Analysis of Nonlinearities in Superconducting Microstrip Straight Bends; FDTD Method in Comparison with Nonlinear Circuit Modeling

This paper presents the prediction of nonlinearities in the superconducting microstrip straight bends in microwave frequencies based on two different methods; FDTD simulation as a numerical approach, and nonlinear circuit modeling as an analytical method. In the FDTD method, the superconducting microstrip structures are simulated with London’s equations. In the simulation, the penetration depth and normal conducting coefficient are considered as functions of current density of superconductor. To simulate the thin strip of superconductor, a non-uniform mesh has been used. For the nonlinear circuit modeling, we use distributed RLGC parameters for superconducting microstrip transmission lines. These parameters are considered as functions of the current distribution. This yields an equivalent nonlinear circuit model for bends. The final equivalent nonlinear circuit is analyzed using the harmonic balance (HB) method. Different straight bend structures have been considered and the two methods’ results are compared.     

Gibbs free-energy barrier against irreversible magnetic flux entry into a superconductor

We have calculated the Gibbs free-energy barrier against irreversible magnetic flux entry into a superconductor for a long cylinder with elliptical cross section which approximates a long, flat strip. Our model is simplified to the two-dimensional case by assuming magnetic flux to enter in the form of a long, narrow, normal domain parallel to the axis of the cylinder. The following four contributions to the Gibbs free energy have been taken into account: (1) loss of condensation energy and gain of magnetic field energy inside the superconductor, (2) magnetic field energy outside the superconductor, (3) energy of interaction of the domain with an applied magnetic field, and (4) energy of interaction with an applied electrical transport current. Because of the Gibbs free-energy barrier, the critical magnetic field for entry of magnetic flux can be enhanced considerably above that calculated using Silsbee's rule. This enhancement is found to be proportional to the square root of the width of the superconducting cylinder. Important consequences of this are the enhancement of the critical current in a superconducting strip in zero magnetic field at which electrical resistance starts to appear and a corresponding modification of Silsbee's rule. We have demonstrated these effects experimentally through measurements of the onset of the current-induced resistive state in a series of superconducting indium strips of different widths and thicknesses. The experimental results confirm the theoretical predictions. The Gibbs free-energy-barrier effect described here can be interpreted as a novel flux-pinning mechanism, which might be called edge pinning.Work performed in part in the Ames Laboratory and in part in the Argonne National Laboratory of the U.S. Atomic Energy Commission.     

Study of phonon-injection-induced destruction of superconductivity in tin films

We have investigated the phenomenon of the destruction of superconductivity in thin film strips of tin by injecting direct currents through a transverse normal metal (silver) strip in direct contact with the superconducting strip. We observe that the critical current of the superconducting tin strip decreases continuously (in some cases with an intermediate jump discontinuity) with increasing injection current. From a careful consideration of various possible mechanisms, such as current-induced depairing, simple heating, and nonequilibrium superconductivity, we infer that the experimental observations could be mostly due to steady-state nonequilibrium phenomena associated with heat pumping from various locally heated regions.Research carried out at the Tata Institute of Fundamental Research.     

Steady States of Current Carrying Layered Superconducting Film

The model of heat production by the transport current flowing in a layered superconducting film with nonlinear resistivity is proposed. It reflects the main features of the quasi-two-dimensional behavior of the system. The steady states of the superconductor and stability of the superconducting state are investigated within the framework of the model. Diagrams of the steady states of a layered superconductor are presented. The velocity of the resistive zone motion as a fuction of the transport current is calculated for superconductors with different anisotropy.     

Effect of superconductive destruction in YBa2Cu3 δ bulk bridges under the action of strong Joule self-heating

Transition from superconductive to normal state and back into superconductive state in bulk single crystal YBaCuO bridges with twins induced by a strong transport current has been investigated. The current-voltage characteristics (CVC), the temperature and magnetic field dependences of resistance [resistive transition (RT)] in the regime of fixed current nearT _c were studied in detail. A sharp dynamic superconductive transition or switching over effect, nonlinear CVC, and hysteresis were observed. The CVC and RT characteristics can be explained by the thermal bistability phenomena at strong Joule self-heating of bridges, in particular, in the presence of internal local inhomogeneities in superconductor. Electrical characteristics of bridges which can be used as nonlinear elements for cryogenic electronics were obtained.     

