Table of contents

Bulk samples of MgB₂ were prepared with 0, 3, 5, and 10 wt% Bi₂Sr₂CaCu₂O₈ (Bi-2212) particles, added using a simple solid-state reaction route in order to investigate the effect of inclusions of a material with higher T_c than the superconducting matrix. The density, diamagnetic signal, and critical current density, J_c, of the samples change significantly with the doping level. It is found that J_c is significantly enhanced by the Bi-2212 addition. Microstructural analysis indicates that a small amount of Bi-2212 is decomposed into Cu₂O and other impurity phases while a significant amount of unreacted Bi-2212 particles remains in MgB₂ matrix, and these act as effective pinning centres for vortices. The enhanced pinning force is mainly attributable to these highly dispersed inclusions inserted in the MgB₂ grains. Despite the effectiveness of the high-T_c inclusions in increasing superconducting critical currents in our experiment, our results seem to demonstrate the superiority of attractive centres over repulsive ones. A pinning mechanism is proposed to account for the contribution of this type of pinning centre in MgB₂ superconductors.

We report on measurements of the ac susceptibility of sintered YBCO pellets, with ZnO added, as a function of temperature T, for several different ac magnetic field amplitudes, H_ac, in the presence of static bias magnetic field H_b directed along H_ac. For each choice of H_ac, the imaginary part of the ac susceptibility versus T traces a peak, denoted χ''_max, centred at a temperature denoted T_max. We find that χ''_max versus both H_ac and T_max traces a valley. Calculations of hysteresis losses, exploiting the critical state model where the critical current density j_c = α(T)/Bⁿ, reproduce the experimental data quite well.

YBCO films were grown on magnesium oxide (MgO) substrates for fabricating step-edge junction SQUIDs and other Josephson junction-based devices. In-plane 45° grain misorientation was frequently observed in films grown on degraded or contaminated MgO substrates. The appearance of these misoriented grains results in a decrease of the thin-film critical-current density and reduces the device yield. In this work, we investigated the chemical properties of MgO substrates with various surface conditions due to different substrate preparation methods and environmental degradation, by using x-ray photoelectron spectroscopy (XPS). The XPS characteristics of the surface are compared before and after a thermal annealing at 760 °C resembling the thin-film deposition heating cycle. The MgO substrates, after lithographic processing or only weeks of exposure to the laboratory environment, showed surface degradation characterized by the presence of hydroxyl groups, carbonate, and other possible carbon compounds such as bicarbonate, alcohols and carboxyl. Heating of the substrates to 760 °C improves the surface quality to a certain degree with the removal of some of the above contaminants, but is not sufficient to recover the MgO surfaces. A final Ar ion-beam etch cleaning process at low ion energy proved to be very effective in refreshing the MgO substrate surface that had been degraded due to lithographic processing or storage. Films grown on MgO with this pre-treatment showed perfect grain alignment and high critical-current densities.

We fabricated pure and SiC-added MgB₂/Fe composite tapes using a MgH₂ starting powder and applying heat treatments at 600–900 °C and systematically investigated their superconducting properties. For both the pure and SiC-added tapes, the critical temperature (T_c) increased with increasing heat-treatment temperature due to the improved crystallinity of MgB₂.The SiC addition decreased the T_c but increased the slope of the B_c2–T and B_irr–T curves, d B_c2/d T and d B_irr/d T, for all heat-treatment temperatures. The d B_c2/d T and d B_irr/d T of the pure tape decreased with increasing heat-treatment temperature from 600 to 700 °C because of the longer coherence length associated with the improved crystallinity. However, the SiC addition significantly decreased the heat-treatment temperature dependences of d B_c2/d T and d B_irr/d T. At a temperature of ∼20 K, which is easily obtained using a cryocooler, the B_irr is governed by both the T_c and d B_irr/d T. The B_irr of a pure tape at 20 K decreased with increasing heat-treatment temperature from 600 to 700 °C, but the B_irr of the 10 mol% SiC-added tape increased with the temperature. These behaviours can be explained by the heat-treatment temperature dependence of the T_c and d B_irr/d T. At 20 K the highest B_irr of 10 T was obtained under the conditions of a 10 mol% SiC addition and heat-treatment temperature of 900 °C. This B_irr at 20 K is comparable to that of commercial Nb–Ti at 4.2 K. The 10 mol% SiC-added tape heat treated at 900 °C and the 5 at.% SiC-added tape heat treated at 800 °C showed J_c (MgB₂ core) values higher than 10⁴ A cm⁻² at 20 K in 5 T.

