Table of contents

We propose a new approach for the fabrication of MgB₂ tape utilizing an interface diffusion reaction between an Fe–Mg alloy substrate and a boron (B) layer. The Fe–Mg alloy has enough workability to be deformable into a tape form, and the Fe–Mg/(B(+ SiC)) composites were prepared by a lamination method. During the heat treatment at 700 °C, the Mg in the Fe–Mg alloy diffuses towards the interface, forming a thin Mg rich layer along the interface. The Mg rich thin layer acts as a source of Mg for MgB₂ formation, which proceeds to the inside of the B layer. Such a formation mechanism produces a dense structure of MgB₂ with much less MgO impurity, which is difficult to achieve by the conventional powder-in-tube (PIT) process. SiC addition to the B layer is effective for improving the critical current density J_c in applied magnetic fields as in the case of the PIT process, and a J_c value of 3 × 10⁴ A cm⁻² at 5 K and 7 T has been obtained, which is comparable to that of PIT tape with SiC addition. This approach is expected to be a breakthrough in the development of MgB₂ wire with a high quality superconducting layer in the composite.

We report a new in situ reactive deposition thin film growth technique for the production of MgB₂ thin films which offers several advantages over all existing methods and is the first deposition method to enable the production of high-quality MgB₂ films for real-world applications. We have used this growth method, which incorporates a rotating pocket heater, to deposit MgB₂ films on a variety of substrates, including single-crystalline, polycrystalline, metallic, and semiconductor materials up to 4 inch in diameter. This technique allows growth of double-sided, large-area films in the intermediate temperature range of 400–600 °C. These films are clean, well-connected, and consistently display T_c values of 38–39 K with low resistivity and residual resistivity values. They are also robust and uncommonly stable upon exposure to atmosphere and water.

We successfully grew high-quality single-grain (Nd_0.33Eu_0.39Gd_0.27)Ba₂Cu₃O_y pellets using MgO seed crystals, provided a small quantity of ZnO was added. T_c (onset) continuously decreased with growing Zn content. An optimum for ZnO, compromising good superconducting and electromagnetic properties, was 0.035 mol%. With this ZnO content, T_c>92 K, a critical current density of 100 kA cm⁻² was achieved at 77 K and 3 T, and the irreversibility field was >7 T. Perfect facet lines up to the bottom of the sample with the c-axis reflection plane appeared after the melt growth, implying a high density of the material and practically no liquid loss. Very small difference in the trapped field distribution on the top and bottom surfaces confirmed a uniform crystallographic structure and the electromagnetic properties of the pellet.

Superconducting magnetic energy storage (SMES) systems offering flexible, reliable, and fast acting power compensation are applicable to power systems to improve power system stabilities and to advance power qualities. The authors have summarized researches on SMES applications to power systems. Furthermore, various SMES applications to power systems have been described briefly and some crucial schematic diagrams and equations are given. In addition, this study presents valuable suggestions for future studies of SMES applications to power systems. Hence, this paper is helpful for co-researchers who want to know about the status of SMES applications to power systems.

The design of a cylindrical dielectric cavity operated in various TM_nmp modes, suitable for Josephson plasma resonance measurements of small superconducting high-T_c single crystals, is described. Its resonant frequencies are calculated analytically using the perturbation theory and compared with experimental results. An outline of the measurement procedure is provided. Experimental results measured at several frequencies are presented.

Josephson junction oscillators provide a means to determine the dielectric constant (ε_R) and its frequency, temperature and electric field dependence (ε_R(ω,T,E)) in thin-film dielectrics in the frequency range 100–900 GHz. Here, we have applied this technique to a 150 nm thick SrTiO₃ (STO) film and found a variation in ε_R at 4.2 K from 363 ± 5 at one end of the sample to 150 ± 6 at the other end and in the absence of external bias voltage V across the STO. On application of such a bias, the ε_R of the film at 4.2 K was found to change by 30%. The maximum in ε_R(V) was found to be at −1 V instead of the expected 0 V, indicating the presence of an internal electric field in the STO layer, which we attribute to the difference in work functions of the gold and YBa₂Cu₃O₇ electrodes in our structure. ε_R(T) was markedly different to that of bulk STO. In our thin film, ε_R(T) has a broad shoulder in which the temperature dependence is small between about 35 and 50 K, above which it rises again.

