Trapped field and levitation force in melt-textured YBCO doped with Ni and Li

Trapped magnetic fields and levitation forces at 77 K have been studied in bulk melt-textured YBCO with single grains of 26 mm, doped with different content of Li and Ni. Li dopant results in higher trapped field than Ni indicating that Li additions are more effective in increasing pinning capabilities.     

Enhancement of the critical current density in YBCO/Ag composites

YBa₂Cu₃O_7 + xAg?(x = 0.0, ?5.0, ?15.0 and 20 wt%) composite samples have been prepared by the solid state reaction method. The changes in structure are confirmed from the X-ray diffraction analysis and SEM measurements. The critical current density is calculated using Bean's formula from the magnetization measurement. We find that the addition of silver in YBCO enhances the critical current density (?J_C) by a factor of nearly six (for 15% Ag) in comparison to pure YBCO. Enhancement of the pinning force (F_P) by a factor of ten is also reported. The enhancement in ?J_C is investigated over a wide range of magnetic fields. These significant changes in ?J_C and F_P are attributed to the presence of Ag particles as efficient artifical pinning centers in YBCO.     

Enhancement of pinning and the peak effect in Y1–xFexBa2Cu3Oy high-temperature superconductors

Complex structural and magnetic studies of polycrystalline samples of the Y_{1 – x}Fe_xBa₂Cu₃O_y high-T_c superconductor (x = 0, 0.02, 0.05) have been carried out in order to determine the effect of weak doping with a magnetic 3d ion on structural changes and the critical current density J_c. An increase in the Fe content results in a significant change in J_c and its field dependence. The dependences J_c(H) become nonmonotonic and demonstrate a pronounced peak (peak effect); as a result, the J_c increases strongly in wide temperature and magnetic field regions. The analysis of the field dependences of the pinning force reveals a scaling behavior. The parameters of the scaling function correspond to the point magnetic defects under strong effects of a thermally activated flux flow. Thus, the weak Fe doping (several molar percent) is a promising way to significant improvement of transport properties of high-T_c superconducting materials based on 123-type compounds.     

High-frequency oscillations,generated in the resistive state of a superconducting film

Summary The effect of non-Josephson generation (high-frequency oscillations of voltage) in the resistive state of the superconducting film is studied theoretically in the presence of a constant magnetic field. The density of the generation current in the magnetic field is calculated. The consideration is based on the earlier developed model, in which the above effect is due to the bremsstrahlung radiation from the particles with the energy ε>Δ which are injected from the quasi-normal layer of the phase slip centre to the superconducting bank. The quasi-particles pick up the energy in the process of the multiple Andreev scattering at theN-S boundaries, and the bremsstrahlung radiation is only possible if ε-Δ is of the order of the perturbation energy of the off-diagonal potential at theN-S boundary (~ΔΓ ^3/2, where γ is the depairing factor of the super-conductor).     

Critical currents in polycrystalline thin films of high-T c superconductors

Summary It is shown that the behaviour of the temperature dependence of the critical current in polycrystalline thin films of high-T _c superconductors depends crucially on the assumption made concerning the nature of the intergranular material. The usual assumption of a superconductor-insulator-superconductor (=SIS) ?sandwich? between each grain leads to a crossover fromI _c∼(1−T/T _c) toI _c∼(1−T/T _c)^3/2, for temperatures nearT _c (whereI _c is the critical current,T the absolute temperature, andT _c the superconducting transition temperature). Instead, for a superconductor-normal metal-superconductor (=SNS) sandwich the dependenceI _c∼(1−T/T _c)² is found for all temperatures. Consideration is given to the effect of self-magnetic field on the analysis. The comparison between expressions for continuous and granular systems is extended. Due to the relevance of its scientific content, this paper has been given priority by the Journal Direction.     

