Table of contents

Our research targets the study of the interaction and coupling between the two states, magnetic and superconducting, coexisting in the hybrid ruthenocuprates RuSr₂GdCu₂O₈ (Ru-1212). While the magnetic phase arises from RuO₂ layers, and the superconducting (SC) phase originates in the CuO₂ planes, it is worth observing and investigating how the properties of these phases change under the substitution of Ru by Cu in this material. In order to study the effects of the annealing parameters and the substitution of Ru by Cu on the magnetic and SC properties of Ru-1212, five Cu-doped polycrystalline ceramic samples Ru_1−xSr₂GdCu_2+xO_8−δ (x = 0–0.4) were synthesized and annealed under oxygen pressure at temperatures of 1039 and 1058 °C respectively.

A key issue affecting the critical current density of MgB₂ is the lack of full electrical connectivity. In situ MgB₂ wires can be easily fabricated by reacting Mg powder with amorphous B powder, but such powders normally contain impurities such as B₂O₃ as a result of exposure to air. Here we introduce a practical method for removing B₂O₃ prior to making MgB₂. X-ray diffraction and microstructure analysis show that removing B₂O₃ results in a decrease in MgO from 5.3 to 1.5 mol% in the final reacted MgB₂. Normal state transport measurements using the Rowell analysis indicate that the active current-carrying area fraction (A_F) increased from ∼0.25 to ∼0.48 in MgB₂ made with purified B. These results demonstrate that intergranular MgO is an important current barrier in MgB₂ and that it can be significantly reduced by boron purification.

This topical review summarizes various features of magnetic penetration depth in unconventional superconductors. Precise measurements of the penetration depth as a function of temperature, magnetic field and crystal orientation can provide detailed information about the pairing state. Examples are given of unconventional pairing in hole- and electron-doped cuprates, organic and heavy fermion superconductors. The ability to apply an external magnetic field adds a new dimension to measurements of penetration depth. We discuss how field-dependent measurements can be used to study surface Andreev bound states, nonlinear Meissner effects, magnetic impurities, magnetic ordering, proximity effects and vortex motion. We also discuss how measurements of penetration depth as a function of orientation can be used to explore superconductors with more than one gap and with anisotropic gaps. Details relevant to the analysis of penetration depth data in anisotropic samples are also discussed.

Significant efforts can be found throughout the literature to optimize the current-carrying capacity of Nb₃Sn superconducting wires. The achievable transport current density in wires depends on the A15 composition, morphology and strain state. The A15 sections in wires contain, due to compositional inhomogeneities resulting from solid-state diffusion A15 formation reactions, a distribution of superconducting properties. The A15 grain size can be different from wire to wire, and is also not necessarily homogeneous across the A15 regions. Strain is always present in composite wires, and the strain state changes as a result of thermal contraction differences and Lorentz forces in magnet systems. To optimize the transport properties, it is thus required to identify how composition, grain size and strain state influence the superconducting properties. This is not possible accurately in inhomogeneous and spatially complex systems such as wires. This article therefore gives an overview of the available literature on simplified, well-defined (quasi-)homogeneous laboratory samples. After more than 50 years of research on superconductivity in Nb₃Sn, a significant amount of results are available, but these are scattered over a multitude of publications. Two reviews exist on the basic properties of A15 materials in general, but no specific review for Nb₃Sn is available. This article is intended to provide such an overview. It starts with a basic description of the niobium–tin intermetallic. After that, it maps the influence of Sn content on the electron–phonon interaction strength and on the field–temperature phase boundary. The literature on the influence of Cu, Ti and Ta additions will then be summarized briefly. This is followed by a review of the effects of grain size and strain. The article concludes with a summary of the main results.

The electrical resistivity of the MgB₂ superconductor is calculated from the generalized Bloch–Gruneisen equation, and an overlap repulsive potential is used to generate the acoustic and optical phonon frequencies. Due to two inherent energy gaps, the elastic scattering of electrons from impurities was estimated first. Within a two-band picture, the impurity-limited resistivity due to π band carriers ρ₀^π is larger than the contribution from σ band carriers. The interband impurity scattering is assumed to be negligible compared with intraband scattering. The inelastic scattering of σ band carriers with acoustic phonons dominates over that of the π band carriers with optical phonons below T≈θ_D/2. At room temperature the slope dρ_π/dT is larger. The temperature dependence of resistivity of acoustic phonons with a Debye temperature θ_D≈450 K and high-energy optical phonons with an Einstein temperature θ_E≈966 K, along with the limited resistivity of impurity scatterers, when subtracted from single-crystal data, infers a power temperature dependence over most of the temperature range (40 K≤T≤300 K). The quadratic temperature dependence of ρ_diff = [ρ_exp−{ρ₀+ρ_e−ph(= ρ_e−ph^π+ρ_e−ph^σ)}] is understood in terms of inelastic electron–electron scattering. The comparison of transport parameters with single-crystal data appears to be consistent within the two-band scheme for resistivity that we have presented.