A theoretical model is evolved to account for the anomalies reported for the thermal conductivity (κ) of the electron doped cuprate superconductor Nd_1.85Ce_0.15CuO₄. The lattice thermal conductivity by incorporating the scattering of phonons with defects, grain boundaries, electrons, and phonons in the model Hamiltonian is evaluated as a first step. Later on, the scattering of electrons with impurities for both s and d wave symmetry of the order parameter is also analysed. As a next step, the scattering of magnons with phonons, defects, grain boundaries and magnons is investigated in order to assess their role in thermal conduction. It is noticed that at very low temperatures (T<10 K), κ increases and shows almost T^3/2 dependence on the temperature, which is attributed to spin-wave transport. However, the inclusion of phonon–impurity and the carrier–impurity scattering reduces the temperature dependence of κ and a power temperature dependence is revealed for T<10 K. Further, at T_c, κ develops a broad peak and then decreases as the temperature is increased. The anomaly in the vicinity of T_c and above its value is well accounted for in terms of interaction among the phonon–impurity, the magnon–impurity and the carrier–impurity channels of thermal conductivity. Conclusively, the temperature dependence of thermal conductivity is determined by competition among the several operating scattering mechanisms for the heat carriers and a balance between electron, magnon and phonon contributions. Numerical analysis of thermal conductivity from the present model shows similar results to those revealed from experiments.

The axial tensile stress dependences of AC transport current losses and magnetization losses in YBCO conductors, made by the IBAD/PLD (ion beam assisted deposition/pulse laser deposition) process, were investigated in the stress range 0–770 MPa. Measured transport current losses showed a slight stress dependence, which can be explained by the stress dependence of the critical current of the conductor I_c. The magnetization losses were less sensitive to the stress than the transport current losses. The tensile stress dependences of those losses can be estimated from the data for those losses at zero stress and stress dependent I_c.

For a binary alloy s-wave superconductor in the Bardeen–Cooper–Schrieffer regime (weakly attractive electron interaction), the superconducting critical temperature, T_c, is calculated within the coherent potential approximation as a function of the impurity concentration c and the random potentials ε_i for different band fillings. In contradiction to Anderson's theorem (AT), we find that T_c is dramatically sensitive to c and ε_i. Our results show that for low impurity concentrations and weak on-site energies ε_i, AT is valid, while in the strong scattering limit even for low impurity concentration, T_c is very small for the clean system and on increasing c it is completely suppressed; hence AT is violated in this regime.

Hysteresis loss in the superconducting film of a coated conductor is computed for the case where the superconductor carries an ac transport current while being acted upon by a relatively large ac magnetic field applied perpendicular to the film surface. It is shown that the contributions of the ac transport current and ac magnetic field to the loss strongly interact with each other. The importance of the scalar potential due to surface charge on the conductor is pointed out. In general the electric field in a given section of the conductor is the sum of a contribution due to the changing flux density enclosed by the circuit and the electric field of the surface charge. The position of zero electric field in the conductor oscillates back and forth across the conductor width. An alternative expression for the loss is given in terms of a magnetic moment and a dynamic resistance term.

Bi₂Sr₂Ca_1−yY_yCu₂O_x (22 nm)/ Bi₂Sr₂CaCu₂O_x (150 nm) double layers were grown on an MgO substrate. Y atom confinement in the middle Bi₂Sr₂Ca_1−yY_yCu₂O_x layer after an 875 °C heat treatment was confirmed using secondary ion mass spectrometry. Cross-sectional transmission electron microscopy micrographs show a c-axis lattice constant of the Bi₂Sr₂Ca_1−yY_yCu₂O_x layer shorter than that of the bottom Bi₂Sr₂CaCu₂O_x layer, a feature of Bi₂Sr₂Ca_1−yY_yCu₂O_x. A 2 µm × 4 µm intrinsic-Josephson-junction mesa was also fabricated. The critical currents of eight junctions are approximately half those of the remaining junctions. These results demonstrate that artificial critical-current-modulation of intrinsic Josephson junctions is possible by controlled partial atom substitution.

The resistive transitions of (Bi,Pb)₂Sr₂Ca₂Cu_3−xCr_xO_y silver sheathed tapes with x = 0.0, 0.0005, 0.001, 0.002, 0.004 have been measured under applied magnetic fields oriented perpendicular to the tape plane from 0 to 7 T. The transitions are in good agreement with the thermally activated flux creep model by Anderson and Kim: R(T,H) = R₀exp[−U(H)/k_BT] in the low resistance range. Under lower applied magnetic field, the calculated activation energies U(H) of flux motion increased drastically in samples with the proper amount of Cr-ion defects. For our samples, the values of U are all well fitted with the law . With the doped Cr-ion defects, the values of α increase from 0.66 to about 1. In particular, the tape with x = 0.004 has a different α value in a different magnetic field range: α is about 0.67 when H<0.133 T, and about 0.95 when H>0.133 T. This markedly changed value of α indicates flux creep according to various mechanisms in these tapes.

The effect of the properties of starting boron powders on the superconducting properties of MgB₂ has been studied. The 92% and 96% pure powders produce lower surface reactivity and larger particle size than the 99% boron powder, as can be seen from Brunauer–Emmett–Teller (BET) and scanning electron microscopy (SEM) results, indicating that the low purity powders cannot be used to archive the same superconducting properties as those of samples made from pure 99% boron powder. However, the purity of 92% and 96% boron powders can be improved by using a simple chemical process, leading to enhanced magnetic critical current densities J_c. From x-ray diffraction (XRD) measurement, oxide impurity has been observed, which might be originated from the B₂O₃ phase in the boron powders. In order to get high performance MgB₂, it is obviously important to control the phase composition and microstructure of amorphous boron starting powders and solid reaction conditions.