Enhanced flux pinning in pulsed laser deposited Y Ba2Cu3O7−δ : BaTiO3 nanocomposite thin films

The effect of incorporation of BaTiO₃(BTO) nanoparticles on the flux pinning properties of pulsed laser deposited YBCO:BTO thin films was studied. Substantial increase in the critical current density (J_C) and the pinning force density (F_p) of the nanocomposite thin films was observed. At 77 K, and zero applied magnetic field, the value of J_C for YBCO and YBCO:BTO (2%) thin films were 2.93 MA/cm² and 6.43 MA/cm², respectively. At the same temperature and an applied magnetic field of 4 T, the value of J_C increases from 3.6×10⁴ A/cm² for YBCO thin film to 2.7×10⁵ A/cm² for YBCO:BTO (2%) nanocomposite thin film. The study of temperature and field dependence of of YBCO and YBCO:BTO thin films indicates similar type of pinning. The lattice mismatch between YBCO and BTO seems to introduce more defects resulting in the improvement of flux pinning properties.     

The field-theoretic description of dynamics of interfaces in disordered media

The time evolution of an interface in a disordered media is described by using the propagator method. The method enables one to represent the perturbation expansions of different quantities characterizing the interface by means of diagrams which are familiar from the field theory. By the analysis of the divergences in the vicinity of the critical dimension d_c = 4 we found that the regularization of the theory demands the renormalization of the mobility and all moments of the disorder correlator excepting the zero one. The renormalization group (RG) calculations of the average velocity of the interface, the roughness, and the functional RG equation of the disorder correlator are presented to order ? = 4 - d. The latter coincides with the result obtained by D. S. Fisher in the equilibrium case. The RG equations have a pole at the value of the driving force, which coincides with the value of the threshold below which the interface becomes pinned as predicted by Bruinsma and Aeppli. The behavior of the mobility in the vicinity of the pole is discussed.     

Driven quasi-one-dimensional classical electron gas in the presence of a constriction: Pinning and depinning

We studied the properties of a quasi-one-dimensional system of charged particles in the presence of a local Lorentzian-shaped constriction. We investigated the response of the system when a time-independent external driving force is applied in the unconfined direction. Langevin molecular dynamics simulations for different values of the drive and temperature are performed. We found that the particles are pinned unless a threshold value of the driving force is reached. We investigated in detail the depinning phenomenon. The system can depin “elastically”, with particles moving together and keeping their neighbors, or “quasi-elastically”, with particles moving together through a complex net of conducting channels without keeping their neighbors. In the case of elastic depinning the velocity vs applied drive curves is characterized by a critical exponent β consistent with the value , while in the case of quasi-elastic depinning the critical exponent β has on average the value 0.94. The model is relevant e.g. for electrons on liquid helium, colloids and dusty plasma.     

Fabrication of Y123 disk by the seeded infiltration and growth method

The seeded infiltration and growth process was used to prepare disks of Y123. This process produces a net-shape product with a uniform distribution of Y211 with a majority of the particles under 2 μm. The Y211 area fraction remained constant at about 27% for different cooling rates in the range 0.5–4°C/h. The critical current density at 77 K was determined to be in excess of 100 000 A/cm² under zero field and about 20 000 A/cm² under 1.5 T magnetic field, which indicates that this method is a good replacement for the conventional melt-texturing process.     

Single domain generation in YBCO textured samples

This paper discusses the partial melting technique applied to texture cylindrical YBa₂Cu₃O_6+x samples with Y₂BaCuO₅ additions and the experimental procedures developed for obtaining large single domains. The thermal texturing methods produce liquid migration which hinders the stable and continuous solidification process. In the present work different methodologies for avoiding capillary liquid absorption are discussed. The mechanism of single domain selection is studied and an interpretation proposed explaining the oblique angle that the (001) plane of the single domain forms with the sample axis. This mechanism is also compatible with the shape of the recrystallization front shown by the samples studied. In order to impose axial orientation of the (001) crystallographic planes, solidification experiments with textured YBCO seeds are carried out.     

Bond percolation for homogeneous two-dimensional lattices

Bond percolation is studied for the three homogeneous two-dimensional lattices: square lattice (SL), triangular lattice (TL) and honeycomb lattice (HL). An expanding cell technique is used to obtain percolation thresholds and other relevant information for different cell sizes. We extend the analysis as to include slightly asymmetric cells in addition to the usual symmetric cells to get more points in the scaling analysis. Exact percolation functions are obtained for each size. Then, the percolation threshold is obtained by means of two complementary methods: one based on the well-known renormalization techniques and the other one introduced here which is based upon determining the inflection point of the percolation curves. A comparison of the results obtained by these two methods is performed. The study includes iterations to extrapolate numerical results towards the thermodynamic limit. Critical exponents ν, β and γ are obtained. Values are compared with numerical results and expected theoretical estimations; present results show agreement and even improvement (in the case of γ) with respect to some numeric values available in the literature. Comparison tables are provided.     