Theory of the AC Josephson effect in triplet superconducting junction

The quasiparticle and AC Josephson current in superconductor / insulator / superconductor (S/I/S) junctions including a triplet p-wave superconductor are calculated for arbitrary transmission of the junction. As a triplet pairing state we select one of the unitary pairing states which is a promising candidate for the superconducting state in Sr ₂ RuO ₄ . In p-wave superconductor / insulator /p-wave superconductor junction, both quasiparticle current and AC Josephson current are enhanced near zero-bias voltage due to the existence of zero energy state of p-wave superconductors. For the s-wave superconductor/ insulator / p-wave superconductor junction, the quasiparticle current at low voltage is suppressed due to the energy gap of s-wave superconductor. The first Fourier component of the AC Josephson current vanishes due to the difference of the parity between the two superconductors.     

P型半导体金刚石膜的磁阻

研究了p-型异质外延和同质金刚石膜的在不同温度和磁场下的磁阻,磁阻器件的结构为条形和圆盘形,实验结果表明磁阻强烈依赖于磁场、温度和样品的几何形状,圆盘结构的磁阻大于条形结构,条形结构的磁阻还取决于不同的长-宽比。利用F-S薄膜理论,计算磁场为5T时条形和圆盘结构的磁阻分别为0.38和0.74,讨论了霍耳效应对磁阻的影响。给出了形状效应的可能机制。     

Nonlinear current-voltage characteristics of normal-superconducting boundaries

The voltage drop at a normal-superconducting boundary is considered in the case when the current flowing across the boundary is of the order of the critical current of the superconductor. The finite supercurrent decreases the distance the electric field penetrates into the superconductor, and leads to a nonlinear current-voltage characteristic. Explicit calculations are carried out for a dirty superconductor close to the transition temperature.Supported in part by National Science Foundation Grant DMR 78-21068.     

Numerical Simulation of Magnetic Flux Penetration and AC Loss in?HTSC Coated Conductor Tapes

The critical state model developed nearly 50 years ago by Bean (Phys. Rev. Lett. 8:250, 1962) allows finding analytical solutions for the magnetization of a superconducting slab in a parallel field and for a thin strip in perpendicular field, as well as the transport of AC current by a tape with elliptic or strip-like cross-section. Direct application of these models to the currently available HTSC coated conductor tapes is problematic because of several factors:

• Dependence of critical current on the magnitude of magnetic field and its orientation
• Non-uniformity of superconductor properties across the tape width or thickness
• Magnetism of the substrate

Plausible solution is a numerical model for the process of magnetic flux penetration and associated time evolution of current density distribution. State-of-the-art commercial codes for finite element could cope with this problem. The numerical method based on the relation between the current density and the change of vector potential of magnetic field in two-dimensional geometry is presented and the results obtained for coated conductor tapes are reported. Influence of mesh parameters (density and shape of elements) as well as the thickness of boundary between opposite current densities on the calculated AC loss is analyzed. The recommendations for practical use of simulation method are formulated.     

薄金属带材连续高速电镀异型阳极

为了提高金属带材连续高速电镀的速度、质量和降低能耗,控制金属电沉积的电流密度和阳极形状是关键.考虑到金属带材高速电镀时传输的电流大,在镀区内的金属带材不是等电势体,导致金属电沉积的电流密度分布不均匀.为此,建立了恒定电流密度电镀的数学模型,推导出异型阳极形状的计算公式,为金属带材连续高速电镀的电流密度控制提供了理论依据.     

ZnSe薄膜的增强吸收光双稳

报道了用常压ＭＯＣＶＤ生长的ＺｎＳｅ薄膜的光学非线性。在７７Ｋ下用纳秒脉冲激光观测到ＺｎＳｅ薄膜的增强吸收光双稳特征，非线性机理可能是由激子Ｃｏｕｌｏｍｂ相互作用屏蔽引起的。     

Vortex in Holographic Two-band Superconductor

A single static vortex in a holographic two-band superconductor is constructed. We investigate the effect of the interband coupling to the condensate profile. We estimate the first critical magnetic field, and compute characteristic lengths like penetration and coherence lengths and conclude both bands in our superconductor behave type II.     

Development of a superconducting fault current limiter using various high-speed circuit breakers

The authors constructed and tested a model superconducting fault current limiter (SFCL) using a high-temperature superconducting film according to a design that includes a vacuum interrupter with an electromagnetic repulsion mechanism. The superconductor and the vacuum interrupter are connected in parallel with a bypass coil. If a fault occurs and current flows through the system, the superconductor is quenched and the current is transferred to the parallel coil because of the voltage drop in the superconductor. This large current in the parallel coil actuates the magnetic repulsion mechanism of the vacuum interrupter. On opening the vacuum interrupter, the current in the superconductor is interrupted. This model is expected to exhibit very low-energy consumption by the superconductor. The authors succeeded in interrupting the current flowing in the superconductor within a half-cycle using a prototype SFCL. An improved SFCL with higher voltage and current ranges was used to carry out current-limiting tests and to investigate the possibility of adapting our SFCL in a power system. The authors also carried out a currentlimiting test using a conventional high-speed vacuum circuit breaker (HSVCB) as a new method for realising our concept.     

