Table of contents

Bi_2.2-xPb_xSr₂Ca_2.2Cu₃O_y silver sheathed ((Bi, Pb)-2223/Ag) tapes with different Pb doping amounts, x = 0.4, 0.5, 0.6, are prepared by the powder-in-tube method. X-ray diffraction analyses manifest that the tape with x = 0.5 has more Pb-rich phase than that of a tape with x = 0.4. For the tape with x = 0.6, there is a large amount of Ca₂PbO₄ phase and a small amount of Pb-rich phase. Scanning electron microscopy and transmission electron microscopy observations show that the Pb-rich phase with nanometre order of magnitude is embedded in the (Bi, Pb)-2223 grain matrix of samples. The magnetic field dependence of the critical current density of the tape with x = 0.5 is noticeably improved at 77 K, and its irreversibility line is also shifted to higher temperatures and magnetic fields in comparison with the tape with x = 0.4, which is usually considered as the standard Pb doping level in preparing (Bi, Pb)-2223/Ag tapes. The resistivity measurement of the normal state reveals that the anisotropy of the tape with x = 0.5 is also decreased compared with the tape with x = 0.4. The enhanced flux pinning seems to mainly originate from the fine Pb-rich phase particles themselves and the extra defects induced by the larger Pb-rich phase particles.

Bi_1.8Pb_0.4Sr₂Ca_2.2-xPr_xCu₃O_y silver sheathed ((Bi, Pb)-2223/Ag) tapes with x = 0.0 (un-doped), 0.001 and 0.002 (Pr doped) have been investigated. X-ray diffraction analyses and transmission electron microscopy observation show that in the Pr-doped tapes there are many defective regions induced by the partial Pr substitution for Ca with a size of about 10 nm. As compared with the un-doped tape, the magnetic field dependence of critical current densities of the Pr-doped tapes are noticeably improved at 77 K. Their irreversibility lines are also shifted to higher temperatures and magnetic fields, and their activation energies of flux motion are increased drastically. The pinning force densities of the tapes are fitted with a improved law from the Dew-Hughes model; the results of this fitting show that the flux pinning enhanced in Pr-doped tapes mainly originates from the fine normal-like defects.

Magnetic hysteresis loops for the superconductor (Y, Nd)Ba₂Cu₃O_7-δ have been measured using a SQUID and a vibrating sample magnetometer. The field dependence of the current density and volume pinning force showed well developed maxima and their temperature-dependent characteristics were used to construct different scaling types. The measurement of magnetic hysteresis loops at different sweep rates enabled the field dependence of the characteristic pinning energy to be determined. In all cases a remarkable consistency with the Perkins et al (1996 Phys. Rev. B 54 12 551) model was found, while the field dependence of the characteristic pinning energy appeared to be inconsistent with the collective pinning theory predication.

An important predicted, but so far uncharacterized, property of the new superconductor MgB₂ is electronic anisotropy arising from its layered crystal structure. Here we report on three c-axis oriented thin films, showing that the upper critical field anisotropy ratio H_c2^∥/H_c2^⊥ is 1.8 to 2.0, the ratio increasing with higher resistivity. Measurements of the magnetic field-temperature phase diagram show that flux pinning disappears at H*≈0.8H_c2^⊥(T) in untextured samples. H_c2^∥(0) is strongly enhanced by alloying to 39 T for the highest resistivity film, more than twice that seen in bulk samples.

The oxygen nonstoichiometry, structure, magnetic and calorimetric data for La doped Ba-site PrBa_2-xLa_xCu₃O_z systems (0⩽x⩽1 and 6.881⩽z⩽ 7392) are reported. The x-dependence of the oxygen content increases monotonically and two structural phase transitions are observed, confirmed by powder x-ray Rietveld analysis, from the orthorhombic 1212C-phase (space group: Pmmm) to tetragonal 1212 (P4/mmm) at x = 0.45, and then to another orthorhombic (Cmmm) phase around x = 0.7. Magnetic susceptibility and heat capacity measurements show that the anomalous Néel temperature T_N(Pr) decreases monotonically from 18 K for x = 0 to 2.8 K for PrBa_1.3La_0.7Cu₃O_7.236 and to below 2 K for x⩾0.8 for samples prepared under the same conditions, while T_N(Pr) increases with an increase of oxygen content for the same compound. The increasing Pr-O bond length observed with decreasing T_N(Pr) indicates that this unusual Pr magnetic ordering is closely related to the wave-function overlap between the Pr-4f orbital and eight O-2p_π orbital in the CuO₂ bi-layer. No superconductivity has been detected for these insulating cuprates.

