Extraction of Critical Current Density and Flux Creep Exponent in the Magnetic Superconductor Ru-1212 Using the ac Magnetic Susceptibility Measurements

The ac susceptibility data was employed to extract the temperature dependence of the critical current density, J _c(T), as well as the variation of flux-creep exponent n(T,H _ac) with temperature and ac field amplitude in bulk samples of polycrystalline magnetic superconductor RuSr₂GdCu₂O₈ (Ru-1212). The critical state models and the collective flux-creep approximation model were successfully accounted to describe such behavior below the transition temperature. The calculated values of n(T,H) are well fitted to a power law of the following form: n(T,H)=n ₀(H)T ^−s(H), where s is field dependent exponent whose values varied from −2.4, −1.01 for field amplitudes ranging from 0.5 G and 3.8 G. The power law describing the frequency dependence of χ′ is found to be consistent with the results of the current-dependent effective activation energy of the form U(J)=U ₀ln (J _c/J). Additionally, the dependence of the current density is found to scale according as: J _c(T)=J _c0(1−T/T _c) ⁿ , where the exponent n values varied from 1.05 to 1.25. Such dependence is an indication of intergrain coupling that could be ascribed in terms of superconductor–insulator–superconductor junctions. The derived temperature dependence of J _c(T) is in good agreement with the data obtained from the measurements using the traditional “loss-maximum” approach. Furthermore, the flux-creep effect increased with increasing both ac fields and temperatures except at about 15–25 K below the onset of T _c, where a slowing down of the flux creep was observed.     

Influence of the Trapped Field on the Levitation Performance of the Magnetized Bulk High-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Superconductor

A magnetized bulk high-T _c superconductor (HTSC) magnet is a good candidate to improve the levitation performance of the high-T _c superconducting (HTS) maglev system. Compared with the unmagnetized bulk HTSC, the magnetized bulk HTSC magnet can supply stronger levitation or guidance force above a permanent magnet guideway (PMG). Different from the permanent magnet, the magnetic field of a magnetized bulk HTSC magnet is sustained by the induced superconducting current produced during the magnetizing process. Given that the induced superconducting current within the magnetized bulk HTSC magnet is very sensitive to the magnetic field, the levitation performance of the magnetized bulk HTSC magnet is directly related to its own trapped field and the magnetic field of the PMG. This article discusses the influence of trapped and external magnetic fields on the levitation performance of a magnetized bulk HTSC magnet by experiments, and the Critical State Model is used to analyze the test results. The analyses and conclusions of this article are useful for the application of magnetized bulk HTSC magnet in practical HTS maglev systems.     

Flux Concentrator Optimization of PMG for High-Temperature Superconducting Maglev Vehicle System

The permanent magnetic guideway (PMG) composed of permanent magnet (PM) and steel is developed under flux concentration principle, which is the crucial component of high-temperature superconducting (HTS) maglev vehicle system. Optimum PMG design is an effective way to increase levitation force and associated stiffness for improving the load capability of HTS maglev vehicle. In order to realize higher vertical field component B _z in upper surface, three PMG demonstrators with three different forms of flux concentrator are fabricated with same volume of magnet. The levitation performances of onboard HTS bulks array over them are studied. The experimental results indicate that the PMG with a permanent magnet as the flux concentrator would produce biggest levitation force, levitation stiffness and trapped flux when interacting with HTS superconductor.     

Synthesis,Characterization and A.C. Susceptibility of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> / Polyethylene Composites

A series of flexible composites are fabricated by mixing a high-T _c YBa₂Cu₃O_7−x superconductor with Low Density Polyethylene (LDPE). The synthesized polymer composites are characterized by X-ray diffraction technique. Superconducting nature of the composites is confirmed by the Meissner effect. The surface morphology of the composites is studied by scanning electron microscopy. The crystallization temperature of the polymer and its composites is determined by using differential scanning calorimetry (DSC). The composites showed a large diamagnetic susceptibility that increases with increasing volume fraction of superconductor filler. Susceptibility measurements showed that the intrinsic diamagnetic properties of the superconducting materials are preserved in the composites and there is no change in the transition temperature of the superconductor. Potential applications in the areas of shielding and levitation are discussed.     