We report on the first scanning tunnelling spectroscopy measurements on the edge of aluminium-doped magnesium diboride (Mg_1−xAl_xB₂,x = 0.085) single crystals. To target their edge (∼30 µm thick) we have developed a low temperature STM facility featuring a home built compact STM head coupled to an inertially driven carriage. This allows linear submicronic positioning over millimetric distances. We demonstrate the capabilities of our system by showing reproducible tunnelling spectra obtained both parallel and perpendicular to the crystalline c axis. Tunnelling on the edge reveals the gap originating in the highly 2D σ band but in contrast with pure MgB₂ does not show the signature of the π gap.

MoSi₂-doped MgB₂ tapes with different doping levels were prepared through the in situ powder-in-tube method using Fe as the sheath material. The effect of MoSi₂ doping on the MgB₂/Fe tapes was investigated. It was found that the highest J_C value was achieved in the 2.5 at.%-doped samples, more than a factor of 4 higher than the undoped tapes at 4.2 K, 10 T, then further increasing the doping ratio caused a reduction of J_C. Moreover, all doped tapes exhibited improved magnetic field dependence of J_C. The mechanism of J_C enhancement by MoSi₂ doping is also discussed.

The thermal runaway conditions of the composite Bi₂Sr₂CaCu₂O₈ superconductor cooled by liquid helium or liquid hydrogen are compared. The study based on the static analysis of thermoelectric modes was made when the volume fraction of the superconductor in a composite was varied. Some specific trends underlying the onset of thermal runaway in superconducting composites cooled by liquid coolants are discussed. It is stated that the operating modes of superconducting composites may be characterized by stable states during which the current-carrying capacity of a superconductor is not effectively used even with a high amount of superconductor in the composite. These states are possible due to the corresponding temperature variation of the resistivities of the matrix and the superconductor in the high operating temperature range. They have to be considered in experiments when the critical current of a superconductor is determined or when the optimal stable operating modes of the current-carrying elements based on the Bi₂Sr₂CaCu₂O₈ superconductor, which is cooled by liquid coolant, are defined.

Large, single grains of Y–Ba–Cu–O (YBCO) bulk superconductor composed of a YBa₂Cu₃O_7−δ (Y-123) matrix with Y₂BaCuO₅ (Y-211) inclusions have been fabricated using a seeded infiltration and growth (IG) process. Y-211 precursor pellets prepared from powders containing different Y-211 particle sizes were processed by the seeded IG technique to investigate the effect of Y-211 size on the single grain microstructure. The superconducting properties of the single grains fabricated in this study are observed to correlate well with the variation in Y-211 particle size in the fully textured Y-123 phase matrix. The critical current density, J_c, in particular, has been observed to increase from 68 000 A cm⁻² for a reference sample to 94 000 A cm⁻² at 77 K in self-field for a sample prepared using refined Y-211 precursor powder with added platinum oxide.

The fabrication of self-shunted SNS (superconductor/normal conductor/superconductor) Josephson junctions for rapid single flux quantum (RSFQ) logic could potentially facilitate increased circuit density, as well as reduced parasitic capacitance and inductance over the currently used externally shunted SIS (superconductor/insulator/superconductor) trilayer junction process. We report the deposition, fabrication, and device characterization of Josephson junctions prepared with Nb_1−yTi_yN electrodes and Ta_xN barriers tuned near the metal–insulator transition, deposited on practical large-area oxide-buffered silicon wafers. When scaled to practical device dimensions, this type of junction is found to have an I_cR_n product of over 0.5 mV and a critical current (I_c) and normal resistance (R_n) of magnitudes suitable for single flux quantum digital circuits. A longer than expected normal-metal coherence length (ξ_n) of 5.8 nm is inferred from the thickness dependence of J_c at 4.2 K for junctions fabricated using a barrier resistivity of 13 mΩ cm. Although not well understood and not quantitatively predicted by conventional theories, this results in a sufficiently high I_c and I_cR_n to make the junctions suitable for practical applications. Similar observations of unexpectedly large Josephson coupling currents in SNS junctions have been documented in other systems, particularly in cases when the barrier is near the M–I transition, and have become known as the giant proximity effect. The temperature dependence of ξ_n, I_cR_n, and J_c are also reported. For this technology to be used in practical applications, significant improvements in our fabrication process are needed as we observe large variations in I_c and R_n values across a 100 mm wafer, presumably as a result of variations in the Ta:N stoichiometry and the resulting changes in the Ta_xN barrier resistivity.