Single core MgB₂/Fe and MgB₂/Nb composite tapes of different aspect ratios (width to thickness—b/a) were prepared by an in situ process. Critical currents measured in parallel and perpendicular field directions revealed that I_c anisotropy increases with the tape aspect ratio b/a and also with applied temperature T. The effect of aspect ratio is saturated for b/a>5, but increasing the temperature systematically increases I_c anisotropy. Large I_c anisotropy makes the application of flattened MgB₂ composite tapes for windings more problematic. The application of a Nb inner sheath and nano-sized additions into MgB₂ reduced the anisotropy effectively.

A HTS induction motor, with a HTS squirrel-cage rotor, is analysed using an electrical equivalent circuit. The squirrel-cage winding in the rotor consists of rotor bars and end rings, and both are considered to be made of YBCO film conductors. A wide range of electric field versus current density in YBCO film is formulated based on the Weibull function, and analysed as a non-linear resistance in the equivalent circuit. It is shown that starting and accelerating torques of the HTS induction motor are improved drastically compared to those of a conventional induction motor. Furthermore, large synchronous torque can also be realized by trapping the magnetic flux in the rotor circuit because of the persistent current mode.

Fe-sheathed MgB₂ tapes were prepared by the in situ powder-in-tube technique using nanometre Si/N/C, SiC whiskers and SiC as doping materials, respectively. The doping effect on phase composition, microstructure and critical current properties was investigated. Heat treatment was performed at 650 °C for 1 h under an argon gas atmosphere. All the doped tapes were found to have significantly enhanced critical current density J_C at 4.2 K in magnetic fields up to 14 T compared with their undoped counterparts. Moreover, the tapes doped with nano-SiC had the best pinning performance, while the SiC whiskers and Si/N/C powders showed a similar improved field dependence of J_C compared to undoped samples. At 4.2 K and 10 T, J_C for the nano-SiC doped samples increased by a factor of 32. Even for Si/N/C doped tapes, a 16-fold improvement in the magnetic field J_C was observed. It is inferred that the different chemical properties of the Si and C elements in SiC, SiC whiskers and Si/N/C led to the J_C–B difference.

We have found that naturally occurring grain boundaries in Tl₂Ba₂CaCu₂O₈ (Tl-2212) thin films grown on MgO substrates have significantly higher critical current values (J_c^gb) than expected. In particular, films grown on clean MgO are bi-epitaxial, containing almost exclusively 45° tilt grain boundaries with J_c^gb values as high as 10⁶ A cm⁻² at 77 K. We have used high resolution electron backscatter diffraction (EBSD) to analyse the structure of both 'natural' grain boundaries in Tl-2212 films grown on MgO substrates, and 'artificial' grain boundaries forced to form in Tl-2212 films grown on lattice-matched bicrystal substrates such as LaAlO₃. Polycrystalline, c-axis aligned Tl-2212 films on 'dirty' MgO contain diffuse or highly dissociated grain boundaries, thus explaining their high J_c values. Artificial grain boundaries, however, show a much more abrupt change in orientation at the grain boundary. The bi-epitaxial 45° grain boundaries are also abrupt; therefore, the high J_c^gb values suggest that the local structure or chemistry at these grain boundaries is different from those of both artificial and other natural grain boundaries in polycrystalline films.

High purity (Tl_0.5Pb_0.5)(Ba_0.2Sr_0.8)₂Ca₂Cu₃O_x superconducting 20–60 µm thick films with T_{c (0)}-values of between 110 and 115 K and transition widths of 2–4 K were fabricated on polycrystalline 9 mol% Y₂O₃ doped ZrO₂ (YSZ) substrates by screen-printing. The microstructure and the phase composition at the interface of the superconducting Tl-1223 film and the substrate were analysed using transmission electron microscopy, scanning electron microscopy, selected area electron diffraction and energy dispersive x-ray fluorescence analysis. The interface between the substrate and the superconducting film consisted of several phases formed by the reaction between the thallium superconductor and zirconium dioxide. Analyses of the substrate/film interface performed using energy dispersive x-ray fluorescence showed the presence of a Ba–(Pb–Sr)–Zr–O compound close to the substrate. This phase was responsible for the very good adhesion of the superconducting films to the YSZ substrates. The additional phases found at the interface were CaO, calcium strontium cuprates and barium cuprate.