薄带连铸用侧封板材料的研究现状

钢铁薄带连铸生产技术是轧制技术的一个重要发展方向,侧封板材料起着约束金属液体,促进薄带成型,保证薄带边缘质量等重要作用,影响着整个薄带连铸系统的稳定性以及成本的高低。介绍了薄带连铸技术侧封板的性能要求,综述了国内外侧封板材料的研究现状,分析了国内外所采用侧封板的材质种类及性能参数。结果表明,传统耐火材料不适合用作薄带连铸侧封板材质,以BN为基质的新型复合陶瓷材料是适合用于制备薄带连铸侧封板的材质。     

传导薄板的非线性磁弹性振动问题

研究了磁场环境中传导薄板的非线性磁弹性振动问题。由虚功原理,给出了磁场中薄板的磁弹性耦合运动方程。并根据薄板薄壳的磁弹性基本假设及麦克斯威尔方程,得到了方程中电磁力及力矩的表达式。作为具体问题,采用多尺度法求出了横向磁场中条形板非线性振动的近似解析解。通过算例,分析了磁场环境对振动周期和幅值的影响。从而证实了,通过改变磁场因素,可达到控制该磁场环境中传导薄板振动的目的。所得的结论,对工程实际将有较大的应用参考价值。     

Vortex dynamics at the initial stage of resistive transition in superconductors with fractal cluster structure

The effect of fractal normal-phase clusters on vortex dynamics in a percolative superconductor is considered. The superconductor contains percolative superconducting cluster carrying a transport current and clusters of a normal phase, acting as pinning centers. A prototype of such a structure is YBCO film, containing clusters of columnar defects, as well as the BSCCO/Ag sheathed tape, which is of practical interest for wire fabrication. Transition of the superconductor into a resistive state corresponds to the percolation transition from a pinned vortex state to a resistive state when the vortices are free to move. The dependencies of the free vortex density on the fractal dimension of the cluster boundary as well as the resistance on the transport current are obtained. It is revealed that a mixed state of the vortex glass type is realized in the superconducting system involved. The current-voltage characteristics of superconductors containing fractal clusters are obtained and their features are studied.     

Reversible Magnetic Behavior of Superconductors Forced by a Small Transverse AC Magnetic Field

Recently it was shown that with thin superconductors placed in a transverse dc magnetic field, the application of a small ac magnetic field perpendicular to the dc field causes rapid relaxation of the irreversible magnetization. We show here that for a thin superconducting strip in a transverse dc field the application of a weak ac field perpendicular to the dc field generates a dc voltage in the strip. This voltage leads to the decay of the critical currents circulating in the strip, and eventually the equilibrium state of the superconductor is established. This relaxation is not due to thermally activated flux creep but to the walking motion of vortices in the two-dimensional critical state of the strip with in-plane ac field. Our theory explains the shaking effect that was used for detecting phase transitions of the vortex lattice in superconductors with pinning. Some recent experiments are discussed.     

三向轮胎力作用下路面动力响应分析

摘 要：采用两自由度非线性汽车模型和地基上双层薄板建立三向轮胎力作用下的车-路系统。将非线性Gim模型与垂向点接触轮胎模型相结合对三向轮胎力进行描述。利用伽辽金方法推导三向轮胎力作用下路面响应的解析解,研究制动移线工况下各轮胎分力对路面响应的贡献,比较只考虑垂向轮胎力作用和考虑三向轮胎力作用时路面响应的差别,并分析系统参数对此差别的影响。研究成果有利于揭示车辆对道路的三向作用机理,为道路响应计算和道路寿命预测提供新的研究思路。     

1Cr18Ni9Ti不锈钢双辊薄带凝固组织区的形成机理

在双辊薄带连铸实验和薄带凝固组织特征分析的基础上,结合对薄带凝固组织区的模拟预测结果,研究了1Cr18Ni9Ti不锈钢双辊薄带凝固组织区(特别是等轴晶区)的形成机理.结果表明:1Cr18Ni9Ti不锈钢双辊薄带凝固组织中的等轴晶区不但在凝固类型为半固态时形成,在轧制或理想型时也能形成.其形成机理为,熔池中悬浮游离晶体的沉积、聚集以及在枝晶生长前沿的长大和薄带离开二铸辊最小间隙(铸辊出口)后,薄带/空气界面换热系数骤然降低抑制了柱状枝晶的生长,并促使薄带中部未凝固熔体中游离晶体的择优长大.