Numerical calculations using the commercial finite-element code FEMLAB have been carried out to find the distribution of current density in a wire of elliptical cross-section from a hard superconductor carrying the critical current. Dependence of the critical current density on local magnetic field and its orientation has been taken into consideration in these calculations. The results of critical current calculations for wires of different shapes and dimensions clearly revealed how this quantity can be influenced by the distribution of the self magnetic field. The outcome found for wires with cross-sections resembling tapes made from a high-temperature superconductor Bi-2223 have been approximated by two engineering formulae: one can be used to cancel out the influence of the tape width, and the other allows us to perform a correction on the aspect ratio. These expressions would allow the quality of superconducting properties expressed by the self-field critical current density to be compared also for wires of different cross-sections.

After giving all known contributions to coupling losses in superconducting flat structures with twisted filaments, we argue that in some structures the contribution, caused by the induced currents between filaments or striations on the same side of a flat cable or tape, generally cannot be neglected in calculating the total coupling losses. This contribution was not considered in previous calculations. As was demonstrated recently, this type of contribution can be decisive in determining the coupling losses for flat cables with well-conducting edge layers. We show that an analogous contribution should determine the coupling losses for flat cables with a central insulating layer and also for coated conductors with stripes or striations. In addition, it can restrict the maximum allowable twist pitch of superconductors with reliable loss generation and electromagnetic stability.

The total AC losses of YBCO conductors (Y-Cs) and a BSCCO multifilamentary tape (Bi-MT) that have the same engineering current densities and carry an AC transport current in an AC transverse magnetic field with various orientations were compared based on experimental results. The measurements were carried out by using a 3.9 mm wide Bi-MT and 10 mm wide YBCO film fabricated on sapphire simulating a Y-C. If we assume that this YBCO film is fabricated on a metal substrate and that the total thickness of the assumed Y-C is 0.11 mm, the engineering current densities of the assumed Y-C and the Bi-MT are almost the same. Based on the fact that the total AC loss of Y-Cs that carry AC transport current in an AC transverse magnetic field with various orientations is proportional to the width of the Y-Cs, the total AC loss of the Y-C whose width is the same as that of the Bi-MT was estimated from the measured data of the 10 mm wide Y-C. The total AC losses of the 10 mm wide and 3.9 mm wide Y-Cs were compared with those of the Bi-MT in transverse magnetic fields with various orientations.

The calculation of trajectories for sedimenting diamagnetic particles shows that superconducting matter in the Meissner or vortex state can be captured sidewise to a ferromagnetic wire magnetized perpendicular to its length. Capture is possible for externally applied fields lower than typical critical fields H_c1(c), H_c1(a,b) of cuprates. For single crystalline particles in the vortex state, the magnetic anisotropy may reduce the capture force because of alignment. Theoretical predictions were confirmed experimentally for suspensions of polycrystalline and single crystalline particles of YBa₂Cu₃O_x (size range: 2–125 µm) in liquid nitrogen. As a general conclusion we find that for the extraction of superconducting particles out of combinatorial ceramic reaction mixtures, separation in the Meissner state might be most effective because of the presence of an excess of normal state matter featuring a much lower diamagnetic susceptibility.

Superconducting cables can be used to transport large amounts of energy with small losses in considerably smaller volume compared to conventional cables. At present, the first YBCO cable demonstrations are under development and they are expected to outperform BSCCO cables. Due to extremely high current densities in very thin superconducting films these cables are sensitive to overcurrents and therefore cable designers should be able to predict their behaviour during faults. In this paper, the temperature distribution in various 1 kA YBCO cable geometries was computed with several fault current waveforms in order to study the cable stability. FEM models were used to determine simultaneously both the current density and the temperature distributions of the cable as functions of time. Real temperature dependent properties of the cable materials and the strong magnetic field and current density dependence of the superconductor resistivity were taken into account.