Inclined substrate deposition (ISD) is an effective method for the fast production of highly textured template films for YBa₂Cu₃O_x (YBCO) coated conductors (CCs). By electron-beam evaporation of MgO targets, we deposited biaxially textured ISD-MgO films with ϕ-scan full-width at half maximum (FWHM) of ≈10° on flat metallic tapes. Cube-on-cube epitaxial growth was demonstrated for YBCO grown on ISD-MgO buffered metallic tapes with an SrRuO₃ buffer. A transport critical current density of J_c> 1.6 × 10⁶ A cm⁻² was measured at 77 K in self-field on short-length flat-tape CCs produced with the ISD architecture. We have grown biaxially textured ISD-MgO template films on round wire surfaces. An in-plane FWHM of 18° was measured on ISD-MgO deposited on cylindrical surfaces. CC wires with a low aspect ratio and, therefore, low AC losses, are highly desirable in order to facilitate cable winding without compromising the mechanical integrity or engineering current density of the CCs. This work discusses the prospects of using the ISD process to produce low aspect ratio round-wire CCs.

The specific heat of a melted-textured-growth (MTG) YBa₂Cu_2.99Li_0.01O_x crystal has been measured from 1.8 to 30 K in different fields up to 9 T parallel to the c-axis by the thermal relaxation technique. The defects induced by Li-doping and the paramagnetic centres have a strong effect on the low-temperature specific heat. The electronic specific heat C_e(T,H)/T at lower temperatures (T<5 K) follows αT at 0 T and AH^1/2 in the magnetic field, and can be scaled on a line, C_e(T,H)/TH^1/2 = F(T/H^1/2). These results are consistent with that of the d-wave superconductor predicted theoretically by Volovik, indicating that a Li-doped MTG-YBa₂Cu_2.99Li_0.01O_x crystal should be a d-wave superconductor.

The phase transition from vortex liquid to vortex slush has been confirmed in resistance measurements for YBCO crystal superconductors, on the basis of scaling analysis. The temperature dependence of the resistivity under various magnetic fields collapses onto two branches in the scaling behaviour, associated with the vortex slush and the vortex liquid states. The lower branch, for temperatures below the transition point, has a negative curvature, while the upper one shows a plateau above the transition point. The critical exponents are estimated from the scaling result. The phase diagram in the H–T plane is presented and compared with previous reports.

The influence of the quality of boron precursor powder on the microstructure and superconducting properties of MgB₂ bulk samples and tapes was investigated. The nominal purity specified by the suppliers considers only metallic impurities and is not sufficient for the characterization of the boron precursor powder. Oxygen impurities and the grain size of the B precursor powder were found to affect T_c and the microstructure of the MgB₂ tapes. The microstructure was investigated by SEM and TEM. Grains in the boron precursor powders were either nanocrystalline or crystalline, with grain sizes varying between 110 and 500 nm. MgB₂ precursor powder was prepared by mechanical alloying, which resulted in a small, 20–60 nm, MgB₂ grain size of bulk samples. Bulk samples showed the highest MgB₂ phase fraction and a critical current density of 4.7 × 10⁴ A cm⁻² (at 20 K, 1 T) if boron precursor powder with small grain size and small fraction of metallic impurities was used. Such powder also yielded compact tapes and required lower annealing temperatures for the MgB₂ phase formation. The typical critical current densities of the tapes were 5.0 × 10⁴ A cm⁻² (at 20 K, 3 T) and were significantly better than those of samples reported recently. These results underline the importance of mechanical alloying for enhancing the critical current density of MgB₂ tapes. Summarizing, the phase content, the density and the superconducting properties of MgB₂ bulk and tapes depend on the choice of boron precursor powder.

The presence of impurities in the precursor metal carboxylate solutions for the preparation of epitaxial thin films by metal organic decomposition (MOD) is substantially avoided by the use of acid anhydrides. In particular, trifluoroacetic anhydride (TFAA) was used for the synthesis of the starting Y, Ba and Cu trifluoroacetates used in YBa₂Cu₃O_7−x (YBCO) preparation by the MOD process. In this way, highly stable organometallic precursors and a short pyrolysis process could be used leading to YBCO films with high critical currents (J_c ≥2–4 MA cm⁻² at 77 K). Furthermore, the reproducibility of the results has been ascertained.

We have considerably improved the critical current density, J_c, in applied magnetic fields, B, of Al-sheathed MgB₂ tape. Our MgB₂/Al tape, made with MgB₂ powder obtained from the core of an in situ powder-in-tube (PIT) processed tape, was fabricated via an ex situ PIT method without heat treatment and additive compounds. The highest J_c obtained at 10 T and 4.2 K was 880 A cm⁻². This value exceeds that of conventional ex situ fabricated MgB₂/Al tapes made from commercial MgB₂ powder. The improvement in the J_c-B performance of this ex situ fabricated MgB₂ tape is considered to be due to a significant improvement in the magnetic irreversibility field of the MgB₂ cores. Both the MgB₂ and the Al sheath show a low density and a low induced radioactivity. Our experimental data obtained using Al as the sheath material of the MgB₂ tape indicate the great potential of the tape for specific applications, such as in fusion reactors and on-board magnets.

