Flux penetration in high-Tc superconductors: Implications for magnetic suspension and shielding

Two new effects associated with flux penetration and pinning in superconducting YBa₂Cu₃O_x are demonstrated. One of these is focussing of magnetic field lines by the superconductor and the other is magnetic suspension. The existence of stable equilibrium in suspension is shown to be related directly to hysteresis observed in the force-separation relation for a magnet and the superconductor. We also observe that the levitation height of a magnet increases with the magnet volume, contrary to results in the literature.     

The collective nature of a flux creep in high-T c superconductor YBa2Cu3O x as observed in microwave absorption experiments

Studies of the flux lattice dynamics via microwave absorption measurements with the help of an ESR spectrometer are presented. The experiments give evidence that for the temperatures below 35 K the absorbed power depends on the input microwave power in a non-linear way. For the case of absorption due to the motion of vortexes via thermally activated flux creep this corresponds to the dependence of the activation barrier on the current density. The measurements lead to the conclusion, that the activation barrier should grow with decreasing current. Fitting of the expression for the dissipated power obtained in terms of the collective creep theory to the obtained experimental data enable us to determine the character of pinning and creep at small amplitudes of the external exciting current. The obtained numerical values give evidence that the flux lattice is frozen in a vortex-glass state and in the limit of zero current the resistance at finite temperatures vanishes.     

Thermally induced oscillatory relaxation of dc resistivity and ac susceptibility in Y−Ba−Cu−O during N−S transition

Dc resistivity and ac susceptibility have been measured simultaneously on Y–Ba–Cu–O specimens under the effect of a temperature gradient. During cooling process, an intermediate (normal-superconducting) state of samples forms in the vicinity ofT _c and an artificial, local, step-like change of temperature induces an oscillatory relaxation of the measured parameter. This phenomenon has been interpreted in the framework of a vortexglass model as a collective relaxation of vortices inside a domain volume defined by vortex-glass correlation length. Close to theT _c the domain relaxation time and the correlation length have been determined as 51 s and 2 mm, respectively.     

永磁体辅助下单畴GdBCO超导体和永磁体之间的磁悬浮力研究

通过对永磁体辅助下单畴GdBCO超导体和圆柱形永磁体在液氮温度、零场冷、轴对称情况下磁悬浮力的测量,研究了两种不同组态下辅助永磁体对超导体磁悬浮力特性的影响.实验结果表明,当长方体辅助永磁体水平磁化、且磁极N指向超导体时,超导体的最大磁悬浮力从没有引入辅助永磁体的29.8 N增加到61.5 N,增加为没有引入辅助永磁体时的206%.当长方体辅助永磁体的N极与圆柱形永磁体的N极反平行时,超导体的最大磁悬浮力从没有引入辅助永磁体的29.8 N减小到19.6 N,减小为无辅助永磁体时的65.8%.这些研究结果说明,通过科学合理地设计超导体和永磁体的组合方式,能有效地提高超导体的磁悬浮力.该研究结果对促进超导体的应用具有重要的指导意义. 关键词： 单畴GdBCO 永磁体 磁悬浮力     

条状永磁体的组合形式及间距对单畴GdBCO超导体磁悬浮力的影响

通过对由条状永磁体组成的组合磁体与单畴GdBCO超导体在零场冷情况下磁悬浮力的测量,研究了5种不同组态下组合磁体之间距离的变化对超导体磁悬浮力的影响.结果发现,当条状永磁体之间的距离D从0 mm增加到30 mm时,超导体的磁悬浮力大小与组合磁体排列形式有着密切关系(以Z=5 mm为例):1)对由3个条状永磁体组成的组合磁体,当中间磁体的磁极N向上、两侧磁体的磁极N均水平指向中间磁体时,超导体的磁悬浮力从22.8N减小到9.7N;当中间磁体的磁极N向上、两侧磁体的磁极N均向下时, 关键词： 单畴GdBCO块材 磁体组合形式 磁悬浮力     