A new Zn-rich high-temperature superconductor Cu_0.5Tl_0.5Ba₂Ca₂Cu_0.35Zn_2.65O_10−δ has been synthesized. The structure of this compound is tetragonal, with a- and c-axes lengths 3.354 and 14.402 Å, respectively. The substitution of Zn at the CuO₂ planar site in the formula unit Cu_0.5Tl_0.5Ba₂Ca₂Cu_3−yZn_yO_10−δ gives superconductivity for all the doping concentrations with y = 0.75,1.5,2.25, and 2.65. The zero-resistivity critical temperature [T_c(R = 0)], quantity of diamagnetism and critical current density [J_c(H = 0)] are found to increase with the increased concentration of Zn in the final compound. The onset temperature of superconductivity in these samples was observed at 127 K and the zero-resistivity critical temperature was at 121 K. The material prepared by this method is extremely reproducible.

YBCO films were prepared by pulsed laser deposition from nanocrystalline targets doped with different concentrations of BaZrO₃ ranging from c = 0.9 to 9.0 wt%. The critical temperature of the films decreases almost linearly with increasing BaZrO₃ content whereas the critical current density shows a maximum near 3.9 wt%. In comparison with undoped YBCO films the accommodation field B^* is considerably enhanced and the critical current density is improved in high fields in the films doped with BaZrO₃, e.g. for c = 3.9 wt% by factor of 4.5 in a field of 5 T at 5 K. In the doped films the BaZrO₃ particles grow epiaxially with YBCO. Transmission electron microscopy results show that the density of the BaZrO₃ particles increases with increasing doping but the particle size remains practically the same (5–10 nm).

Large-scale commercial applications of high-temperature superconducting (HTS) YBa₂Cu₃O_7−x (YBCO)-based second generation coated conductors require simple and cost-effective process technologies to fabricate the buffer layer(s) and YBCO superconducting layer. Sm_0.2Ce_0.8O_2+x (SCO) thick films have been epitaxially deposited on roll-textured Ni substrates as the single buffer layer for HTS coated conductors by using high-rate photo-assisted metal–organic chemical vapour deposition (PhAMOCVD) at a relatively low deposition temperature of 600 °C. YBCO superconducting films were then successfully deposited on these thick SCO single buffer layers by the same high-rate PhAMOCVD process, and yielded critical current densities (J_c) of ∼0.52 MA cm⁻² at 77 K and zero applied field. X-ray diffraction and scanning electron microscope analyses of SCO/Ni samples revealed very good crystalline structure and surface morphology for the SCO single buffer layers. These results suggest that SCO single buffer layer, as well as the YBCO conductors, fabricated by the high-rate PhAMOCVD technique may offer great potential for manufacturing YBCO coated conductors.

We have investigated the in-plane thermal conductivity κ_ab(T,H) of large single crystals of optimally oxygen-doped (Y_1−x,Sm_x)Ba₂Cu₃O_7−δ (x = 0, 0.1, 0.2 and 1.0) and YBa₂(Cu_1−yZn_y)₃O_7−δ (y = 0.0071) as functions of temperature and magnetic field (along the c axis). For comparison, the temperature dependence of κ_ab for as-grown crystals with the corresponding compositions is presented. The nonlinear field dependence of κ_ab for all crystals was observed at relatively low fields near half the critical temperature (T_c). We make fits of the κ(H) data to an electron contribution model, providing both the mean free path of quasiparticles and the electronic thermal conductivity κ_e, in the absence of a field. The local lattice distortion due to the Sm substitution for Y suppresses both the phonon and electron contributions. On the other hand, the light Zn doping into the CuO₂ planes affects solely the electron component below T_c, resulting in a substantial decrease in .