The aim of this report is to compare the trapped field distribution under a local heating created at the sample edge for different sample morphologies. Hall probe mappings of the magnetic induction trapped in YBCO bulk samples maintained out of thermal equilibrium were performed on YBCO bulk single domains, YBCO single domains with regularly spaced hole arrays, and YBCO superconducting foams. The capability of heat draining was quantified by two criteria: the average induction decay and the size of the thermally affected zone caused by a local heating of the sample. Among the three investigated sample shapes, the drilled single domain displays a trapped induction which is weakly affected by the local heating while displaying a high trapped field. Finally, a simple numerical modelling of the heat flux spreading into a drilled sample is used to suggest some design rules about the hole configuration and their size.

We present a modified normalized least-mean-square algorithm for SQUID-based magnetocardiography data processing with a new error function, in which the instantaneous signal component represented approximately by an average of near past error data has been eliminated. In this way, the rebounds of the weight vector W from its optimal value in parameter space due to the signal component can be well avoided.

The flux pinning behaviour of (Nd_1/3Sm_1/3Gd_1/3)Ba₂Cu₃O_7−δ samples, having different concentrations x of Gd₂BaCuO₅ secondary-phase nanoparticles, was studied using magnetoresistivity data collected in the presence of 0, 0.5, 1, 2, 4, 6, 8 T magnetic fields (H) along and perpendicular to the c axis, with the transport current lying in the ab plane. Significant enhancement of the flux pinning and hence also the irreversibility field H_irr(T) was found. H_irr(T) extrapolated to 77 K, H_irr^77 K(T), was determined from the H_irr(T) plots using best fits to an empirical formula, H_irr(T) = H_irr(0)(1− T/T_c)^a, where a is a constant. H_irr^77 K(T) for axis (a) remains almost constant up to x = 0.2, (b) increases beyond x = 0.2, (c) peaks at x = 0.4 and (d) falls beyond x = 0.4. Further, H_irr^77 K(T) values are above 15 T, the maximum being 243 T for the 40 wt% sample.

In this paper, issues related to the optimization of superconducting passive interconnects are discussed. Results of the microwave optimization of bends, via connections and crossings of superconducting microstrip lines (SMSLs) are reported. The optimum design of the SMSL cross gives more than 95% of transmission and can be well used in a two-bus cross design with up to 14 signal wires. The results have been confirmed by time-domain simulations and measurements.

An alternative formalism that does not require the assumption of the deconfinement phase of a U(1) gauge field is proposed for the slave particle mean field theory. Starting from the spin-fermion model, a spinon field, which is either fermion or boson, is introduced to represent the localized spin moment. We find a d-wave superconductive state in the mean field theory in the case of the fermion representation of the localized spin moment that corresponds to the slave boson mean field theory of the t–J model, whereas the d-wave superconductive state is absent in case of the Schwinger boson representation of the localized spin moments.

The application of superconducting rapid single-flux quantum (RSFQ) digital electronics for highly sensitive measurement of magnetic fields can provide significant advantages in the use of conventional analogue SQUIDs, especially in terms of operation speed and dynamic range. Furthermore, utilizing an unconventional generalized single-flux-quantum (SFQ) logic with a bidirectional operation principle allows an additional decrease in effort in superconducting electronics. Our novel fully digital SQUID based on the SFQ technique can be assumed to be operating at frequencies in the gigahertz range corresponding to slew rates of several 10⁹ Φ₀ s⁻¹. We present first experimental results for the proper digital function as a preliminary stage for a digital SQUID magnetometer device. The measurements presented are performed for a reliable low temperature superconductor technology at liquid helium temperature; nevertheless the very low complexity of the superconducting digital circuitry holds promise as regards prospects for a working digital SQUID based on high temperature superconductor technology.