Magnetothermal instabilities in an organic superconductor

We have studied the occurrence of magnetothermal instabilities in a single crystal during field sweep magnetization experiments, equivalent to short time relaxation studies. We find instability behaviour in good agreement with a recent model by Mints, for a non-linear E(J) characteristic. In particular, we find that a decrease of the dynamic relaxation rate, characterizing the effective activation energy, precedes the unstable regime. We point out formal analogies between such instabilities and the general predictions for flux avalanches. Received: 11 December 1997 / Revised: 15 January 1998 / Accepted: 31 March 1998     

Numerical study on an isolated vortex of the type-II superconductor in the presence of a twin plane

Based on the Ginzburg-Landau theory and using the numerical relaxation approach, we have carried out a numerical analysis of an isolated vortex structure for a type-II superconductor in the presence of a twin plane (being a normal region) situated parallel to the external magnetic field. Meanwhile, the lower critical field is also calculated and is found to decrease as compared to the twin-free case, which is consistent with the intuitive theoretical understanding and the fact that twins are easy channels for the flux penetration.     

空心圆柱形永磁体内径对单畴GdBCO超导块材磁悬浮力的影响

通过对空心圆柱形永磁体与单畴GdBCO超导体磁悬浮力的实验测量,研究了空心圆柱形永磁体内径(d)的变化对超导体磁悬浮力的影响.结果发现,当空心圆柱形永磁体内径从0 mm增加到26 mm时,超导磁悬浮力大小与空心圆柱形永磁体内径有着密切关系(最小测量间距Z=2 mm),所有超导磁悬浮力曲线都存在磁滞现象.随着空心圆柱形永磁体内径的增大,最小间距处超导磁悬浮力逐渐减小,从d=0 mm时的14.8 N减小为d=26 mm时的-0.1 N,d≥20 mm时,最小间距处超导磁悬浮力出现负值;当0 mm≤d5 mm时,超导体最大磁悬浮力出现在最小间距处,d≥5 mm时,超导磁悬浮力先增大后减小,最大超导磁悬浮力产生的位置随着内径的增大而变大.研究表明:只有科学合理地设计永磁体结构参数,才能获得较大的磁场强度,提高超导磁悬浮力特性.该结果对设计并优化磁悬浮轴承系统、环形轨道和超导体的实际应用具有一定的指导意义.     

Some parameters of superconductors with an extreme singularity in the density of state: (s+d) model

We study some parameters of superconductors with δ-function type singularities in the electronic density of states (DOS), exhibiting (s+d)-wave symmetry of the order parameter. Starting with a pure s-wave pairing potential V_s, the critical temperature T_c at first slightly increases with increasing the d-wave interaction potential V_d, being determined by this interaction only for stronger V_d values. The ratio R=2|Δ(0)|/k_BT_c of the mean value of the zero temperature energy gap |Δ(0)| to T_c increases with increasing V_d, reaching a maximum which depends on the mixing interaction term. The maximum values of R are comparable with very high values obtained in some gap measurements. The jump in the specific heat at critical temperature is by a factor 2.4 higher for the extreme singularity of pure s-wave symmetry, as compared with the BCS theory with constant DOS. Such higher values of the jump are in agreement with the experimentally observed values, as well as with the calculations determined by extended saddle points in the electronic bands. By switching the d-wave channel, the value of the jump decreases. The results show the usefulness of calculations with δ-type singularities as a limiting case of very strong singularities in the DOS.     

Vibrations of a magnet levitated over a flat superconductor

Based on a dipole-dipole interaction model, we discuss the levitation force and related vibration problems to understand the effects of flux pinning in type-II superconductors, and the applications of levitation in superconducting systems.     

