Ceramic HTSC SQUID-based galvanometer

A highly sensitive galvanometer operating at liquid-nitrogen temperature for direct and low-frequency current measurements was developed on the basis of a HTSC SQUID. The direct-current sensitivity of the galvanometer is approximately 0.5 nA at an internal resistance of about 20 . Its energy resolution in the white noise region is 2 × 10^–21 J/Hz.     

Dimensional Crossover in the Fluctuation Magnetization in YBa2Cu3O7- x and its Evolution with the Content of Oxygen

We have studied the field-induced dimensional crossover in the fluctuation magnetization of three single crystals of YBa₂Cu₃O_7-x, with superconducting transition temperatures, T_c = 62.5, 52, and 41 K. The dimensional crossover is observed by studying the diamagnetic vortex fluctuations of the lowest-Landau-level type which occur in isochamps magnetization curves, MvsT, for temperatures close to the transition temperature T_c(H). The study was accomplished by obtaining isochamps magnetization curves as a function of temperature for fields in the range of 0.1–5 T. Magnetization curves for each sample when plotted together show two distinct well resolved crossing points, one formed by low field curves and located at a higher temperature than the other formed by high field curves. A lowest-Landau-level scaling analysis is applied to the curves forming the crossing points and it is verified that lower field curves obey the three-dimensional form of this scaling while the higher field curves obey the two-dimensional form. The results allow to observe the evolution of the dimensional crossover field H_cross, 3D–2D, with the content of oxygen in YBa₂Cu₃O_7-x. It is observed that the evolution of the field H_cross with the content of oxygen in each sample qualitatively agrees with theoretical predictions and allow us to estimate the ratio of the anisotropy among the studied samples.     

Propagation of a.c. magnetic field through high-T c coatings

Studies on the propagation of AC magnetic field through plasma-sprayed superconducting Y₁Ba₂Cu₃O_7−x coatings show that complete shielding is achieved up to a certain critical magnetic field strengthH ₀. Increase in the thickness orJ _c of the specimen increases theH ₀ value. Flux-trapping occurs in the specimen at high frequencies and the frequency at which it occurs increases with increase in specimenJ _c .     

Effect of chemical degradation on superconducting properties of the HTSC ceramics

The effect of humid atmosphere (59 and 86%) on phase composition and properties of YBa₂Cu₃O_7−y powder and ceramics prepared by hot pressing has been studied. It has been found that with increase of water content in the powder to 1·6 mass.% the superconducting properties of the corresponding ceramics gradually improve; on further increase of the water content the ceramics degrade resulting in a complete loss of superconductivity. The observed phenomenon can be explained by a plastifying effect of water on the powder by hot pressing on the one hand and by a solid-phase reaction of the products of decomposition (1-2-3) at a high temperature annealing, on the other.     

AC Losses in HTSC Bulk and Tapes: A Review

After the advent of high temperature superconductors (HTSC), much research and development have been carried out and currently these materials are on the verge of being used in the power engineering applications at a commercial scale. In such applications, the ac losses of superconductors are regarded as one of the most important design parameters that guide the fabrication of the devices. There are different methods for the determination of such losses. This article describes some of the widely used methods and discusses the results on the ac losses of bulk HTSC compounds and single and multifilamentary tapes. The nature and origin of the losses in these materials and their dependence on various physical entities like applied field, frequency, and temperature have also been dealt with in detail. Gap areas have also been identified in which further research is needed in order to facilitate the design and fabrication of commercially usable devices with superior performance.     

Critical magnetic field in layered superconductors

The nature of the dependence H _c2(T) is greatly affected by the presence of magnetic impurities and their ordering at low temperatures. The scattering leads to a change in the sign of the curvature, and consequently to an increase in the value H _c2. The theory allows one to explain recent experimental data with the overdoped cuprates which exhibits behavior that is drastically different from the conventional picture. The calculations are in excellent agreement with the data.The authors are grateful to A. Mackenzie, J. Cooper, and S. Wolf for fruitful discussions. One of us (YNO) wishes to acknowledge the support of the Humboldt Foundation. The research of VZK is supported by the U.S. Office of Naval Research under contract no. N00014-94-F0006.     

Long multifilament Bi-2223 Ag-sheathed superconducting tapes and solenoids

Multifilament Ag-sheathed BiPbSrCaCuO (2223) superconducting tapes containing 49 filaments were fabricated by the powder-in-tube route and the roll-anneal process. The transport critical current densityJ _c was 1.3×10⁴ A cm^–2 at 77 K and 7×10⁴ A cm^–2 at 4.2 K in self-field. A 12-m-long tape was used to construct superconducting solenoids (50, 28, and 14 mm internal diameters) generating dc fields 380–1070 G at 4.2 K. Measurements of the variation ofJ _c with field (0–1.6 T) and bend strain (0–5%) are used to explain the performance of the solenoids. The critical bend strain of tapes was about 1.5%.     