We performed simulations with the numerical CUDI-CICC code on a typical short ITER (International Thermonuclear Experimental Reactor) conductor test sample of dual leg configuration, as usually tested in the SULTAN test facility, and made a comparison with the new EFDA-Dipole test facility offering a larger applied DC field region. The new EFDA-Dipole test facility, designed for short sample testing of conductors for ITER, has a homogeneous high field region of 1.2 m, while in the SULTAN facility this region is three times shorter. The inevitable non-uniformity of the current distribution in the cable, introduced by the joints at both ends, has a degrading effect on voltage–current (VI) and voltage–temperature (VT) characteristics, particularly for these short samples. This can easily result in an underestimation or overestimation of the actual conductor performance. A longer applied DC high field region along a conductor suppresses the current non-uniformity by increasing the overall longitudinal cable electric field when reaching the current sharing mode. The numerical interpretation study presented here gives a quantitative analysis for a relevant practical case of a test of a short sample poloidal field coil insert (PFCI) conductor in SULTAN. The simulation includes the results of current distribution analysis from self-field measurements with Hall sensor arrays, current sharing measurements and inter-petal resistance measurements. The outcome of the simulations confirms that the current uniformity improves with a longer high field region but the 'measured' VI transition is barely affected, though the local peak voltages become somewhat suppressed. It appears that the location of the high field region and voltage taps has practically no influence on the VI curve as long as the transverse voltage components are adequately cancelled. In particular, for a thin conduit wall, the voltage taps should be connected to the conduit in the form of an (open) azimuthally soldered wire, averaging the transverse conduit surface potentials initiated in the joints.

Recent advances in the manufacturing of multifilamentary high temperature superconducting composite wires allow for wider practical applications of the conductors, e.g. in power transmission cables, transformers and motors. The wires, based mainly on BSCCO-2223 and YBCO-123 cuprates, are used in the forms of variously shaped coils; therefore they are subjected to different kinds of mechanical stresses and strains. These, in turn, lead to some changes in the physical parameters of the superconducting material, mainly in the critical current density, and thus in the dissipated electromagnetic energy, when subjected to changing magnetic fields and transport currents. In this work we report some experimental results related to the AC loss characteristics of Bi-2223/Ag multifilamentary tapes and their dependences on bending strains. These losses are compared to the losses of virgin, straight tapes. The total AC losses, i.e. transport current and magnetization losses, in the Bi-2223/Ag tapes, were measured by means of the electrical and calorimetric methods. The experimental data obtained are compared with the critical state model predictions for AC loss behaviour in the experimental conditions presented here.

The use of an Ag alloy sheath is effective for improving the mechanical strength of Bi2223 wires although it may easily decrease J_c through undesired reactions with Bi2223 superconductor. In this study, Bi2223 tapes sheathed with Ag–Mg alloy, which is less reactive with the superconducting phase, were fabricated, and the effect of processing on the microstructure and J_c properties was investigated. The Ag–Mg alloy sheathed Bi2223 tapes showed many Bi2223 crystal outgrowths when they were fabricated with the same pattern as was used for fabricating pure Ag sheathed tape, and the J_c value of the tapes became poorer: about 9 × 10³ A cm⁻². This degradation of J_c is attributed to the poorer crystal alignment seen as many outgrowths, which would be caused by the reduction of the conversion rate of Bi2223. To improve the crystal alignment and J_c values, the rolling and sintering patterns were improved. It was found that the addition of intermediate rolling and careful selection of the sintering time could be used to suppress the formation of outgrowths effectively and improve the J_c value to 1.4 × 10⁴ A cm⁻² through the improvement of the crystal alignment. This means that the timing of the rolling step(s) should be selected at effective point(s) in the phase development of Bi2223 crystal.

ErBa₂Cu₃O_y (ErBCO) thin films with BaZrO₃ (BZO) nanorods were prepared by a PLD method for an enhancement of the critical current density J_c. The values of J_c for the ErBCO thin film containing 1.5 wt% BZO (Er15) in magnetic fields are higher than those for the ErBCO thin film containing 0.5 wt% BZO (Er05). The peaks of J_c have been observed in the angular dependence of J_c in both the films when the magnetic field is applied parallel to the c-axis. It has been found that the peak is attributed to the flux pinning by BZO nanorods oriented parallel to the c-axis. The vortex glass–liquid transition temperature T_g and the pinning parameter m were derived by fitting observed electric transport properties to the theoretical expression based on the percolation transition model. The value of T_g of Er15 is higher than that of Er05. This result indicates that the vortex glass phase extends to a higher temperature region on increasing the fraction of BZO. The peak of m has been found in the magnetic field dependence. This fact is probably due to matching the density of BZO nanorods with that of fluxoids, which was confirmed by TEM observations.