The thermomagnetic instability of the critical state in superconductors is analysed with an account of the dissipation and dispersion. The possibility of the existence of a nonlinear shock wave describing the final stage of the instability evolution in a superconductor is demonstrated. The problem of nonlinear wave stability with respect to small thermal and electromagnetic perturbations is studied. It it also shown that only damped perturbations correspond to the space-limited solutions, which means that the nonlinear thermomagnetic wave is stable.

Tunnelling measurements have been carried out on heavily underdoped, slightly overdoped and partially Ni-substituted Bi₂Sr₂CaCu₂O_8+x (Bi2212) single crystals by using a break-junction technique. We find that in-plane tunnelling spectra below T_c are a combination of the incoherent part from the pseudogap and coherent quasiparticle peaks. There is a clear correlation between the magnitude of the pseudogap and the magnitude of the superconducting gap in Bi2212. Analysis of the data suggests that the tunnelling pseudogap in Bi2212 is predominantly a charge-density-wave gap on dynamical charge stripes. The tunnelling characteristics corresponding to the quasiparticle peaks are in excellent agreement with theoretical predictions made for a quasi-one-dimensional topological-excitation liquid. In addition, analysis of the data measured by different techniques shows that the phase coherence along the c-axis is established at T_c due to spin fluctuations in local antiferromagnetic domains of CuO₂ planes.

The influence of flux creep on I_c measurement was studied by numerically solving the nonlinear flux creep equation at different dI/dt, V_c and n*, where dI/dt is the sweeping rate of the applied current (I), I_c, V_c and n* are the critical current, the criterion and the material parameter, respectively. It is shown that the V-I curve consists of two parts converging at I_p, V_p and (dI/dt)_p at which the current fully penetrates the sample. In the segment where I<I_p, the V-I curve is parabolic, V∝I², and independent of n*, whereas in the segment where I>I_p, the curve is a power law, V∝I^n*, reflecting the material equation. It is suggested that the appropriate region in the V-I curve to determine n* is where I>I_p. Based on the V-I curve, it is concluded that if dI/dt>(dI/dt)_p, I_c decreases with increasing dI/dt. On the other hand, if dI/dt<(dI/dt)_p, I_c is independent of dI/dt and is therefore suitable. The three critical parameters ((dI/dt)_p, V_p and I_p) are dependent on each other. The parabolic V-I relation can be observed in the giant flux creep state (small n*), whereas in the critical state (large n*), V in the parabolic V-I relation is too small to be detected by a standard voltmeter. This also indicates that the pulsed method may underestimate I_c in the case of high temperatures or in strong applied fields, but it will not affect I_c in the case of low temperatures, weak applied fields and strong pinning.

Two factors affect critical current anisotropy in multifilamentary Ag/(Pb,Bi)₂Sr₂Ca₂Cu₃O_10+x tapes - the intrinsic material anisotropy and the geometry. Experimental results on the magnetic field dependence and anisotropy of the critical current in a multifilamentary Ag/(Pb,Bi)₂Sr₂Ca₂Cu₃O_10+x tape after correction for self-magnetic field effects were found to fit the anisotropic Kim relation. Based on this relation a finite-element-method numerical code for solving the nonlinear Poisson equation for vector magnetic potential was adopted. It allowed the experimental data to be reproduced by back calculation and made possible the study of the interplay of self and external magnetic fields in different cases with well defined physical parameters of the material. The model was used to analyse the distribution of the critical current in individual filaments as well as to evaluate the influence of their geometrical arrangements on the critical current of the tape. The self-field critical current of an individual filament `extracted' from the tape was compared with the critical current of the overall tape. The effect of the self-magnetic field on critical current distribution obtained by the cutting method was determined. The critical currents of the tapes with different cross sections were calculated and compared with experiments and the influence of the self-field was analysed. The anisotropic properties of a low anisotropy architecture of a multifilamentary Ag/(Pb,Bi)₂Sr₂Ca₂Cu₃O_10+x conductor were studied. The dependence of critical currents (normalized to self-field critical currents) on external magnetic field corrected for the self-field was found to follow nearly the same curves as those for tapes with different critical current densities (in the range 20-70 kA cm^-2 in a self-field), which makes the numerical model applicable to different tapes.