Magnetization curves and Bean''s model in high-Tcsuperconductors

High-T _c superconductors have critical currents that decay sharply with increasing magnetic field. We solve Bean's model forJ _c decaying exponentially withH and obtain qualitative agreement with existing magnetization data. We show thatH _c1 cannot be obtained from the linear part of the magnetization curve; it can only be inferred from a low-field anomaly in the hysteresis curve. Presently quoted values ofH _c1 ,based on the linear part of the magnetization curve, are gross overestimates.     

Magnetic field enhancement of high-Tc superconducting pancake coils by Mu metal sheets

High permeability ring-shaped mu metal sheets have been used to enhance magnetic fields in the bore of Bi-based high-T _c superconducting coils. The central magnetic fieldB ₀, generated by pancake coils placed between mu metal sheets, was measured at liquid nitrogen temperature 77 K. The increasing rate (IR) of the central magnetic fieldB ₀ for single pancake coils was from 15 to 63% by using a 0.1 mm thick mu metal sheet. IR was decreased when the originalB ₀ value increased. The thickness of mu metal sheets also affected IR. It was found that IR increased as the number of mu metal sheets was increased. In a test double-pancake coil IR reached 101%,B ₀ was enhanced from 902 to 1815 G, when the total thickness of mu metal sheets on the top and bottom surface of the coil was 2.0 mm. These results indicate that the well-designed high-permeability materials can significantly enhance the magnetic fields generated by high-T _c superconducting coils and magnets.     

Deformation of silver-sheathed oxide superconducting tapes by trapped gases

A simple model was developed to describe the deformation in long silver-sheathed oxide superconducting tapes caused by the thermal expansion of air trapped within the conductor and by carbon dioxide released by the decomposition of SrCO₃ in the precursor powder during heat treatment. The analysis presented can be extended to include other carbon-containing compounds, impurities and gases released during the annealing process. The model assumes a zero-strain deformation process of the thin silver sheath in response to the gas pressure developed within the conductor due to heating. Results show that swelling (uniform deformation along the conductor length) increases linearly with temperature. For a carbon-free tape, swelling increases with decreasing precursor powder packing density. The reverse is true for tapes that contain carbon which is distributed uniformly within the tape. A carbon content of ≈0.001% by weight produces swelling similar to that produced by air. Increasing carbon content increases swelling. Carbon content in excess of 0.5% by weight will deform the conductor to the extent that elastic/plastic deformation can occur. Discrete SrCO₃ particles dispersed among the precursor powder produce bubbles (localized deformation). The size and concentration of these bubbles depend on the SrCO₃ particle size and concentration. For a given carbon content, small SrCO₃ particles form a large number of bubbles, which merge together resulting in swelling along the conductor length. Large SrCO₃ particles, on the other hand, form only a few discrete bubbles whose size increases with particle size.     

History effects in low-field magnetoresistance of BPSSCO polycrystals

Low-field (H<40 G) magnetoresistance measurements have been made on Bi_1·6Pb_0·4Sr₂Ca₂Cu₃O₁₀ polycrystals at several temperatures between 80 and 105 K. Considerable hysteresis in ρ(H) is found in a zero-field-cooled sample when the applied field is increased from 0 to a maximum value and then lowered back to 0 at all temperatures. The observation of hysteresis is taken as an evidence for field trapping in the grains. We show that the hysteresis in ρ(H) occurs for applied fields much lower than that at whichdρ(H)/dH exhibits a discontinuity. In addition, we find that when the applied magnetic field (H _a) is lowered from a maximum field, the effective intergranular field,H _eff, becomes zero forH _c>0, which gives rise to a minimum in ρ(H).     

AC losses in HTS bulk and their influence on trapped magnetic field

A trapped magnetic field in a bulk is decayed and even erased by the application of an AC external magnetic field whose amplitude is much smaller than the trapped magnetic field. In the paper an analytic model to describe behavior of the trapped magnetic field is proposed assuming that its decay is caused by thermal effect due to AC losses in the bulk. The proposed model well explains AC loss characteristics, temperature rise and the decay of the trapped magnetic field of the bulk. Based on the proposed model, methods to suppress the decay of the trapped are shown and the effectiveness of the methods is shown.     

Knight Shift Data and Superconducting Tc Modeling Based on an Effective Hole Model

The superconducting T _c of cuprates have been modeled on a linear scaling with hole concentration per CuO₂ plane and a deleterious influence of bond resonance with the apical system (effective hole formalism). In cases where distribution between various hole reservoirs is not trivial, Knight shift can provide actual hole concentrations. It is shown here that when Knight shift data are used in an effective hole algorithm, satisfactory T _c predictions can be made, corroborating the deleterious influence on T _c of apical O and earlier assumptions concerning hole distributions. For the case of stacking of more than two CuO₂ planes, the inner plane has to be treated as an infinite layer analog in the effective hole model. A separation into inner and outer planes with different dopings is indeed observed by Knight shift, with higher doping in the latter. This is explained here as being due to a tendency to equalize an effective doping degree, as the outer planes lose holes in resonance of bonding with the apical system.     