A series of composite samples of YBa₂Cu₃O_7−x and linear low density polyethylene (Y-123/LLDPE) with volume percentage ranging from 0 to 75% was prepared. The crystallinity of the composites was studied using x-ray diffraction (XRD) patterns. It is found that the percentage of crystallinity in the composite samples increases with increasing volume of the LLDPE. A four-phase system for the composite materials may be inferred from a combination of XRD and density data. Repulsive force measurements showed that the diamagnetic properties were preserved in the composites and the samples exhibited appreciable magnetic levitation forces and this force increases with increasing volume fraction of the superconductor filler.

Since high-temperature superconductors were discovered, several studies have been made on their physical properties, attempting to associate them to the origin of superconductivity. Obviously, the oxygen atoms interstitially dissolved in the matrix have an important role in superconductivity, since they move easily in the lattice. In addition, they contribute to hole creation in the CuO₂ planes. Anelastic spectroscopy (internal friction) measurements are sensitive tools for the study of defects in solids, in particular for oxygen mobility. In this paper, Bi₂Sr₂CaCu₂O_8+y samples with several different amounts of interstitial oxygen were analysed by means of anelastic spectroscopy measurements. The measurements were performed by using a torsion pendulum operating at a frequency of about 40 Hz. Complex relaxation structures were observed and attributed to the shift of the oxygen interstitial atoms in BiO chains.

The quality of the contact between a YBCO layer and the protective silver coating is an important parameter affecting the current transfer between YBCO and the normal metal. We studied experimentally the quality of this contact in 6 mm wide YBCO-coated conductor with a critical current of ∼60 A at 77 K. The measured current transfer length for the original sample of a YBCO-coated conductor covered by ∼3 µm thick silver and for the same sample additionally laminated by a 25 µm thin copper layer was 0.19 and 0.47 mm, respectively. The contact resistivity determined from these experiments was between 3.7 × 10⁻¹¹ and 7.0 × 10⁻¹² Ω m². A direct measurement of the resistance between two overlapped tapes soldered by indium yielded values between 2.5 × 10⁻¹² and 5 × 10⁻¹² Ω m². The boundary resistance and current transfer length are important parameters for the design of the optimal tape architecture for coils and windings, especially for stability issues.

We have investigated the rf SQUID (radio-frequency superconducting quantum interference device) and its coupling to tank circuit configurations to achieve an optimal front-end assembly for sensitive and high spatial resolution magnetic imaging systems. The investigation of the YBCO rf SQUID coupling to the conventional LC tank circuits revealed that coupling from the back of the SQUID substrate enhances the SQUID signal while facilitating the front-end assembly configuration. The optimal thickness of the substrate material between the SQUID and the tank circuit is 0.4 mm for LaAlO₃ resulting in an increase of the SQUID flux–voltage transfer function signal, V_spp, of 1.5 times, and 0.5 mm for SrTiO₃ with an increase of V_spp of 1.62 times compared to that for direct face to face couplings. For rf coupling with a coplanar resonator, it has been found that the best configuration, in which a resonator is sandwiched between the SQUID substrate and the resonator substrate, provides a V_spp about 3.4 times higher than that for the worse case where the resonator and the SQUID are coupled back to back. The use of a resonator leads to a limitation of the achievable spatial resolution due to its flux focusing characteristics. This resulted in a favouring of the use of the conventional tank circuits when considering the desired high spatial resolution. The effect of the YBCO flip chip magnetic shielding of the SQUIDs in the back-coupling with the LC tank circuit configuration has also been investigated, with a view to reducing the SQUID effective area to increase the spatial resolution and also for studying the effect of the coupling of various kinds of transformers to the SQUIDs. It is revealed that there is no very considerable change in the flux–voltage transfer function signal level with respect to the effective shield area, while the lowest working temperature of the SQUIDs was slightly shifted higher by a couple of degrees, depending on the shield area.

We have measured the isothermal magnetization loops of top-seeded melt processed (Sm_0.5Eu_0.5)Ba₂Cu₃O_7−δ+10 mol% (SmEu)BaCuO₅+10 wt% Ag crystals in a magnetic field up to 8 T parallel to the c-axis from 50 to 87 K using a vibrating sample magnetometer. All of the isothermal J_c(H) curves show a second peak. The scaling of the F_p–H curves indicates that the flux pinning force F_p comes from the collective pinning of two kinds of pinning centres, F_p = F_p^H+F_p^L. One (F_p^H) is the ΔT_c-type pinning centre originating from the RE–Ba and RE–RE substitution responsible for the second peak; the other (F_p^L) is the normal pinning contributed by the surface and point pins. The H_c2–T diagram implies two critical temperatures. The crossover occurs at about 88.2 K.

