Magnetic Levitation With High-Tc Superconducting Thin Films

The levitation force between a permanent magnet and a superconducting thin film was investigated experimentally. The configuration consisted of a cylindrical NdFeB permanent magnet placed above a circular YBa₂Cu₃O_7– disk with common cylinder axis. Precise measurements were made of the vertical force F _z and the magnetic stiffness _z as a function of the magnet–superconductor separation at 77 K. Several features contrasting the levitation force produced using bulk superconductors were observed. Thin films produced very high values for F _z and _z per unit volume of superconducting material. The hysteretic behavior of F _z during decreasing and increasing separation resulted in loops of nearly symmetrical shape, which also contain a peak in the repulsive force branch. The observations are analyzed and explained with good quantitative agreement using recent theories for flux penetration in thin superconductors in transverse magnetic fields.     

Characteristic of the Guidance Force in the High-T c Superconducting Levitation System with Pre-Load Process

The stable levitation above permanent magnet is an important characteristic of the bulk high-T _c superconductor (HTS). When an external force pushes the bulk HTS up, down or sideways, or tries to tilt it, a restoring force can return it to its initial position. The HTS Maglev relied on this characteristic can overcome the external force from wind or pass the curve lines successfully. The change of guidance force (GF) during many times lateral movement is studied. Experiments show that GF increases during the lateral movement, no matter what kind of PMG or HTS is used, and the change of the GF slows down after 5 times lateral movement. The pre-load method can reduce the levitation force decay during lateral movement. So the influence of GF by the pre-load method is needed to be studied. It is found that the pre-load method can increase GF and reduce the change of the GF during lateral movement. The Halbach permanent magnetic guideway (PMG) can offer much more GF but the change is larger just as the levitation force decays. The GF of cylindrical bulk HTS increases more than of the rectangular bulk HTS in the pre-load case. The characteristics of the GF during the lateral movement are explained. These results are important for further HTS Maglev vehicle system designs.     

The Maximum Levitation Force of High-T c Superconductors

In this paper we present the dependence of the maximum levitation force (F _z ^max) of a high-T _c superconductor (HTS) on the structural factors of high-T _c superconducting systems based on the Bean critical state model and Ampère’s law. A transition point of the surface magnetic field (B _s ) of a permanent magnet (PM) is found at which the relation between F _z ^max and B _s changes: while the surface magnetic field is less than the transition value the dependence is subject to a nonlinear function, otherwise it is a linear one. The two different relations are estimated to correspond to partial penetration of the shielding currents inside the superconductor below the transition point and complete penetration above it respectively. The influence of geometric properties of superconductors on the dependence is also investigated. In addition, the relation between F _z ^max and the critical current density (J _c ) of the HTS is discussed. The maximum levitation force saturates at high J _c . An optimum function of the J _c and the B _s is presented in order to achieve large F _z ^max.      

The Influence of Excitation Frequency on Magnetic Levitation Systems with a High-T c Superconductor

In this paper we present a numerical analysis of dynamic features of the levitation system generated by an interaction between a levitated permanent magnet (PM) and a high-T _c superconductor (HTSC) excited by an oscillatory external source. The obtained results show that the value of the frequency (f _free) of the PM displacement in the case of the levitation system generated by an interaction between a levitated PM and a fixed HTSC is equal at the resonance frequency (f _re) of the levitation system generated by an interaction between a levitated PM and HTS excited by an oscillatory external source and the resonance frequency (f _re) is mainly dependent upon the cooling position (Z ₀) and the mass of the PM. The numerical problem in this paper is solved by using the control volume method (CVM).     

Nonlocal in-plane Resistance in High-T c Superconducting Films in the Vicinity of a Kosterlitz-Thouless-Type Transition

We have measured the nonlocal in-plane resistance in high-T _c superconducting Yba ₂ Cu ₃ O _7–d films. The results obtained in zero magnetic field show a negative nonlocal resistance in the vicinity of the Kosterlitz-Thouless-type vortex-antivortex unbinding transition, as it was predicted recently by Wortis and Huse [Phys. Rev. B 54 , 12413 (1996)]. We also found that the negative nonlocal resistance takes place within a certain domain of the current-temperature diagram. Our findings indicate that nonlocal measurements in planar geometry can be a new method to study vortex dynamics in superconductors.     

