-E-V分布方法的计算结果总是补偿赝势法计算结果的过度偏差. 由N-E-V 分布方法所得结果最特异之处在于: 在低温条件下, 弱磁场中弱相互作用费米系统存在一相变温度t_c, 其正处于费米系统发生玻色-爱因斯坦凝聚(BEC)和费米原子形成库珀对的超流状态(BCS)相变及BEC-BCS跨越的温度范围内, 且不随反映弱相互作用大小和特征的散射长度a (a<0引力, a>0斥力)变化, 但随弱磁场的加强而降低, 即弱磁场可调节该相变温度. 磁场为零时, 相变温度最高, 为费米温度的0.184倍.     

Dynamic coupling-decoupling crossover in the current-driven vortex state in Tl2Ba2CaCu2O8 probed by the Josephson plasma resonance

We have used terahertz spectroscopy to measure the Josephson plasma resonance in the superconductor Tl2Ba2CaCu2O8+delta. This allows us to probe the longitudinal ordering of pancake vortices as a function of applied ab-plane current in a 2.5 kG c-axis magnetic field. With increasing current in the low temperature vortex solid phase, we observe a decrease in the interlayer phase coherence consistent with a progressive misalignment of the pancake vortices in neighboring layers. In the high temperature vortex liquid phase, an increase in the longitudinal ordering occurs above a certain threshold current. Our results show evidence of a current-driven coupling-decoupling crossover in the pinned liquid phase.     

Flux vortices and transport currents in type II superconductors

This article is concerned with the mechanisms by which type II super-conductors can carry currents. The equilibrium properties of the vortex lattice are described and the generalized driving force in gradients of temperature and field is derived using irreversible thermodynamics. This leads to expressions for thermal cross effects which can include pinning forces.

The field distributions which occur in a range of situations are derived and a number of useful solutions of the critical state given. In particular, the distribution in a longitudinal field is obtained, and the conditions under which force-free configurations can break down by the cutting of vortices discussed.

The effects of lattice rigidity on the summation of pinning forces is considered and it is shown that a summation based on statistical arguments uses the same approximations and leads to the same results as a dissipation argument. Theoretical expressions are derived for the vortex pinning interaction to a number of different metallurgical defects. The theoretical models are compared critically with experimental measurements of pinning forces and other related phenomena, such as flux creep, low amplitude vortex oscillations and vortex lattice defect effects. Finally, the implications for technological materials are assessed.     

Ba1-xKxFe2As2单晶(Tc=38.5K)磁通钉扎力与钉扎机理研究

铁基超导体是在2008年由Hosono发现的一种新型超导材料, 由于其具有上临界场高、各向异性小、临界电流密度大等优点, 在世界范围内引起了广泛关注. 以Ba_1-xK_xFe₂As₂为代表的FeAs-122系超导体具有结构简单、合成温度低、单晶容易制备等优点, 是物理学家和材料学家关注的焦点. 本工作在获得最优化掺杂的Ba_1-xK_xFe₂As₂单晶(T_c = 38.5 K)基础上, 通过分析其在不同磁场条件下电阻温度变化关系、不同温度条件下的磁滞回线等数据, 系统的研究了Ba_1-xK_xFe₂As₂单晶磁通钉扎力和磁通钉扎机理. 研究发现Ba_1-xK_xFe₂As₂超导体具有非常高的磁通钉扎势, 其中9 T的外场条件下, 其在H//c轴和H//ab面的钉扎势分别为5800 K和8100 K, 展示出良好的应用前景; 通过进一步分析发现, 其磁通钉扎机理应是由于晶格内部的小尺寸缺陷引起的电子平均自由程变化而导致的δl钉扎.     

Dependence of the magnetization relaxation time of single-domain ferromagnetic particles on damping in the Brown model

Analytical expressions for the magnetization relaxation time τ of single-domain ferromagnetic particles with cubic or uniaxial anisotropy in a static transverse magnetic field are derived. The derivation is based on calculating the escape rate of a Brownian particle from a potential well; this technique is applicable at any damping and is generalized to the case of magnetic relaxation of superparamagnetic particles. The validity of the expressions obtained for τ is checked against a numerical solution of the Landau-Lifshitz-Gilbert equation over the whole range of damping (very low, intermediate, and high damping and the crossover region between low and intermediate damping).     

Crossover effects in scheelite DyLiF4

The Zeeman effect, magnetization, and differential susceptibility of a DyLiF₄ crystal in a pulsed magnetic field are studied experimentally and theoretically. It is found that Dy³⁺ ion levels in DyLiF₄ approach each other and a crossover occurs in a magnetic field H ‖ [001], which leads to a smearing of peaks in the differential magnetic susceptibility dM/dH and to inflection points in the magnetization curves M(H) at low temperatures. It is demonstrated that magnetic anomalies that accompany the crossover in DyLiF₄ in a field H ‖[001] are sensitive to the electronic structure of the Dy³⁺ ion. Therefore, these anomalies can be used to refine the crystal-field parameters. The effects of variations in the crystal field and temperature and of a deviation of the direction of the magnetic field from the symmetry axis on the magnitude and character of the magnetic anomalies associated with the crossover are investigated. The crystal field and crossover effects in the scheelite structure are compared with those in the zircon structure.     

Crossover from glassy to inhomogeneous-ferroelectric nonlinear dielectric response in relaxor ferroelectrics

The temperature dependence of the dielectric nonlinearities in a PMN single crystal and in 9/65/35 PLZT ceramics has been determined by measuring the first and third harmonic response as well as the dielectric behavior as a function of the dc electric field. In zero field a paraelectric-to-glass, and, in a high enough dc field, a glass-to-ferroelectriclike crossover in the temperature dependence of the nonlinear response have been observed. Both crossovers agree with the predictions of the spherical random-bond-random-field model. Relaxors thus undergo in zero field a transition to a spherical glass, while above the critical field a transition into a ferroelectric state occurs.     

Direct evidence for an intrinsic square vortex lattice in the overdoped high- T(c) superconductor La(1.83)Sr(0.17)CuO(4+delta)

We report here the first direct observations of a well ordered vortex lattice in the bulk of a La(2-x)Sr(x)CuO(4+delta) single crystal (slightly overdoped, x = 0.17). Our small angle neutron scattering investigation of the mixed phase reveals a crossover from triangular to square coordination with increasing magnetic field. The existence of an intrinsic square vortex lattice has never been observed in high-temperature superconductors and is indicative of the coupling of the vortex lattice to a source of anisotropy, such as those provided by a d-wave order parameter or the presence of stripes.