Practical applications of high-temperature superconductors require long length and complex shapes of the textured ceramic material. The limitation of processing the REBa₂Cu₃O_y (RE-123, RE = Y, Sm, Nd, etc) superconductors in desired designs requires the development of joining techniques. In this paper we report on various attempts to join sections cut from single-domain YBa₂Cu₃O_y (Y-123) monoliths. The lower peritectic temperature rare earth Yb-123 compounds were used as solder for melt texturing at the interface for joining. The various factors influencing the formation of defect structures during the growth of the solder phase have been investigated. Transport current measurements across the joints of the samples and also of a typical configuration of a brick wall structure prepared under optimized conditions are presented.

The pinning energy of a vortex in a long periodically modulated Josephson contact is calculated. An algorithm is proposed for the exact numerical solution of a system of difference equations describing the shape and energy of the vortex in the stable and unstable states. The magnetic and Josephson energies of a vortex are different. The magnetic and Josephson components of the pinning energy are close in magnitude and have opposite signs, so that the total pinning energy is an order of magnitude lower than its components. A theoretical analysis is carried out, confirming this behaviour. A technique is proposed for analytical calculation in terms of the continuous vortex model, retaining the difference terms in the expression for energy. The magnetic energy E_H calculated by this method differs from the Josephson energy E_J, and the magnetic component of the pinning energy has a sign opposite to that of the Josephson component. A comparative analysis of the possibility of calculating the energies by approximate methods in the framework of the continuous vortex model with difference terms retained or replaced by derivatives is made. The analysis shows that the continuous vortex model fails to provide correct values of the Josephson and magnetic components of the pinning energy. The exact values of these parameters are tens and hundreds of times greater than those obtained using the continuous vortex model. This notwithstanding, the exact values of the total pinning energy differ insignificantly from those obtained by applying the continuous vortex model because of the opposite signs of the Josephson and magnetic components of the pinning energy.

(Bi_0.93Pb_0.17)₂Sr_1.9Ca_2.05(Cu_1.02 ⁵⁷Fe_0.01)₃O_y and (Tl_0.74Bi_0.25)(Ba_0.2Sr_0.8)₂Ca₂(Cu_0.99 ⁵⁷Fe_0.01)₃O_y superconductors were synthesized and investigated by Mössbauer spectroscopy. The samples were carefully characterized by x-ray diffraction, by scanning electron microscopy in combination with energy dispersive x-ray fluorescence (EDX) and by electrical resistivity measurements. Introduction of small quantities of Fe led to a modest decrease in the critical temperatures but the samples remained superconductors. EDX analysis confirmed that iron entered the superconducting phase.

The ⁵⁷Fe Mössbauer spectra of these compounds could be evaluated based on two doublets. The two doublets were assigned to Fe located in the square planar fourfold oxygen coordinated Cu sites and to Fe in the square pyramidal fivefold oxygen coordinated Cu sites in the 2223 and 1223 lattices. The Mössbauer parameters derived in this study were compared with the results obtained for (Tl, Pb)-1223 and (Hg, Pb)-1223 compounds measured earlier. Based on the combined set of data, information on the different chemical bonding of the apical and basal oxygen in the square pyramids and on the layered electronic structure of these superconductors was derived.