The properties of superconductors are very sensitive to the existence of structural defects. Long-range strains which appear around structural defects lead to the formation of surface superconducting domains localized at these defects at temperatures significantly higher than the bulk superconductor critical temperature. For a spherical structural defect, the critical temperature T_c of origination of a superconducting domain localized near a void, the non-monotonic dependence of the third critical field on the defect radius caused by magnetic field quanta entrapping, and the magnetization have been calculated, taking into account the anisotropic dependence of the critical temperature on pressure.

Long length textured MgO template on Hastelloy C276™ (HC) has been successfully deposited in a reel-to-reel (R2R) electron beam (e-beam) evaporation system by inclined substrate deposition (ISD). High deposition rate up to 10 nm s⁻¹ with exposure length of 7 cm has been realized. The MgO template showed good in-plane texture of 9.5°–11.5° measured from the (002) phi scans. Experimental results reveal that MgO in-plane texture is formed by the preferred growth direction of normal and one of the MgO {200} planes rotates to the in-flux direction. A new expression, termed the 'two-thirds relationship', between the inclination angle α and the tilted angle of the (00l) plane from the substrate normal, β, has been summarized. YBa₂Cu₃O_7−δ (YBCO) film deposited by pulsed laser deposition (PLD) on strontium ruthenate (SRO) buffered ISD MgO showed T_c of 91 K with transition width of 1 K. Critical current measurement indicated an I_c of 110 A cm⁻¹ at 77 K in self-field for 0.68 µm YBCO film, corresponding to a J_c of 1.6 MA cm⁻².

YBCO films with different levels of excess yttrium were prepared on single-crystal LaAlO₃ with metal–organic deposition using trifluoroacetates (TFA-MOD). X-ray diffraction and transmission electron microscope measurements revealed excess yttrium in YBCO in the form of nanoscale Y₂O₃ with (400) preferred orientation. The field dependence of J_c demonstrated that YBCO films with Y₂O₃ doping had enhanced J_c in comparison with stoichiometric YBCO films in the magnetic fields. We think the reason for this is that the Y₂O₃ nanoparticles act as pinning centres. YBCO films with 60% yttrium excess display 43% increased J_c compared to stoichiometric YBCO films at a magnetic field of 1 T.

The lateral current distribution and the magnetic self-field induced by a transport current were measured independently in three multifilament Bi-2223/Ag tapes with qualitatively different filament layouts. The current and field data from these two experiments, magnetic knife and scanning Hall probe, are compared with each other after straightforward 'forward' or 'inverse' current-to-field calculations. Both datasets show the critical current to be maximal in the central part of the tapes, while the comparison demonstrates that the drop of the critical current distribution at the tape edges is an intrinsic material property and is not caused by the tape's magnetic self-field. Small differences between the datasets reveal the effect of filament bridging.

The flux pinning properties of (Bi, Pb)-2212 superconductors modified by the addition of rare earth (RE) elements La, Gd and Yb with varying concentrations are studied. Samples were prepared by a conventional solid state route with initial stoichiometry of Bi_1.7Pb_0.4Sr₂Ca_1.1Cu_2.1RE_xO_y (where RE = La, Gd, Yb and x = 0.0 (pure), 0.1, 0.2, 0.3, 0.4). The samples were characterized by powder x-ray diffraction, scanning electron microscopy, R–T measurements and superconductivity measurements at 64 K. It is found that the addition of these elements considerably enhances the flux pinning strength of the system, and there is also an increase of critical temperature (T_C) and critical current density (J_C). These values become maximum at optimum concentrations (x = 0.2–0.3), and at higher levels of addition, these values decrease. The results are explained based on the replacement of Sr²⁺ and/or Ca²⁺ by RE³⁺ ions and the associated distortion of the lattice and change in carrier concentration of the system. La and Yb are found to be more effective than Gd in obtaining better flux pinning properties.

In order to integrate superconducting qubits with rapid-single-flux-quantum (RSFQ) control circuitry, it is necessary to develop a fabrication process that simultaneously fulfils the requirements of both elements: low critical current density, very low operating temperature (tens of millikelvin) and reduced dissipation on the qubit side; high operation frequency, large stability margins, low dissipated power on the RSFQ side. For this purpose, VTT has developed a fabrication process based on Nb trilayer technology, which allows the on-chip integration of superconducting qubits and RSFQ circuits even at very low temperature. Here we present the characterization (at 4.2 K) of the process from the point of view of the Josephson devices and show that they are suitable to build integrated superconducting qubits.