Scaling analysis of the magnetocaloric effect in Gd5Si2Ge1.9X0.1 (X=Al, Cu, Ga, Mn, Fe, Co)

The field dependence of the magnetic entropy change has been studied for a series of doped Gd₅Si₂Ge₂ alloys, which possess a magnetic phase transition that is either entirely second order or a combination of primarily second-order mixed to a very minor degree with a first-order transition arising from a magneto-structural phase change. By analyzing the field scaling of the refrigerant capacity as well as of the reference temperatures used for constructing a universal scaling curve, a procedure for estimating the values of the critical exponents for the alloys was developed. For the cases where the transition is entirely second order, the results obtained from this procedure are comparable to the values obtained from the Kouvel–Fisher method. For the case of Fe-doped alloys which partially possess a first-order phase change, the Kouvel–Fisher method is inapplicable. However, their critical exponents determined by our developed procedure can be used to estimate the Curie temperature of the orthorhombic majority phase.     

Determination of the field dependence of the intergranular critical current in a YBCO tube by horizontal traversals

The magnetic flux density in the cavity of a sintered tube of Yttrium Barium Copper Oxide (YBCO) is measured as a function of H, the axial magnetic field impressed and removed after zero field cooling (ZFC). We note that, in a short thick-walled hollow cylinder of a weak-linked polycrystalline high-T_c superconductor, the magnetic field in the cavity of the tube, H_z hole, rises above the applied field H when it is initially increasing. Consequently the standard approach to determine the critical current density from an ascending sweep of H alone, or a descending sweep alone cannot be exploited. However the field dependence of the intergranular critical current density J_cm can be reliably obtained from the horizontal traversals of H_z hole vs H. Comparison of the experimental data with model calculations for the horizontal traversals of the hysteresis curves show that the intergranular critical current density in the YBCO tube has a Kim-like field dependence. The magnetization of the grains has a significant influence on the flux density in the cavity of the tube.     

Bi2Sr2CaCu2O8掺杂MgB2的磁通钉扎特性

为了研究超导材料中高温超导相颗粒的钉扎行为，在Ar气保护条件下，采用固相反应法制备了质量百分比为0，3，5和10％BizSr2CaCu2O8含量的MgB2块状样品．用X射线衍射和扫描电子显微镜对样品进行了显微结构分析；用物理性能综合测试系统振动样品磁强计（最大磁场9T）测量了所有样品在不同磁场下的直流M（T）曲线，并测量了不同温度下的准静态磁化曲线，通过Bean临界态模型分析出Jc（H）曲线．随着掺杂量的增大掺杂后Tc基本不变，转变宽度略为增大；相比于未掺杂样品，掺杂量为3wt％样品抗磁信号和临界电流密度有较大提高．显微结构分析结果表明，部分Bi2Sr2CaCu2O8分解为Cu2O和其它杂相，有部分Bi2Sr2CaCu2O8颗粒保留在样品内部，成为有效的钉扎中心．最后本文对超导体中的高温超导相颗粒的钉扎行为进行了分析．     

金属芯尺寸对CORC缆REBCO带材临界性能影响

为推动CORC缆应用于CICC导体及高场磁体,研究金属芯尺寸对CORC缆上的ReBCO带材临界性能影响具有一定的重要性。手工绕制了多根单层CORC缆样品,研究了在77 K和自场条件下,芯部尺寸对上海超导生产的YBCO带材的临界电流的影响。随着缆芯尺寸减小,YBCO带材的临界电流产生衰退现象。当缆芯尺寸低于4.3 mm时,临界电流明显下降。当缆芯尺寸为4 mm时,临界电流为127 A,n值为32.5,临界性能衰退了21%。     