AC susceptibility for superconducting slab with regions of different critical-current densities

Based on the flux creep model, the temperature T and the amplitude of the AC field B₀ dependence of the complex AC susceptibility as a function of temperature are numerically calculated in a superconducting slab composed by three parts connected by two regions of lower critical-current density J_c which characterize the non-uniform J_c distributions in inhomogeneous superconductors. By a comparison with the analytical results of a homogeneous superconductor with a constant J_c, the numerical results show that the fundamental AC susceptibility of the composite slab is obviously different from the case of the homogeneous superconductor.     

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).     

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.     

Remagnetization effects due to lateral displacement above a PMG on bulk HTS magnet

For a high-T_c superconducting (HTS) maglev system with large force requirements, the use of magnetized bulk high-T_c superconductor magnets (MBSCMs) is a good candidate because of its strong flux pinning ability and corresponding high trapped flux. Different from the rare-earth permanent magnet (PM), the trapped flux of a MBSCM is sustained by the supercurrent produced by a magnetizing process, so the trapped flux is sensitive to variations of the supercurrent. The lateral displacement of a MBSCM above a PM guideway (PMG) will provide disturbance of the applied field and then alter the supercurrent as a process of remagnetization. Different magnetization histories will bring different remagnetization characteristics and consequently diverse levitation performances for a MBSCM during the lateral displacements. When the MBSCMs are applied into the HTS maglev system, the influence of lateral displacements on levitation performance should be taken into consideration. This article investigates the remagnetization characteristics of a MBSCM when it is subject to the lateral displacements above a PMG with different trapped magnetic flux and opposite magnetization polarities. Relevant analyses about the internal supercurrent configuration based on the critical state model are also included to better understand the remagnetization characteristic of a MBSCM.     

How Grain-Boundaries Influence the Intergranular Critical Current Density of Cu1−x Tl x Ba2Ca3Cu4O12−δ Superconductor Thin Films?

The insulating and metallic behavior of the grain-boundary weak links has been studied in thallium rich and the samples with small amount of thallium in the charge reservoir layer of Cu_1−x Tl _x Ba₂Ca₃Cu₄O_12−δ superconductor thin films. The influence of the nature of grain boundaries on the inter-granular critical current density (J _c) has also been investigated. From the power law dependence of H _ac∼(1−T _p/T _c) ⁿ , it was observed that n=1 gives a best fit for the J _c of thallium rich samples and n=2 provides a best fit for the J _c of the samples with small amount of thallium. The polycrystalline thin film samples showing the power law dependence of J _c as n=1 make superconductor-insulator-superconductor (SIS) type while the samples with n=2 follow superconductor-normal metal-superconductor (SNS) types of Josephson junctions. The insulating grain boundaries decrease the inter-granular Josephson coupling and hence the transport properties are suppressed.      

Magnetic Properties and Phase Transitions in a Diluted Ferromagnet: Ising,XY, and Heisenberg Models

The magnetic properties and phase transition of a ferromagnetic spin-S disordered diluted thin film with a face-centred cubic lattice are investigated using the high-temperature series expansion technique extrapolated with Padé approximant method for Heisenberg, XY and Ising models. The reduced critical temperature of the system t_c=\frack_BT_c2S(S+1)J_b\tau_{\mathrm{c}}=\frac{k_{\mathrm{B}}T_{\mathrm{c}}}{2S(S+1)J_{\mathrm{b}}} is studied as the function of the thickness of the thin film and the exchange interactions in the bulk, and within the surfaces J _b,J _s and J _⊥, respectively. It is found that τ _c increases with the exchange interactions of surface. The magnetic phase diagrams (τ _c versus the dilution x) and the percolation threshold are obtained. The shifts of the critical temperatures T _c(L) from the bulk value (\fracT_c(￥)T_c(L)-1)(\frac{T_{\mathrm{c}}(\infty )}{T_{\mathrm{c}}(L)}-1) can be described by a power law L ^−λ , where λ is the inverse of the correlation length exponent.     

Two-Peak Temperature Dependence of Microwave Surface Impedance of Single-Crystalline YBCO Thin Films: Role of Pairing Symmetry and Defect Structure

Temperature dependencies of microwave surface impedance were measured for c-oriented highly perfect YBCO thin films deposited by off-axis dc magnetron sputtering onto CeO₂-buffered r-cut sapphire substrates. A distinct two-peak structure of R _s(T) and X _s(T) dependencies with peaks at 28–30, K and 50, K has been revealed. The peaks become smeared at higher frequencies or in applied dc magnetic field, while the peak positions remain almost unchanged. The two-peak Z _s(T) behavior is believed to be an intrinsic electron property of extremely perfect quasi-single-crystalline YBCO films. A theoretical model is suggested to explain the observed anomalous Z _s(T) behavior. The model is based on the Boltzman kinetic equation for quasiparticles in layered high-T _c superconductors (HTS) cuprates. It takes into account the supposed s + d wave symmetry of electron pairing and strong energy-dependent relaxation time of quasiparticles, determined mainly by their elastic scattering on extended defects parallel to the c-axis.     