高温超导磁悬浮测试系统

介绍一种高温超导磁悬浮测试系统,该系统主要包括液氮低温容器,永磁体轨道,数据采集和处理,机械传动和自动控制四部分,采用了能放量高温超导体块材的薄底液氮低温容器（杜瓦容器）,解决了将高温超导体块材置于永久磁体之上的磁悬浮测量问题,能实时检测多块或单块高温超导体块材的各种高温超导磁悬浮性能。     

Electromagnetic Forces of High-T c Superconducting Coated Conductor Coils Subjected to Sinusoidal Traveling Magnetic Fields

In order to optimize the structure of high temperature superconducting coils for linear motor applications, three separate coils with different shapes made of Re-BCO coated conductor were studied: circular shaped single pancake, circular shaped double-pancake and racetrack shaped single-pancake. The thrust and vertical forces of the three coils above a conventional flat linear three-phases winding were investigated experimentally. With the aid of the experimentally obtained values, it was found that single-pancake coil in the shape of a racetrack was the best selection for a flat single-sided linear motor system. Studies were also made on the frequency characteristics of the vertical force of the racetrack shaped single-pancake coil.     

Correlation between Superconducting Gap and Pseudogap in High-T c Cuprates

We studied the low-temperature energy gap 2 ₀ on Bi ₂ Sr ₂ CaCu ₂ o ₈₊ (Bi2212) and La _2–x Sr _x CuCO ₄ (La214) systematically over a wide range of doping level p using STS, break junction tunneling spectroscopy, Raman scattering and low-T electronic specific heat data. We have also studied the electronic specific heat of La214 in the normal state at T > T _c , and confirmed that pseudogap behavior appears at around T*, below which the in-plane resistivity and magnetic susceptibility tend to be slightly suppressed. Similar suppression appears in and of Bi2212 below the onset temperature of pseudogap T*. It is pointed out in the present study that 2 ₀ is closely related to T* in both Bi2212 and La214 systems; T* 2 ₀ /4.3k _B . It is also pointed out that 2 ₀ is in almost linear proportion to k _B T _max ( T*), where T _max is the temperature exhibiting a broad peak in –T curves and k _B T _max can be considered to give a measure of the effective antiferromagnetic exchange energy J _eff. The factors in 2 ₀ k _B T _max J _eff are 1 for La214 and 2 for Bi2212, respectively. We also report that in both Bi22l2 and La214 systems T _c roughly scales with p ₀ except in highly doped samples, where T _c 2 ₀ .     

Study of Superconducting Properties in Bismuth-Based Cerium Doped High-T c Superconductors

Superconductivity in bismuth-based high-T _c superconducting materials attracts the researchers for their unique properties. Bismuth-based superconductors commonly called BSCCO have great importance among the superconducting family. These are divided into three phases among them 2223 phase is highly studied in order to investigate its superconducting properties by substitution of different elements. We have studied the substitution of cerium (Ce) on the calcium site of bismuth-based Bi(Pb)Sr(Ba)-2223 high-T _c superconductor. The nominal compositions of Bi_1.6Pb_0.4Sr_1.6Ba_0.4(Ca_1?x Ce _x )₂Cu₃O _x ceramic superconductor were prepared by the sol–gel method. X-ray diffraction (XRD) was done at room temperature for structural analysis and different parameters were calculated. Surface morphology was done by scanning electron microscopy (SEM). DC resistivity measurements for the transition temperature of synthesized superconducting samples were taken by the standard four-probe method, apparatus for which was developed in our laboratory. Current density measurements were also taken by the same apparatus. The synthesized superconducting samples were also characterized by thermogravimetric analysis (TG) and Fourier transformation infrared radiations (FTIR). It is observed that the substitution of cerium on the calcium site favors the formation of single high-T _c 2223 phase.     

How the Surface Resistance R s of Patterned High-T c Superconducting Thin Film Is Affected by the Film's Edge

This paper defines an effective microwave surface resistance $R_{\text{s}}^{{\text{eff}}}$ for the nonuniform distribution of microwave surface resistance R _s in the strip of a microstrip. It is proved that $R_{\text{s}}^{{\text{eff}}}$ is equivalent to the expression of R _s used in experiments, and that the $R_{\text{s}}^{{\text{eff}}}$ is dominated by the edge part, i.e., the area of width λ²/2t from the strip edge, where λ is the magnetic penetration depth and t is the film thickness. Under the assumption that $R_s \sim \left( {H_{{\text{rf}}}^y } \right)^n$ where $H_{{\text{rf}}}^y$ is the component of rf magnetic field along the film thickness and n is an integer, the ratio of the contributions of the edge part and the rest of the strip to $R_s^{{\text{eff}}}$ is calculated by using an approximate analytical expression of the surface current density distribution J _s in the strip and $H_{{\text{rf}}}^y$ calculated by the London equation. The effect of film's edge on R _s was studied using a microstrip resonator. It is found that the perfectness of the edge could affect the magnitude of the power dependence of R _s significantly, which agreed with our analysis.     