Gd₂Ba₄CuMoO_y (Gd2411) single phase was synthesized in air and added into GdBa₂Cu₃O_7−δ (Gd123) bulk superconductors with the addition amount ranging from 0.2% to 20%, in molar ratio to Gd123. Both microstructure and superconducting properties of each bulk were investigated from the seed position to the boundary of a single domain. Although a band structure with the alternative distribution of Gd₂BaCuO₅ (Gd211) and Gd2411 appeared in all samples, the range of this inhomogeneity only appeared within 5 mm from each side of the seed. The critical current density (J_c) shows that Gd2411 plays an important role in the control of the local superconducting properties. The highest J_c reached 84 500 A cm⁻² at 77 K under the self-field in the sample with 0.4% addition of Gd2411.

A procedure is developed to determine the twin wall and domain widths of YBa₂Cu₃O_7−δ samples through x-ray diffraction. Use is made of easily implemented simulations of diffraction spectra based on an existing model of the twin wall strain. It is shown that within the range of typical values of the wall width, that is a few unit cells, the extraction of the domain width can be carried out independently of the wall strain profile considered.

The differential resistances R_d = d V/d I(V) of point contacts between a normal metal and a c axis oriented YNi₂B₂C film (T_c = 15.2 K) in the superconducting (SC) state have been investigated. R_d(V) contains clear 'gap' features connected with processes of Andreev reflection at the boundary between a normal metal and a superconductor that allow the determination of the SC gap Δ and its temperature and magnetic field dependence. A distribution of Δ from Δ_min≈1.5 meV to Δ_max≈2.4 meV is revealed; however, the critical temperature T_c in all cases corresponded to that of the film. The value 2Δ_max/k_BT_c≈3.66 is close to the Bardeen–Cooper–Schrieffer (BCS) value of 3.52, and the temperature dependence Δ(T) is BCS-like, irrespective of the actual Δ value. It is supposed that the distribution of Δ can be attributed to a gap anisotropy or to a multiband nature of the SC state in YNi₂B₂C, rather than to the presence of nodes in the gap.

High temperature superconductor (HTS) thin films have been applied in making a low loss RF receiver coil for improving magnetic resonance imaging image quality. However, the application of these coils is severely limited by their limited field of view (FOV). Stringent fabrication environment requirements and high cost are further limitations. In this paper, we propose a simpler method for designing and fabricating HTS coils. Using industrial silver alloy sheathed Bi_(2−x)Pb_xSr₂Ca₂Cu₃O₁₀ (Bi-2223) HTS tapes, a five-inch single-turn HTS solenoid coil has been developed, and human wrist images have been acquired with this coil. The HTS tape coil has demonstrated an enhanced FOV over a six-inch YBCO thin film surface coil at 77 K with comparable signal-to-noise ratio.

The out-of-plane plane coherence length ξ_c of YBa₂Cu₃O_7−δ superconductor has been evaluated by analysing the thermally activated magnetic vortex flux creep across superconducting CuO₂ layers. Current flow along CuO₂ layers has been ensured by patterning (103)/(013) oriented films into several macro-bridges positioned at different angles with respect to the crystalline planes. The bridge in which vortex creep occurs across the intrinsic potential of CuO₂ layers has been selected on the basis of the superconducting transition and current–voltage characteristics. Current–voltage characteristics of the selected bridge were analysed in the framework of the theoretical model of Barone et al (1990 J. Supercond.3 155), which accounts for a vortex creep in a periodic potential. The estimated out-of-plane coherence length ξ_c ranges between 0.11 and 0.30 nm for films with different oxygen and structural defect contents, consistently with values evaluated with different techniques and reported in the literature. The ξ_c value changes in different ways with changing critical temperature T_c, depending on the dominating type of oxygen vacancies and defects.

A theoretical model of the microwave response of superconducting thin films with trapped magnetic flux is proposed. The model developed allows us to calculate the energy losses and other measurable quantities for films with arbitrary distributions of the trapped flux (Abrikosov vortices) and ac current. It is shown that the response of the film for rather low frequencies is a nonmonotonic function of both the trapped flux and the temperature. The nonmonotonic dependence of the losses obtained is discussed and compared with available experimental data.