We report the epitaxial growth and superconducting properties for NdBa₂Cu₃O_7−δ (NdBCO) films grown on rolling-assisted, biaxially-textured substrates (RABiTS) by pulsed laser deposition. At the optimum deposition temperature of 760 °C, the critical current densities, J_c, at 77 K, self-field, of NdBCO films ranging in thickness from 0.13 to 0.25 µm were found to be in the range 2.2–3.4 MA cm⁻², with a lower thickness corresponding to a higher J_c. Compared to epitaxial YBa₂Cu₃O_7−δ (YBCO) films, these NdBCO samples are found to have superior field and angular dependences of J_c. In an applied field of 1 T, NdBCO of the same thickness has more than twice the J_c of YBCO films with a significantly enhanced J_c peak for a field applied parallel to the c-axis. These results suggest the presence of an increased density of c-axis correlated pinning centres within NdBCO films compared to YBCO films.

The strain dependence of the critical current in Y-123 tapes has been studied as a function of field and temperature up to strain values of 0.81%. For the particular tape configuration with a Ni–W substrate, the critical current was found to be reversible up to 0.51%. The I_c(ε) curves have been measured at 4.2, 50 and 77 K at fields up to 19 T. In the reversible region, the decrease of I_c(ε)/I_c(ε = 0) with ε was found to be more pronounced at higher fields and temperatures: at 17 T the decrease of I_c/I_co at ε_irr was of the order of 9% and 25% at 4.2 and 50 K, respectively. At 77 K and 6 T, the corresponding decrease was 12%. Using the Walters spiral for tapes up to 0.80 m length, the V–I curves have been measured over three decades. At low electric field (0.01 µV cm⁻¹) the n value is constant up to 0.81% strain, while at higher electric field it decreases with the applied strain.

High-quality CeO₂ buffer layers are requisite for the successful growth of YBCO thin films with excellent properties on sapphire substrates. In this study, we evaluated the effect of surface modification of the CeO₂ layers on the properties of the YBCO thin films prepared by large-area pulsed laser deposition (PLD), in particular the critical current density J_c and defect microstructure. High-temperature annealing (1050 °C) has been found to significantly smoothen the very rough and granular surfaces of the as-grown CeO₂ layers (surface roughness rms∼5–10 nm) to atomic flatness (rms∼0.5 nm). However, a rather unique characteristic of the CeO₂ layers deposited by large-area PLD is the development of pores when subjected to prolonged high-temperature annealing. For very short annealing periods (10–20 min), the surface morphology becomes atomically flat, along with the appearance of a high density of 'nanopores' that are ∼40–100 nm in diameter and ∼3–5 nm in depth. Extending the annealing period to 60 min or more results in the development of a surface subtended with enlarged pores ∼0.2–0.5 µm in diameter. Compared with the YBCO thin films deposited on as-grown CeO₂, YBCO thin films on annealed CeO₂ exhibited better homogeneity of J_c and better crystalline texture. Among the YBCO thin films deposited on annealed CeO₂, higher self-field and in-field J_c was obtained for YBCO thin films deposited on CeO₂ with smooth surfaces but interspersed with nanopores. Investigation of the defect microstructure via the etch pit method in conjunction with atomic force microscopy (AFM) of the YBCO thin films revealed a high density of linear defects in the form of screw and edge dislocations, which correlated well with a high density of nanopores on annealed CeO₂. Transmission electron microscopy (TEM) further confirmed the presence of threading dislocations clearly emanating from the nanopore sites. Angular dependence of J_c revealed enhanced flux pinning for YBCO thin films deposited on CeO₂ containing a high density of nanopores.

The temperature dependences of the enhanced critical current in wide and thin Sn films exposed to a microwave field have been investigated experimentally and analysed. It was found that the microwave field stabilizes the current state of a wide film with respect to the entry of Abrikosov vortices. The stabilizing effect of irradiation increases with frequency. Using the similarity between the effects of microwave enhancement of superconductivity observed for homogeneous (narrow films) and inhomogeneous (wide films) distributions of the superconducting current over the film width, we have succeeded in partially extending the Eliashberg theory to the case of wide films.