Fabrication of high performance Y-123/Y-24Nb1/Ag single grain composites

Y₂Ba₄CuMO (Y-2411, where M is a transition metal element) has been recognized as a potential candidate for enhanced flux pinning in Y–Ba–Cu–O (YBCO) bulk superconductors. However, the non-uniform distribution of Y-2411 in the superconducting phase matrix and the presence of a–c micro-cracks in the sample microstructure limit the magnetic field trapping ability of bulk YBCO containing these inclusions. Recent attempts to add Ag to the YBCO matrix have been observed to enhance the microstructural properties of Y-2411 containing materials. In this work we report the fabrication of high performance YBCO composite containing Y-24Nb1 (Y₂Ba₄CuNbO_y) and metallic Ag. The average value of critical current density, J_c, is compared for samples containing different concentrations of Y-24Nb1. The microstructural features of these samples have been compared with a YBCO single grain processed with Y-24Nb1 and metallic Ag.     

单源进液金属有机化学气相沉积法工作气压对YBCO薄膜制备的影响研究

基于金属有机化学气相沉积法(MOCVD),在沉积有Y2O3/YSZ/ CeO2( YYC)多层过渡层的Ni - W_at.5％金属基带上沉积YBa2Cu3O7-x(YBCO)超导薄膜.通过对单源进液系统的优化,使金属有机源连续稳定地输送到蒸发皿进行闪蒸.优化总气压并通入N2O气氛,以获得高质量的YBCO薄膜.在优化的温度条件下,总压为380Pa,氧气和N2O分压为200Pa(氧气、笑气流量比为5:3),获得了较高的临界电流密度Jc=0.2MA/cm2.在延续织构方面,由CeO2的面外半高宽3.6.降到YBCO(005)峰半高宽的1.8.,由CeO2的面内半高宽5.2.降到YBCO( 103)面半高宽的4.8.,织构获得较大改善.     

Two-parameter analysis of the scaling behavior at the onset of chaos: tricritical and pseudo-tricritical points

We discuss the so-called tricritical points at the border of the period-doubling transition to chaos and examine to what extent the associated universality applies to 2D dissipative maps. As a concrete example, the Ikeda map is studied together with its 1D analog. For the approximate 1D map, the tricritical points appear as the terminal points of Feigenbaum's critical curves in the parameter plane. For the 2D map the same type of critical behavior does not occur in a rigorous sense. It may be observed as a kind of intermediate asymptotics, however, when one considers a finite number of period doublings. We refer to the associated points in the parameter plane as pseudo-tricritical. For the Ikeda map, we present estimates of the number of period doublings, after which the departure from the tricritical universality becomes essential.     

Irreversibility line and flux pinning properties in iron-based high-Tc superconductor SmFeAsO0.85

The irreversibility line and flux pinning properties of high-T_c superconductor SmFeAsO_0.85 were studied using DC magnetization data. Polycrystalline SmFeAsO_0.85 was prepared in a high pressure synthesis apparatus under the pressure of 6 GPa. The results of DC susceptibility showed the superconducting transition at about 55 K. A critical current density J_c(B) was calculated using Bean’s critical state model. At low temperatures (20 K), J_c(B) showed a relatively high value with weak dependence on an applied magnetic field. At higher temperatures, a stronger dependence of the magnetic field was observed, which resulted from decrease in a critical current density probably due to the flux creep effect. The irreversibility line (IL) agreed well with the flux creep theory of Matsushita et al. A comparison of normalized pinning force density with the theoretical models showed that the irreversible behavior in SmFeAsO_0.85 is dominated mainly by normal point pinning (δT_c) and surface pinning mechanisms.     

Angular dependence of the critical current density and the temperature scaling of the flux pinning force in YBCO thin film

In this paper, the critical current J_c(Θ) have been investigated as a function of magnetic-field angle Θ. Θ is the angle between the c-axis and the applied magnetic field direction. This investigation concerned three temperature values (60?K, 78?K and 81?K). The normalized pinning force f_p versus the normalized magnetic field h was also studied (f_p?=?F_p / F_pmax and h?=?H / H_max). The F_p expression was determined based on the Kramer model.The studied sample was a single crystal of YBaCuO thin film deposited by the ablation laser method on the surface (100) of a SrTiO₃ substrate.The results of this work show the existence of point core pinning of the normal centers in the low field regime and the occurrence of the flux creep in high field regime.