To study the possibility of enhancing the pinning forces in YBa₂Cu₃O_7−δ films through the introduction of nanosized precipitates, quasi-multilayers of YBa₂Cu₃O_7−δ and a transition metal (TM = Ti,Zr,Hf) were deposited on single-crystal SrTiO₃ substrates using pulsed laser deposition. The transition metal layer thickness was chosen to be less than one unit cell, resulting in separated nanoscale islands that form inclusions with perovskite structure BaTM O₃ during film growth. These inclusions grow biaxially textured within the film. Whereas the Ti-doped films show a very strong decrease in the critical temperature T_c and, hence, a strong decrease in the critical current density J_c with increasing TM amount for all temperatures, Hf and Zr doping show an increase in J_c for the smallest amounts of doping. An irreversibility field as high as 10.3 T at 77 K was observed in the case of low Hf content.

The operating thermal and electric modes of a high-T_c superconducting composite in partially and fully penetrated states induced by the charging current are investigated. They were studied under conditions in which the current charging rate, the volume fraction of the superconductor in a composite or the temperature of the cooling bath were changed. The transient behaviour of the voltage–current dependence, which is characteristic during stable and unstable increases in electric field inside the composite under a continuous current charging, is discussed. Simulations were done using zero- and one-dimensional steady and unsteady thermoelectric models with a power equation describing the virgin voltage–current characteristic of a superconductor. It is found that some thermoelectric trends underlie the shape of the voltage–current characteristic of the high-T_c superconducting composite. These have to be considered during experiments in which the critical or quench currents are defined. First, in the initial stage of the fully penetrated regime (in the low voltage range), the electric field distribution does not have a uniform character. These states depend on the volume fraction of the superconductor and the current charging rate: the higher these quantities, the higher the heterogeneity of the electric field. Second, during the stable over-critical regime (in the high voltage range) occurring in complete penetration modes, the evolution of the electric field may depend on the relevant temperature increase of a composite according to the corresponding increase in its temperature-dependent heat capacity. Consequently, the shape of the voltage–current characteristic of a composite high-T_c superconductor during continuous current charging, both before and after thermal runaway, has only a positive slope. Moreover, it is proved that the growth of the fully penetrated part of the voltage–current characteristic becomes less intensive when the current charging rate or the coolant temperature increase. That is why the voltage–current characteristic of a high-T_c superconductor cannot determine the boundary of onset of thermal runaway. It is also confirmed that there is a thermal degradation mechanism of the current-carrying capacity of the composite. In particular, according to this mechanism, the quench currents do not increase proportionally to an increase in the amount of superconductor in a composite.

Polycrystalline MgB₂ samples, with added 0, 2, 4 and 6% nano-Co₃O₄, synthesized by vacuum (10⁻⁵ Torr) annealing at 750 °C for two and a half hours each, are found to be nearly single phase with the presence of only a small quantity of Mg/MgO in the pristine sample in addition to the Co₂O₃ in the doped compounds. All the samples exhibited clear and sharp diamagnetic transitions at around 38 K, in zero-field-cooled (ZFC) magnetic susceptibility measurements with a sizeable signal. The field-cooled (FC) measurements, though having sharp transitions, showed a very small signal, indicating a high level of pinning centres in these samples. Further, some of the doped samples exhibited the paramagnetic Meissner effect (PME) in an applied field of 5 Oe. The critical current density (J_c), estimated by invoking Bean's model for the pristine compound, increases by nearly an order of magnitude for 2% and 4% nano-Co₃O₄ doping and then decreases sharply for the 6% sample at nearly all studied temperatures and applied fields. Further, the increased J_c (∼10⁸ A cm⁻²) is coupled with fluxoid jumps (T≤20 K and H≤1 T). Fluxoid jumps are not seen in the relatively low J_c pristine or 6% sample. This means that the fluxoid jumps are intrinsic only to the high-J_c samples.

We have studied the evolution of superconducting and normal state properties of neutron irradiated Mg(B_0.962C_0.038)₂ wire segments as a function of post-exposure annealing time and temperature. The initial fluence fully suppressed superconductivity and resulted in an anisotropic expansion of the unit cell. Superconductivity was restored by post-exposure annealing. The upper critical field, H_c2(T = 0), approximately scales with T_c, starting with an undamaged T_c near 37 K and H_c2(T = 0) near 32 T. Up to an annealing temperature of 400 °C the recovery of T_c tends to coincide with a decrease in the normal state resistivity and a systematic recovery of the lattice parameters. Above 400 °C a decrease in ordering along the c-direction coincides with an increase in resistivity, but no apparent change in the evolution of T_c and H_c2. To a first order approximation, it appears that carbon doping and neutron damage affect the superconducting properties of MgB₂ independently.