The magnetic responses of granular (NdBa₂Cu₃O_7-x and YBa₂Cu₃O_7-x) and monolithic (PbIn) superconducting plates, cooled in a fixed magnetic field H_z parallel to their plane and, later, subjected to cycles of a transverse magnetic field H_y with an amplitude H_y,max>H_z, are investigated both experimentally and theoretically. Magnetization curves, generated as H_y with H_z constant applied and of the order of the full penetration field H_p, are interpreted by employing two competing models (the double critical-state model and the two-velocity hydrodynamic model) for describing flux-line cutting effects on the magnetic behaviour of superconductors. Both models qualitatively and semi-quantitatively reproduce the experimental curves. Unlike the double critical-state model, the two-velocity hydrodynamic approach predicts the appearance of zones with homogeneous magnitude of the magnetic induction B at relatively small values of H_y as |H_y| is increased. However the effect of these zones is not appreciable since they disappear when |H_y| reaches a sufficiently large value: |H_y| ≳ H_z≈H_p.

Superconducting phases, superstructural modulations and their relationship in La₂CuO_4.12 were studied. By magnetic measurements and x-ray diffraction, two superconducting phases with T_c values of 15 and 41 K were found with the structurally independent correspondents of two orthorhombic Fmmm phases. By electron diffraction, two types of superstructural modulations situated on the ab- and bc-plane were observed, with modulation vectors of 1/6a*±1/2b* and 0.18b*±(0.26-0.28)c*, respectively. These superstructural modulations were identified to be actually originated from two types of charge ordering. In addition, it is indicated that the observed ab- and bc-plane charge ordering could very possibly be correlated to the 15 and 41 K phases, respectively. For the 15 K phase, its depressed superconductivity possibly originates electronically from the ab-plane charge ordering which is compatible with the concept of the `stripe phase'.

We report on the realization of dual mode microwave filters based on Tl₂Ba₂Ca₁Cu₂O₈ (Tl-2212) double-sided superconducting films. The Tl-2212 samples have been grown ex situ on 10×10 mm² LaAlO₃ (100) substrates by a combined approach of metal-organic chemical vapour deposition (MOCVD) and thallium vapour diffusion. Their morphological, compositional and structural natures have been investigated by scanning electron microscopy, energy dispersive x-ray analyses and x-ray diffraction characterization. Both sides of the films are smooth, homogeneous in thickness and composition and show epitaxially-grown Tl-2212 phases. Typical values of T_c = 100 K and J_c = 0.05 MA cm^-2 at 77 K have been inductively measured for both sides. Microwave measurements have been performed at 20 GHz by using a dielectric resonator.

Two planar superconducting single-stage dual mode filters operating in the C-band with 1% and 10% fractional bandwidth have been realized. The basic element of the device is a square patch resonator diagonally crossed by unequal slots providing both a size reduction and a simple and controllable way to couple the two degenerate modes. Device response has been studied at different temperatures, performing power and intermodulation measurements. In view of practical applications, the results are compared with those obtained by a filter based on high-quality YBCO films.

A model of a superconducting fault current limiter on a polycrystalline high-temperature superconductor basis is checked in the dc short circuit regime. Protection of load takes place under the conditions described in this paper. The use of ceramic materials with superconducting-normal metal-superconducting Josephson junction network having an S-type current-voltage characteristic (CVC) is shown to be effective for fault current limiter devices.

(Bi, Pb)-2223/Ag sheathed tapes were prepared using precursor powders with different particle size distributions which were obtained by grinding the calcined powder for different durations. DTA results revealed that the reaction temperature of the precursor powder depends on the particle size and is at a minimum for the finest powder. Phase evolution studies, transport current measurement at 77 K with and without the application of an external field, microstuctural examination etc, were carried out in order to evaluate the relative performance of the tapes. The results showed that the particle size has significant effects on the reaction kinetics, ultimate phase fraction, microstructure and transport critical current density. For the tapes fabricated using coarser powder, the reaction kinetics were found to be slower and the microstructure contained smaller grains with many misalignments and voids. On the other hand use of extremely fine precursors yielded tapes with a large fraction of secondary phases due to the degradation of the crystallinity of the precursor and a consequent change in the reaction sequence. The tape prepared using a precursor with an average particle size of 3-4 µm showed the best results with respect to J_C, high-T_C phase fraction, microstructure and J_C-B characteristics.