Identification of magnet and superconductor contributions to the ac loss in a magnet-superconductor levitation system

By measuring the quality factor of the mechanical oscillator with an attached magnet we investigated the amplitude dependence of the energy loss in a permanent magnet-YBCO superconductor system. When a non-conducting ferrite magnet is used the energy loss is proportional to the amplitude to the 2.79 power. This value is close to the value of 3 expected for the hysteretic motion of flux lines from the Bean model. When a conducting magnet is used the exponent decreased to the value of 2.5. This reflects the contribution of eddy current dissipation in a conducting magnet to the overall loss. We identified a novel mechanism of ac loss in the magnet-superconductor system. According to this mechanism the eddy current in a magnet is induced by the ac field generated by the ac supercurrent in response to the motion of a magnet. This mechanism would take place in a rotary bearing if circumferential inhomogeneity of the rotor magnetization occurred.     

辅助永磁体磁化方式对单畴GdBCO超导块材捕获磁场分布及其磁悬浮力的影响

通过对方形永磁体和方形辅助永磁体在液氮温度下对GdBCO超导体磁化后超导磁悬浮力的测量, 研究了两种组态中方形辅助永磁体对超导体的磁化方式对单畴GdBCO超导块材磁场分布及其磁悬浮力的影响. 结果发现, 方形辅助永磁体的下表面和超导体上表面保持在同一个水平面上, 磁化进程中方形辅助永磁体在GdBCO超导体上表面水平面内沿直径方向的位置x从–15 mm增加到+15 mm时, 超导磁悬浮力大小与超导体的磁化方式有着密切关系(以Z=0.1 mm为例): 1) 当方形辅助永磁体N极垂直向上且场冷后去掉辅助永磁体时, 超导体最大磁悬浮力先从16.7 N增大到23.1 N, 再减小到16.6 N; 2) 当方形辅助永磁体N极垂直向下且场冷后去掉辅助永磁体时, 超导体最大磁悬浮力先从17.7 N减小到7 N, 再增加到17.6 N; 3) 两种组态中最大磁悬浮力不相等, 而且与零场冷下的最大磁悬浮力(17.1 N)也不同. 这些结果说明: 只有通过科学合理地设计超导体和永磁体的组合方式, 才能获得较高的磁场强度, 有效地提高超导体的磁悬浮力特性, 该结果对促进超导体的应用具有重要的指导意义. 关键词： 单畴GdBCO 永磁体 捕获磁场 磁悬浮力     

辅助永磁体的引入方式对单畴GdBCO超导块材磁场分布及其磁悬浮力的影响

通过对永磁体辅助下单畴GdBCO超导体和方形永磁体在液氮温度、 零场冷、 轴对称情况下磁悬浮力的测量, 研究了三种不同组态中辅助永磁体的引入方式对单畴GdBCO超导块材磁场分布及其磁悬浮力的影响. 实验结果表明, 如果处在超导体上方的测量用方形永磁体N极向下, 则在轴对称情况下, 当方形辅助永磁体N极向上与超导体下表面贴在一起时, 超导体的最大磁悬浮力从没有引入辅助永磁体磁化的14.3 N增加到31.8 N, 提高到222%; 当方形辅助永磁体放置在超导体上表面、 N极垂直向上且场冷后去掉辅助永磁体时, 超导体的最大磁悬浮力从没有引入辅助永磁体磁化的14.3 N增加到21.6 N, 增加到151%; 当方形辅助永磁体放置在超导体上表面、 N极垂直向下且场冷后去掉方形辅助永磁体时, 超导体的最大磁悬浮力从没有引入辅助永磁体磁化的14.3 N减小到8.6 N, 减小为无辅助永磁体时的60%.这些结果说明, 只有通过科学合理地设计超导体和永磁体的组合方式, 才能获得较高的磁场强度, 有效地提高超导体的磁悬浮力特性, 该结果对促进超导体的应用具有重要的指导意义.     

Analysis of force characteristics of a superconducting ball in a given magnetic field

The electromagnetic force characteristics along Z direction of a superconducting ball levitated by spherical coils with shaping blocks are calculated based on a semi-analytical method. The calculating results from the semi-analytical method are compared with the finite element analysis (FEA) method through a calculation example. The method can be applied to further analysis of dynamic characteristics and parameter optimization in the suspension system.     