X-ray diffraction, atomic force microscopy, resistivity and susceptibility measurements are used to examine the microstructure and superconducting properties of La_1.85Sr_0.15CuO₄ films grown by pulsed laser deposition on LaSrAlO₄ substrates. The films grow with different degrees of built-in strain, ranging from large compressive in-plane strain to large tensile in-plain strain. While the compressive strain may be attributed to the lattice parameter mismatch between the substrate and the film, the observation of tensile strain is surprising, and the possible origins of the tensile strain are discussed. Films with small built-in compressive or tensile strain, not larger than about 0.02%, grow in quasi two-dimensional (2D) fashion and show sharp superconducting transitions and high transition temperatures. Larger compressive strain induces three-dimensional (3D)-like growth with gradual strain relief across the thickness of the film, and larger tensile strain is observed in the films with columnar growth. Large strain of both types deteriorates superconducting properties.

We present the data of the point contact (PC) Andreev-reflection measurements on the new paramagnetic superconductor Mo₃Sb₇, which were used for finding the energy gap Δ and upper critical field H_{c 2} for this compound. The maximum gap value, reduced to the zero temperature via the Bardeen–Cooper–Schrieffer (BCS) theory, turned out to be  meV, which is slightly smaller than that expected from the BCS theory,  meV. The temperature dependence of the gap obeys the BCS theory approximately. The H_{c 2}(0) value of about 16.5 kOe was obtained from fitting the experimental data to the conventional H(T) dependence, which is quadratic in temperature. This value is in close agreement with the result from magnetization measurements of 17.2 kOe.

MgB₂ thin films were deposited on SiC buffered Si substrates by sequential electron beam evaporation of a B–Mg bilayer followed by in situ annealing. The application of a SiC buffer layer enables the maximum annealing temperature of 830 °C to be reached. Transmission electron microscopy analysis confirms the growth of a nanogranular MgB₂ film and the presence of a Mg₂Si compound at the surface of the film. The 150–200 nm thick films show a maximum zero resistance critical temperature T_C0 above 37 K and a critical current density J_C∼10⁶ A cm⁻² at 11 K.

If a superconducting wire includes a ferromagnetic constituent, then magnet design requires a novel approach. We have previously reported on computing the critical current of coils that include ferromagnetic matrix material. Now we introduce a quench analysis in MgB₂ solenoid and racetrack coils that utilize conductors with a ferromagnetic matrix. The computer code that was developed uses the commercial software MATLAB and FEMLAB. The code is also capable of simulating quench with coils wound of low-temperature superconductor (LTS) and high-temperature superconductor (HTS). The enhancement to this code compared to the earlier reported quench programs is that the non-linear magnetic behaviour due to the ferromagnetic matrix is taken into account. So far, the code does not take into account the conductor AC losses during current decay and the possible quench back due to the thermal interface.

In this study, nanosized Y₂BaCuO₅ (Y211) particles were synthesized by using the microemulsion method. The water-in-oil (w/o) microemulsion system constituted of n-octane, cetyltrimethylammonium bromide (CTAB), butanol and water. The effects of the reaction time and heat treatment on the Y211 particle size were studied. SEM and TEM investigations indicate that the average particle size of the synthesized Y211 was less than 110 nm. Melt-textured superconducting yttrium barium copper oxides (YBCOs) were fabricated by using the synthesized nanosized Y211 powder with and without Pt doping. Trapped field and critical current density (J_C) were enhanced in the melt-textured YBCO as a result of using the nanosized Y211 as the starting precursor.

The effects on transition critical temperature, lattice parameters, critical current density, and flux pinning of doping MgB₂ with carbon nanoparticles, were studied for bulk, wire and tape under a wide range of processing conditions. Under the optimum conditions, magnetic J_c was enhanced by two orders of magnitude at 5 K for a field of 8 T, and by a factor of 33 at 20 K for a field of 5 T for bulk samples, whereas enhancement by a factor of 5.7 was observed in the transport I_c at 12 T and 4.2 K for a wire sample. Samples sintered at high temperature (900 and 1000 °C) exhibited excellent J_c, approximately 10 000 A cm⁻² in fields up to 8 T at 5 K. This result indicates that flux pinning was enhanced by the carbon substitution for B with increasing sintering temperature. Highly dispersed nanoparticles are believed to enhance the flux pinning directly, in addition to the introduction of pinning centres by carbon substitution. Nano-C is proposed to be one of the most promising dopants besides SiC and CNT for the enhancement of flux pinning for MgB₂ in high fields.

Single-core MgB₂ composite wires have been made by the powder-in-tube method using commercial Mg, B and MgB₂ powders (Alfa Aesar) in Fe, Nb and Ta tubes and both in situ and ex situ processes. Prepared wires were subjected to annealing at temperatures ranging from 600 °C up to 950 °C for 30 min in argon atmosphere. Resistive (R(T)) and transport current (I_c(μ₀H)) measurements have shown how the sheath material that was used influences the critical temperature and critical current density. Inter-diffusion and reaction has been observed only for the iron sheath. Niobium has appeared as the best sheath material for MgB₂ wires made by the in situ process but as the worst sheath material for the ex situ method. The reason is mainly due to the large transversal cracks generated in the fully Nb sheathed ex situ wires during the deformation, which are not healed by the subsequent heat treatment.