Critical current densities and irreversibility fields of high-Tc superconductors AuBa2Can−1CunO2n+3 (n=3,4) prepared under high pressure

Critical current densities (J_c) and irreversibility fields (B_irr) of high-T_c superconductors AuBa₂Ca_n−1Cu_nO_2n+3 [Au-12(n−1)n, n=3,4] prepared under high pressure were determined from the hysteresis in DC magnetization loop. Both the J_c and B_irr values of the Au-1234 phase were larger than those of Hg- and Bi-based cupurate superconductors, but smaller than those of the Y-123 superconductor. This result suggests that the electronic structure of Au-1234 is less anisotropic compared with the Hg- and Bi-phases. The Au-1223 phase showed very small J_c values corresponding to its small superconducting volume fraction.     

Structural and Physical Properties of Nd Substituted Bismuth Cuprates Bi1.7 Pb0.3−x Nd x Sr2Ca3Cu4O12+y

BiPb-2234 bulk samples with nominal composition of the compound Bi_1.7Pb_0.3−x Nd _x Sr₂Ca₃Cu₄O_12+y (BSCCO) (0.025≤x≤0.10) have been prepared by the melt-quenching method. The effects of Nd substitution on the BSCCO system have been investigated by electrical resistance (R–T), scanning electron microscopy (SEM), X-ray diffraction (XRD) and magnetic hysteresis measurements. It has been the BSCCO (2212) low-T _c phase is formed for all the substitution levels, together with the BSCCO (2223) high-T _c phase. The results obtained suggest that with increasing Nd³⁺ doping for Pb²⁺ the (2223) phase existing in undoped BSCCO gradually transforms into the (2212) phase and hence all of the samples have a mixed phase formation. The R–T result of the samples show two-step resistance transition; first transition occurs at 100 K and second in an interval of 80–90 K, depending on the Nd concentration. We have found that the magnetization decreases with increasing temperature in agreement with the general characteristic of the high-T _c materials. The samples exhibit weak field dependence particularly after 2 T and changes on the magnetic hysteresis, M–H curve rather small compared to the conventional superconducting materials. The maximum critical current density, J _c, value was calculated to be 8.51×10⁵ at 4.2 K and J _c decreases with increasing temperature and the substitution level.      

New Insights on the Intrinsic Anisotropy of YBCO Films

Careful investigation of the angular dependence of resistivity ρ(θ) (θ is the angle between the magnetic field and the ab-planes) and the temperature dependence of resistivity ρ(T) within the superconducting transition in an applied magnetic field B up to 1 T for a series of YBa₂Cu₃O_7−δ (YBCO) thin films revealed a large variation of intrinsic anisotropy factor γ. The series of films studied included both optimally doped and underdoped samples of different T _c , critical current density J _c , film thickness, and preparation techniques. The variation in the shape and depth of the minimum measured for ρ(θ) near θ=0° could be directly correlated to the intrinsic anisotropy of the YBCO films. The results of fitting of ρ(θ) using Bardeen–Stephen theory allowed a quantitative determination of the value of γ which varies between 7 and 230, and is independent of T _c , film thickness, or J _c . The sharper the minimum in ρ(θ) around θ=0° the larger is the anisotropy. For highly anisotropic film, ρ(θ) showed an identical behavior for B ∥ J and B ⊥ J (i.e., ρ(θ) is independent of the angle θ between B and J for this film). The large variation in γ could be attributed to the “buckling” of the CuO₂ planes.     