Levitation Force Between a Point Magnetic Dipole and Superconducting Sphere

The magnetic interaction between a point dipole and a superconducting sphere in the Meissner state is investigated in detail. It is demonstrated that the special case of perfect flux exclusion can be solved by the method of images when the dipole is vertically magnetized. The incorporation of a nonzero penetration depth is then made by developing the solution within London theory. The analytic solution, given in terms of an infinite series representation, is refined by exactly summing the perfectly diamagnetic response. Throughout, special cases of physical interest are discussed and applications to magnetic force microscopy and levitation are described.     

Effects of Small Fields on the Magnetic Response of Superconducting Thin Films of MgB2

Under the influence of a magnetic field applied perpendicularly to its plane, a superconducting thin film develops dendritic patterns of penetrated regions that coexist with Meissner areas. For a thin film of MgB₂ submitted to an alternate field of moderate amplitude, the AC susceptibility measured while cooling the sample exhibits a quite unusual behavior, reentering and fluctuating with temperature. The effect is more pronounced at frequencies around 1 kHz. Two plausible explanations for the effect are discussed.     

Intermodulation Measurements on High Temperature Superconducting Thin Films

A high temperature superconducting GdBa₂Cu₃O_7– thin film was characterized using a parallel plate resonator at 5.5 GHz and a dielectric puck resonator at 10 GHz, to observe the dependence of surface resistance on microwave power at the fundamental frequency, and the production of intermodulation products using a two-tone measurement setup, respectively. The electromagnetic field pattern of the dielectric puck-HTS thin film mode was modeled using the commercial software package MAFIA. The intermodulation product was compared with a simple RSJ model.     

Thickness-Dependent Current Density and Flux Distribution in Thin Current-Carrying Superconducting Films Exposed to a Magnetic Field

With the rapid development of high-temperature superconductors, it is of great significance to get the precise current density and flux distribution within thin high-temperature superconducting films subjected to a transport current and an applied magnetic field. The transport current distribution and flux density in thin high-temperature superconducting films are calculated by a numerical method based on the Kim model and exponential model in this paper. The influences of transport current, applied magnetic field, width, and thickness of a superconducting film on the current distribution are discussed. The results reveal that the thickness has a significant effect on the critical current density of superconducting films.     

The Edge Effect of a Microstrip Resonator on the Field Dependence of the Surface Resistance of a High-T c Superconducting Thin Film

A new behavior of the field dependence of microwave surface resistance (R _s), which was observed on a microstrip resonator and may be caused by the edge of the center strip, is reported in this paper for epitaxial high-T _c superconducting (HTSC) thin films. The exhibited behavior is that R _s remains almost unchanged below a certain rf magnetic field H _rf, and then increases abruptly at this field, after which it increases in proportion to H _rf. To explain the behavior, the morphology of the microstrip resonator was examined by atomic force microscopy (AFM), which showed that the edge of the resonator was damaged in some regions because of the acid etching. If the damaged edge is considered as a weakened granular superconductor, the observed R _s behavior could be explained well in terms of the high-frequency critical state model. This implies that the edge condition should be considered in studying the field dependence of R _s when the planar resonator technique is used.     

Superconducting Ti/TiN Thin Films for mm-Wave Absorption

Polarization-sensitive detectors at 120–500 GHz are required for the observation of the cosmic microwave background radiation. In this paper, superconducting thin films based on Ti/TiN bilayers are developed to be integrated as electromagnetic wave absorbers in suspended cooled silicon bolometers. The critical temperature (T_c) is tuned in the range of 600–800 mK through the superconductivity proximity effect between Ti and TiN to optimize the absorption of the incident power while minimizing the heat capacity of the system at low temperature. Ti/TiN bilayer samples are fabricated on silicon with two different thicknesses (100/5 and 300/5 nm). Electrical characterizations at low temperature have been performed and revealed the effect of thermal annealing (20–250 °C) on residual stress, T_c, critical magnetic field (H_c) and resistance above T_c. A physical characterization by X-ray photoelectron spectroscopy provides evidences of oxidized states which may explain these effects.     

Deposition and Properties of Superconducting MgB2 Thin Films

The recently discovered superconductor MgB₂ with T _c at 39 K has great potential in superconducting electronics. In this paper, we review the deposition techniques used for MgB₂ thin films in the light of a thermodynamic study of the Mg-B system with the calculation of phase diagrams (CALPHAD) modeling technique. This thermodynamic study identifies a growth window in the pressure–temperature phase diagram, in which the magnesium pressure is very high for likely in situ growth temperatures. A Hybrid Physical–Chemical Vapor Deposition (HPCVD) technique that successfully achieves such a high Mg pressure is shown to produce in situ epitaxial MgB₂ thin films with bulk superconducting properties.     

激光沉积高温氧化物超导薄膜

本文简要介绍激光沉积技术的原理、特点及国外科研机构采用这种方法制作陶瓷氧化物高温超导薄膜的技术参数等。本文还探讨了激光沉积超导薄膜的形貌及激光束与靶材的交互作用情况,并展望了这种技术的应用前景。