We present an analytical thermal model to explain the crosstalk in YBCO edge-transition bolometer arrays. The verification of the model was tested on sample array devices made of 200 and 400 nm YBCO films on LaAlO₃ and SrTiO₃ substrates. The model presented was able to explain the effects of the various physical parameters of the devices, such as the film thickness, operating temperature, and the device separation, which cause different response behaviours based on the variation of the related thermal crosstalk characteristics. In addition, the model is valid above the crosstalk-free modulation frequencies, where the effects of the thermal crosstalk on the response of the devices are negligible.

We report on composite films composed of nanograins made of superconductor embedded in an insulating matrix. The thin films were deposited by the sequential reactive sputtering of three non-continuous granular NbN layers separated by two continuous AlN layers on oxidized Si wafers. This 'multilayer' approach allows for the tailoring of both the Josephson coupling and the tunnelling rate of single electrons between NbN nanograins in the vertical direction. The average in-plane size of NbN nanoparticles was 6.4 nm (as revealed by TEM), and t_AlN = 2–4 nm. On cooling, the R(T) curves show a steep logarithmic increase of the nanocomposite resistance with a ratio of R₈/R₃₀₀>9, which shows that there is no 'metallic' percolation path in a normal state. V–I curves measured at T = 6.15 K exhibit a V∼I³ dependence, which implies the Kosterlitz–Thouless transition in 2D superconductors. At low currents, the V–I curves are linear, which is attributed to the flux flow of unpaired vortices. At higher currents, the V–I curves show superposed oscillations, i.e. small voltage peaks separated equidistantly in current with a period of 0.12 mA. The peaks occur in a wide current range from 0.4 to 4 mA. We assume that this effect is caused by phase slips. The fact that the current period is constant in the whole transition region (6.14–6.81 K) implies the geometrical origin of this effect. Several properties indicate the suitability of NbN–AlN multilayered nanocomposite for use in microbolometers or single-photon detectors. The film should generate electromagnetic waves because the peaks also have a negative slope.

In this paper we present the dependence of the maximum levitation force (F_z^Max) of a high-T_c superconductor on the surface magnetic field (B_s) of a cylindrical permanent magnet, based on the Bean critical state model and Ampère's law. A transition point of B_s is found at which the relation between F_z^Max and B_s changes: while the surface magnetic field is less than the transition point the dependence is subjected to a nonlinear function, otherwise it is a linear one. The two different relations are estimated to correspond to partial penetration of the shielding currents in the interior of the superconductor below the transition point and complete penetration above it, respectively. Furthermore, the influence of the geometrical properties of superconductors on the transition point of B_s is discussed, which shows a quadratic polynomial function between the transition points and the radii and the thickness of superconductors. Some optimum contours of the transition point of B_s are presented in order to achieve large levitation forces.

Polycrystalline samples of NbB_2+x with nominal composition (B/Nb) = 2.0, 2.1, 2.2, 2.3, 2.4 and 2.5 were studied by x-ray photoelectron spectroscopy (XPS). The spectra revealed Nb and B oxides on the surface of the samples, mainly B₂O₃ and Nb₂O₅. After Ar ion etching the intensity of Nb and B oxides decreased. The Nb 3d_5/2 and B 1s core levels associated with the chemical states (B/Nb) were identified and they do not change with etching time. The binding energies of the Nb 3d_5/2 and B 1s core levels increase as boron content increases, suggesting a positive chemical shift in the core levels. On the other hand, analysis of valence band spectra showed that the contribution of the Nb 4d states slightly decreased while the contribution of the B 2p_π states increased as the boron content increased. As a consequence, the electronic and superconducting properties were substantially modified, in good agreement with band-structure calculations.

We are offering a solution for the high-aspect-ratio problem relevant to the numerical simulation of AC loss in superconductors and metals with high aspect (width-to-thickness) ratio. This is particularly relevant to simulation of fault current limiters (FCLs) based on second generation YBCO tapes on RABiTS. By assuming a linear scaling of the electric and thermal properties with the size of the structure, we can replace the real sample with an effective sample of a reduced aspect ratio by introducing size multipliers into the equations that govern the physics of the system. The simulation is performed using both a proprietary equivalent circuit software and a commercial FEM software. The correctness of the procedure is verified by simulating temperature and current distributions for samples with all three dimensions varying within 10⁻³–10³ of the original size. Qualitatively the distributions for the original and scaled samples are indistinguishable, whereas quantitative differences in the worst case do not exceed 10%.