Feshbach Shape Resonance in Multiband Superconductivity in Heterostructures

We report experimental data showing the Feshbach shape resonance in the electron doped MgB₂ where the chemical potential is tuned by Al, Sc, and C substitutions. The scaling of the critical temperature T _c as a function of the Lifshitz parameter z = E _Γ−E _F, where E _F is the chemical potential and E _Γ is the energy of the Γ critical point where the σ Fermi surface changes from the 3D to a 2D topology, is reported. The resonant amplification of T _c(z) driven by the interband pairing is assigned to a Feshbach shape resonance characterized by quantum superposition of pairs in states corresponding to different spatial location and different parity. It is centered at z = 0 where the chemical potential is tuned to a Van Hove-Lifshits feature for the change of Fermi surface dimensionality in the electronic energy spectrum in one of the subbands. In this heterostructure at atomic limit the multiband superconductivity is in the clean limit because the disparity and negligible overlap between electron wavefunctions in different subbands suppresses the single electron interband impurity scattering rate. The emerging scenario from these experimental data suggests that the Feshbach shape resonance could be the mechanism for high T _c in particular nanostructured architectures.     

Fluctuation conductivity of Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CaCu<Subscript>2</Subscript>O<Subscript>8 + δ</Subscript> in a two-band superconductivity model

A modified analytical Aslamazov-Larkin formula for the temperature dependence of the fluctuation conductivity near the critical temperature T _c is obtained for two-band superconducting compounds on the basis of linearized equations of the two-band Ginzburg-Landau theory. The results are compared to experimental data for the high-T _c superconductor compound Bi₂Sr₂CaCu₂O_{8 + δ}.     

High-temperature superconducting microwave devices: Fundamental issues in materials, physics, and engineering

High-T _c superconductivity has generated a great deal of interest because of the challenges it presents in the fields of material science, condensed matter physics, and electrical engineering, and because of the potential applications which may result from these research efforts. Thin-film passive microwave components may become the first high-temperature superconducting (HTS) devices available for widespread use and commercialization. In this article, we review aspects of material science, physics, and engineering which directly impact high-T _c superconducting microwave devices and discuss issues which determine the performance of these devices. Methods of growing HTS thin films on large-area substrates, techniques for fabricating single-level HTS passive microwave components, and the relevant properties of high-T _c superconducting films are discussed, with a focus on thin films of the HTS material YBa₂Cu₃O_7–. Several known mechanisms for microwave loss in both the superconductor and the dielectric substrate are presented. An overview of the general classes of superconducting passive microwave devices is given, and representative microwave devices which have been recently demonstrated are described in detail. Examples of a select few HTS active microwave components are also presented. Potential microwave applications are illustrated with comparisons to current technology.     

Superconductivity in the Pb-based 1222 Cuprate Containing Boron, (Pb0.5B0.5) Sr2(Y1.9−x Ce x Sr0.1)Cu2O z

We previously reported on the discovery of new Pb-based 1222 cuprates containing boron in the (Pb_0.5B_0.5)Sr₂(RE_2−x−y Ce _x Sr _y )Cu₂O _z (RE = Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Y) systems. However, none of the cuprates was a superconductor, but a semiconductor with the conduction process of a three-dimensional variable range hopping at low temperatures, despite annealing under O₂ atmosphere at 100 atm and 400 °C. Recently, we have discovered that some samples showed superconductivity in the (Pb_0.5B_0.5)Sr₂(Y_1.9−x Ce _x Sr_0.1)Cu₂O _z system. Among them, the sample with x=0.45 shows the highest zero resistivity temperature at about 10 K and it shows a diamagnetic signal starting at abut 16 K with lowering temperature. The superconductivity is considered to originate from the 1222 phase.     

A Possible Explanation of Critical Temperature and Isotope Effect Coefficient of High Tc Cuprate Superconductors

The expressions for superconducting transition temperature (T _c) and isotope effect coefficient () have been derived from a generalized integral gap equation for a strongly coupled superconductor, when electrons–phonons and electrons–biexcitons are simultaneously present in high-T _c cuprate superconductors.     

Influence of Re-magnetized HTS Bulk Samples on the Levitation Performances of a Maglev System

High-T _c superconducting (HTS) bulk samples used for a maglev system can be re-magnetized after being subjected to another applied magnetic field which may influence the levitation performance of a HTS bulk over the permanent magnetic guideway (PMG). In order to clarify the relationship between the HTS bulk’s re-magnetization and its levitation performance, a YBCO bulk was re-magnetized at different values and its levitation performance studied experimentally above a permanent guideway (PMG) system. The results show that the increase of trapped magnetic flux inside the HTS bulk is closely related to the magnitude and direction of the applied field during the re-magnetization process. Furthermore, the levitation force changes on the HTS-PMG system affected by the re-magnetized HTS bulk are discussed allowing for optimization improvements to the maglev system load capabilities.