Critical current density of a YBa2Cu3O7 film: Comparison between experiment and collective pinning theory

With the vibrating reed technique we have measured the critical current densityJ _c of a Y 123 200 nm thin film as a function of magnetic field perpendicular to the CuO₂ planes (O T<B8 T) and temperature (20KT60 K). At fieldsB<0.1 TJ _c is magnetic field independent and decreases at higher fields. A comparison with theory indicates that a crossover from a single pinning to a small bundle collective pinning regime may explain the observed behaviour. According to our estimate the main pinning centers are weak point pins due to oxygen vacancies. From the temperature dependence ofJ _c atB0 we obtain a temperature dependence of the thermodynamical critical fieldB _c (1–T/T _c )² forT20 K which agrees with the anomalous temperature dependence ofB _c2 observed recently in highly anisotropic high temperature superconductors.     

Effects of crossed columnar defects on vortex pinning in Bi2Sr2CaCu2O8

To investigate the existence of a splay effect in Bi₂Sr₂CaCu₂O₈ (Bi-2212), vortex pinning has been studied in different configurations of strongly inclined columnar defects (75 from the c axis), installed by heavy-ion irradiation. It is shown that the symmetry of the track setting with respect to the field direction is a more influent parameter than the presence of a dispersion in the track directions. We claim that the enhanced pinning efficiency which is observed in some splayed configurations of columnar defects in Bi-2212 can be interpreted without invoking a splay effect. Received 27 December 1999     

Curvature Effect on Compressible Turbulent Flow over a Wavy Wall

A fully developed compressible turbulent flow in a channel with a lower wavy wall and a upper plane wall is studied using large eddy simulation. We mainly attempt to deal with the curvature effect on compressible turbulent flow over the wavy wall. Some typical quantities including the mean turbulence statistics, dilatation and baroclinic terms in the enstrophy equation, turbulent kinetic energy budgets and the near-wall turbulent structures are analysed. The results obtained in this study provide physical insight into the understanding of the effects of curvature and compressibility on wall-bounded compressible turbulent flow.     

Effect of the characteristics of a superconductor on the levitation properties of the magnet-superconductor system

The results of the experimental and theoretical investigations of the magnetic levitation force appearing at the interaction of the multilayer superconducting block of the YBa₂Cu₃O_7?x melted textured ceramic and a permanent magnet are presented. The maximum repulsive force and maximum attractive force are determined as functions of the thickness of the superconducting block in the superconductor cooling regime in both zero and nonzero magnetic fields. The dependence of the levitation force on the geometric parameters and critical current of the superconductor is found.     

Suppression of guidance force decay of HTS bulk exposed to AC magnetic field perturbation in a maglev vehicle system

Superconducting maglev vehicle was one of the most promising applications of HTS bulks. In such a system, the HTS bulks were always exposed to AC external magnetic field, which was generated by the inhomogeneous surface magnetic field of the NdFeB guideway. In our previous work, it was observed that the guidance force of the YBCO bulk over the NdFdB guideway used in the high-temperature superconducting maglev vehicle system was decayed by the application of the AC external magnetic field. In this paper, we adopted a method to suppress the decay by altering the field–cooled height of the bulk. From the experimental results, it was found that the decay rate of the guidance force was smaller at lower field–cooled height. So we could suppress the guidance force decay of HTS bulk exposed to AC external magnetic field perturbation in the maglev vehicle system by reducing the field–cooled height of the bulk. Furthermore, all the experimental results in this paper were explained based on Bean critical-state model.     

Experimental evidence for anisotropic response of a driven vortex lattice

We present kinetic inductance measurements in current driven Josephson junction arrays. Measurements performed with circular coils indicate that this technique is very sensitive to the vortex mobility. Results obtained with anisotropic detection coils provide experimental evidence for the anisotropic response of a driven vortex lattice. Anisotropic behavior is obtained when rising the temperature or the mean vortex velocity as indicated by the loss of the superconducting shielding capability, first in the direction of vortex motion and then in the perpendicular direction.