We prepared GdBa₂Cu₃O_7−δ (Gd-123) coated conductors by the pulsed laser deposition (PLD) method on PLD-CeO₂/ion-beam assisted deposition (IBAD)-Gd₂Zr₂O₇ (GZO)/hastelloy metal substrate tapes. The Gd-123 film showed a higher critical current density (J_c) in magnetic fields (B) and higher critical temperature (T_c) than those of YBa₂Cu₃O_7−δ (Y-123) film. The Gd-123 film exhibited a high J_c value of 0.19 MA cm⁻² at 3 T () in liquid nitrogen (77 K), and the T_c value was 93.8 K. The J_c value at 3 T was twice as high as that of Y-123 film. Moreover, the angular dependence in magnetic fields of J_c for Gd-123 was also superior to that of Y-123. The reduction of J_c by the magnetic field angle for Gd-123 was less sensitive than that in Y-123. Furthermore, Gd-123 clearly exhibited a peak at 0° () as well as at 90° () in the relationship of J_c on the magnetic field angle curve (J_c–θ), while Y-123 had no peak at 0°. These superior characteristics are considered to be due to the existing defects, such as stacking faults, as observed by transmission electron microscopy. PLD-Gd-123 coated conductors, with pinning-effective defects, high J_c and low anisotropy in J_c–θ, show promise for future applications.

Bi-2212/Ag tapes are fabricated by a combination of dip-coating and partial melt processing. We investigate the effect of addition of MgO particles obtained by Mg combustion. Two different methods of MgO particle embedment in the Bi-2212 matrix are applied. Microstructural studies of processed tapes show that the distribution of MgO particles in the Bi-2212 matrix depends on the methods of tape preparation. If particles were added to the slurry, MgO formed clusters, which depressed Bi-2212 grain alignment and led to degradation of the tape's transport properties.

MgO particles, introduced in the Bi-2212 matrix by direct deposition during Mg combustion, significantly enhance the transport properties of the tapes. This improvement also depends on the magnitude of the magnetic field and operating temperatures. At 25 K, the Bi-2212 tape with the MgO added by direct deposition has 2.5 times higher J_c value compared to the tapes without MgO doping.

This paper investigates the nonlinear effects of high-T_c superconducting (HTS) thin film in high-power applications. A nonlinear model for complex surface impedance has been proposed for the efficient analysis of the nonlinearity of HTS thin films. Further, using the developed model, analysis of HTS-MSR has been done using the spectral domain method (SDM). The SDM formulation has been modified to account for finite conductivity and thickness of HTS films by incorporating a complex resistive boundary condition. The results have been validated with the experiments performed with microstrip resonators (MSRs) based on YBa₂Cu₃O_7−x (YBCO) thin films made by a laser ablation technique on LaAlO₃ substrates, characterized for their characteristics, namely, resonant frequency and quality factor measured as a function of temperature and input RF power. A close agreement between the theoretical and measured results has been achieved validating the analysis.

The irreversible magnetic behaviour at different temperatures of an ex situ Fe-alloy/MgB₂ wire, exhibiting a granular compositional distribution, was studied using an in-field, high resolution Hall probe imaging system. Quantitative information about the local current density was obtained by solving the Biot–Savart inversion problem. The flux penetration and current distribution maps obtained can be attributed to a inhomogeneous compositional 'plum-cake-like' system, consisting of large, isolated MgB₂ agglomerations embedded in a matrix of finely distributed MgB₂+MgO. The critical current densities within the grains and their evolution with the applied magnetic field and temperature have been obtained, and compared to the mean J_c(H,T) in the matrix.

The influence of three processing parameters, temperature, gas flow rate and water pressure, on the YBa₂Cu₃O₇ film growth on LaAlO₃ single-crystal substrates from trifluoroacetate precursors has been investigated and the optimal film processing conditions to achieve high critical currents have been determined. We have found that the growth conditions maximizing the critical current density are those where the nucleation of a-axis oriented grains is minimized, as determined by μ-Raman spectroscopy. Under these conditions the normal state resistivity is very near to that of single crystals because a vanishingly small film porosity is achieved. Transmission electron microscopy analysis of films quenched from the growth temperature gives some hints for understanding the mechanism linking the film porosity with the concentration of a-axis grains. A cross-linked influence of different processing parameters, such as temperature and water pressure, or water pressure and gas flow, has been demonstrated. The optimal growth temperatures are 790–830 °C, but at these growth temperatures, the critical current density is still dependent on the gas flow rate and water pressure. The optimal processing ranges are a compromise between two different competing phenomena influencing the quality of the films: inhomogeneous film formation due to HF gas stagnancy at small nominal growth rates (low gas flow rate or water pressure) and perturbed crystallinity at high gas flow rates or water pressures.