Quantitative analysis of lattice disorder and superconducting transition in epitaxial YBa2Cu3O6.9 films

We show that the average lattice disorder in YBa₂Cu₃O_6.9 films grown by ion-beam sputtering is homogeneous and can be quantified by introducing the lattice coherence lengthr _c that is extracted from the width of X-ray diffraction rocking curves. The superconducting properties of the films are correlated withr _c T _c decreases with increasing disorder forT _c 10 nm, while the width of the resistive transition and the normal-state resistivity increase.     

Statics and dynamics in the system of high-Tcsuperconductor and permanent magnet

Equilibrium and small oscillations in the system of high-T _c superconductor (SC) and permanent magnet (PM) are considered. A phenomenological theory based on the observed magnetization curves for SC is given. The measured forces and oscillation frequencies in the system of YBa₂Cu₃O₇ superconductor and SmCo₅ permanent magnet are compared with the theoretical predictions. Conclusions are made as to the possible applications for studying the SC properties of materials.     

Is Tl2Ba2CuO6 a superconductor-metal array?

From structural analysis we have shown in a previous investigation that the series TlBa₂Ca _n Cu _n+1O_2n+5 and Tl₂Ba₂Ca _n Cu _n+1O_2n+6 are composed of superconductor-semiconductor arrays. In this paper, we demonstrate that the unit cell of the Tl₂Ba₂Ca _n Cu _n+1O_2n+6 series can further be viewed as composed of a composite block of superconductor-metal-semiconductor, where TlBa₂CuO₅ is the fundamental superconductor, TlO the metallic unit, and CaCuO₂ the semiconductor block. It is indicated that theT _c of Tl(2201) can be deduced from that of Tl(1201) based on Kresin's model of composite systems.     

Effects of iodine intercalation into Bi-based copper oxide superconductors

We have investigated the stage-1 iodine-intercalation compounds, whose host materials are Bi₂Sr₂Ca_1–x Y _x Cu₂O₈ (Bi-2212 phase) and Bi_1.6Pb_0.5Sr_1.9–x La _x Cu_1.05O₆ (Bi-2201 phase) with variousx values. In the Bi-2212 phase, it has been found that the change inT _c through the stage-1 iodine intercalation is due to two effects; a change of the two-dimensionality and an increase in the carrier number. The former effect decreasesT _c by about 10 K independent of the carrier number of the host sample. The latter effect shifts thex dependence ofT _c to largerx values by about 0.08, taking account of oxygen release from the host samples during the intercalation process. This result is clear evidence for the occurrence of both the change of two-dimensionality and the increase in carrier number about 0.04 per CuO₂ unit due to the charge transfer through the stage-1 iodine intercalation. In the Bi-2201 phase,T _c decreases by about 10 K through the stage-1 iodine intercalation.     

Lanthanide Substitution Effects in Electron-Doped High-T c Superconductors Studied by Angle-Resolved Photoemission Spectroscopy

We have performed an angle-resolved photoemission study of the electron-doped high-T _c superconductors (HTSCs) Sm_1.85Ce_0.15CuO₄ (SCCO) and Eu_1.85Ce_0.15CuO₄ (ECCO). Around the nodal point , we observed an energy gap at the Fermi level (E _F) for both samples, resulting from the strong effects of antiferromagnetism. The magnitude of this gap in ECCO is larger than that in SCCO, implying that the effects of antiferromagnetism in ECCO are stronger than that in SCCO. Also, we quantitatively confirmed the effects of antiferromagnetism by the band dispersions to tight-binding calculation.     

Pinhole defects in Ag sheath of PIT Bi-2212 tapes

Pinhole defects have been observed in the silver sheath of powder-in-tube (PIT) fabricated Bi-2212 tapes. Material from the oxide core exit through these pinholes. A relatively large area of (Sr, Ca)_x oxide condensate was found to surround the pinholes on the outer surface of the conductor following partial-melt-growth (PMG) heat treatment. The formation of these pinholes is attributed to the PIT mechanical deformation of the precursor powders, which contain a few relatively large (> 30 μm) and hard (relative to silver) particles/aggregates. Results indicate that these pinholes, which are more pronounced in thin tapes (< 100 μm), play a significant role in the mass transfer process between the core and the ambient during annealing. This movement has a detrimental effect on the transport critical current I_c. This paper attempts to quantify the effects of these pinhole defects.     

The Edge Effect of a Microstrip Resonator on the Field Dependence of the Surface Resistance of a High-Tc 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.