The influence of measurement and relaxation time on flux jumps in high temperature superconductors

The influence of the magnetization and relaxation time on flux jumps in high temperature superconductors (HTSC) under varying magnetic field is studied using the fundamental electromagnetic field equations and the thermal diffusion equation; temperature variety corresponding to flux jump is also discussed. We find that for a low sweep rate of the applied magnetic field, the measurement and relaxation times can reduce flux jump and to constrain the number of flux jumps, even stabilizing the HTSC, since much heat produced by the motion of magnetic flux can transfer into coolant during the measurement and relaxation times. As high temperature superconductors are subjected to a high sweep rate or a strong pulsed magnetic field, magnetization undergoes from stability or oscillation to jump for different pause times. And the period of temperature oscillation is equal to the measurement and relaxation time.     

高温超导磁通跳跃过程中的磁致伸缩效应

文中基于超导磁通动力学理论,考虑电磁力与热激活对磁通运动的影响,基本模型包括由等效电阻率随超导体温度和磁场变化的磁通扩散方程,以及比热随超导体温度变化的热传导方程组成.在此基础上,用数值方法求解了这组非线性磁热耦合方程,主要研究了有磁通跳跃状发生状态时环境温度和外磁场速度对于高温超导磁致伸缩的影响.结果表明:磁通进入超...     

Third harmonics of the AC magnetic susceptibility: a method for the study of flux dynamics in high temperature superconductors

The temperature dependence of the 1st and 3rd harmonics ( ) of the AC magnetic susceptibility has been measured on melt grown YBCO samples for different frequencies and amplitudes of the AC magnetic field and intensity of a contemporaneously applied DC field. With the help of critical state models and of numerical simulations [22], we have devised a novel method, based on the combined analysis of the 1st and the 3rd harmonics (specifically on the comparison between and ), that allows to distinguish different temperature ranges dominated by the different dissipative magnetic flux regimes. In particular, we identified three principal zones in the temperature dependence of the real part of the 3rd harmonic: the zone 1, in the temperature range below the peak of the imaginary part of the 1st harmonic, , and the zone 2, characterized by negative values in a temperature region just above , both dominated by the creep regime; the zone 3, just below T _c , in which we revealed the presence of Thermally Assisted Flux Flow (TAFF). By the identification of these zones, an estimation of the value of the pinning potential can be obtained.Received: 23 September 2003, Published online: 19 November 2003PACS: 74.25.Ha Magnetic properties - 74.60.Ge Flux pinning, flux creep, and flux-line lattice dynamics     

Simple algorithm in time-window extending for magnetic relaxation in high Tc superconductors

A one to one map is established between the magnetic-hysteresis-loop (MHL) measurements and the magnetization relaxation measurements performed at the same external field, noticeably keeping no uncertainties of such factors as the microscopic attempt frequency v ₀ and the hopping distance l _hop for pinned flux motion. By utilizing this map, we are able to construct and extend the magnetization relaxation data from the corresponding reported MHL measurements. The present algorithm proves to be reliable and simple for investigations of the U(J) behavior and relaxation effects.     

高温超导体磁通钉扎和磁通动力学研究简介

ψ=ψ0eiφ行为用统一的波函数进行描述,其相位φ在宏观尺度上是相同的.当磁场低于一定值的时候,在超Φ0=h/2e保证最大的界面面积,降低系统能量.该最小的磁通束被称为磁通量子,其磁通量是(h为普朗克超导态是一个宏观量子相干态,其载流子是库珀对.在没有外加磁场和电流的时候,这些库珀对的运动导体的边界处穿透深度内会出现一...     

高温超导体磁通跳跃数值模拟

文中基于超导磁通动力学理论,考虑电磁力与热激活对磁通运动的影响,基本物理模型由等效电阻率随超导体温度和磁场变化的磁通扩散方程,以及比热随超导体温度变化的热传导方程组成。在此基础上,用差分法数值求解了这一复杂非线性系统的磁热耦合控制方程,得到了与实验观测结果基本一致的数值模拟结果。结果还表明：外加磁场变化速度、超导体初始温度以及超导尺寸对于磁通跳跃均产生明显的影响。     

磁场感应式高温超导磁通泵分析

为了解决闭环高温超导磁体的磁场稳定性问题,需要对超导磁体进行电流补偿。针对高温超导磁体励磁和磁场衰减,在对磁场感应式磁通泵原理分析的基础上,提出了高温超导磁通泵的双回路电流模型,建立了高温超导磁通泵模型方程,并对超导泵桥上的磁通运动过程进行了定量分析,获得了磁通泵运行参量对超导磁体负载电流的影响关系。     

Dynamics of local magnetic flux vortexes in superconductors

The dynamics of local magnetic flux vortexes in high-temperature semiconductors is studied with a mechanical method. Physical conditions in a gradient magnetic field are analyzed. Obtained results are treated within a model of free and pinned vortexes. The magnitudes of elastic modula, pinning strength, dissipated energy, viscous friction of the vortexes, and pinning potential were calculated. The method allows the determination of local, rather than integrated, superconductor characteristics. This makes it possible to map pinning centers over the crystal structure.     

Microscopic theory of atomic and electronic stretched exponential relaxation in high temperature superconductors

Measured dimensionless room-temperature conductivity relaxation stretching fractions β in YBCO are in excellent agreement with theoretical predictions, which in 1995 identified two magic fractions, β=3/5 and β=3/7. Thus, relaxation studies provide an absolute measure of “ideality” in these complex materials, independent not only of composition x but even of crystal structure. The relaxation stretching fractions β associated with T_c itself, reported in 2000, are also explained by the magic fraction β=3/5 predicted by microscopic theory. One can infer that the interactions responsible for high-temperature superconductivity are short range, non-magnetic, and primarily associated with resonant trapping centers in semiconductive layers.     

Vortex avalanches and magnetic flux fragmentation in superconductors

We report the results of numerical simulations of nonisothermal dendritic flux penetration in type-II superconductors. We propose a generic mechanism of dynamic branching of a propagating hot spot of a flux flow/normal state triggered by a local heat pulse. The branching occurs when the flux hot spot reflects from inhomogeneities or the boundary on which magnetization currents either vanish, or change direction. The hot spot then undergoes a cascade of successive splittings, giving rise to a dissipative dendritic-type flux structure. This dynamic state eventually cools down, turning into a frozen multifilamentary pattern of magnetization currents.