We analyse the effect of interplay of the different types of inhomogeneities and the thermal fluctuation of the superconducting order parameter on the excess conductivity in a set of YBa₂Cu₃O_7−y/Ag composite thick films. We show that the mesoscopic inhomogeneities arising due to most of the Ag residing at the grain boundaries strongly influence the tailing and the critical regions below and above the mean field transition temperature, T_c, respectively. A small fraction of Ag diffusing into the grains also produces microscopic inhomogeneities. Though these inhomogeneities are not expected to influence the SCOPF, we found that T_LD, corresponding to the transition from the two to three dimensional fluctuation of the order parameter in the mean field region, as well as the inter-planar coupling strength, strongly depend on the Ag content in the composites. The effect of Ag induced inhomogeneities on the temperature window corresponding to different phases in the mean field and the critical regions are depicted in the form of a phase diagram. The large variation of T_LD, and near invariance of T_c with Ag content in the films as seen in the phase diagram, are explained by invoking the role of Ag in modifying the overall electronic structure of the grains.

Single-grain Gd–Ba–Cu–O (GdBCO) bulk superconductors have been grown by a seeded infiltration and growth (SIG) technique under a 1% O₂+N₂ atmosphere using a generic MgO-doped Nd–Ba–Cu–O (MgO–NdBCO) seed placed on the sample surface at room temperature (the so-called the cold-seeding method). Partial melting of the MgO–NdBCO seeds fabricated in air under notionally identical thermal processing conditions, however, limited the reliability of this bulk GdBCO single-grain process. The observed seed decomposition is attributed to the dependence of the peritectic temperature T_p of MgO-doped Nd_1+xBa_2−xCu₃O_y solid solution (MgO-doped Nd-123ss, where ss indicates solid solution) compounds on both oxygen partial pressure during the melt process and the level of solid solution (x). The peritectic decomposition temperature of MgO-doped Nd-123ss, with x ranging from 0 to 0.5 under p(O₂) = 1.00 atm, was observed to remain constant at 1120 °C. T_p was observed to decrease linearly as a function of solid solution level, on the other hand, under oxygen partial pressures of both p(O₂) = 0.21 and 0.01 atm. Based on these results, MgO-doped NdBCO seed crystals should be grown under reduced oxygen partial pressure in order to obtain a stable MgO-doped NdBCO seed crystal suitable for cold-seeding processes of large-grain (RE)BCO bulk superconductors (where RE is a rare earth element).

We report a simple and effective method for healing macrocracks in YBCO bulk superconductors. The samples with cracks were processed in a furnace in flowing oxygen to eliminate problems caused by nitrogen entrapment during the healing process. Samples were heated several degrees above their decomposition temperature to seal the cracks. The results demonstrate that cracks which form during sample growth or subsequent mechanical processing can be successfully repaired. Multiple domains formed by macrocracks are rejoined into a single domain.

In the present work we study extensively the manipulation of superconductivity through ferromagnetism in a new category of hybrids. The studied hybrids consist of antiferromagnetic/ferromagnetic (AF/FM) [La_0.33Ca_0.67MnO₃/La_0.60Ca_0.40MnO₃]₁₅ multilayers (MLs) in contact with a low-T_c Nb superconductor (SC). In these hybrids a relatively thick FM buffer layer was used as a reservoir for the generation of stray fields that influence the SC intensively. Our results show that in the parallel field configuration the SC becomes ferromagnetically coupled to the ML when field-cooled through its T_c^SC. Thus, although the SC should behave diamagnetically in respect to the externally applied magnetic field, its bulk magnetization behaves ferromagnetically and switches together with the magnetization of the ML when its coercive field is exceeded (switching effect). By employing specific experiments, where the ML was selectively exchange biased or not, we clearly demonstrate that the ML structure, separating the FM buffer and the SC layers, inflicts its magnetic properties on the whole hybrid. Thus, in such ML/SC hybrids the exchange bias mechanism can be used for regulating the switching of the SC magnetization. By employing specific experimental protocols for our magnetization measurements we directly uncover that the multidomain magnetic state of the ML strongly suppresses both the transition's height and the critical temperature of the SC. Simple FM/SC bilayers have also been studied. In these samples the switching effect is observed only for zero external field, while it is absent when a magnetic field is applied. This indicates that the ML structure is an essential ingredient for the generic observation of the switching effect. Our experimental results support recent theoretical studies referring to the mutual proximity effect (Kharitonov et al 2006 Phys. Rev. B 73 054511), to the possible formation of spin-triplet superconductivity (Bergeret et al 2001 Phys. Rev. Lett.86 4096), and to the influence of an FM domain state on the properties of an SC (Buzdin and Mel'nikov 2003 Phys. Rev. B 67 020503(R)) in relevant hybrids. Apart from their importance for theory, our results are valuable for the design of spin-valve devices that recently have attracted great interest.

We present a time-resolved magneto-optical (MO) imaging study of high-temperature superconductor (HTS) in a high-frequency alternating current (AC) regime. The evolution of the magnetic flux density distribution in YBa₂Cu₃O_7−δ (YBCO) thin film samples is studied as a function of the phase of the applied AC current. A quantitative analysis of the data shows that the maxima of the AC current density is shifted from the edges further inside the sample, which may be caused by the higher self-induced field in that region. This technique can be used to study magnetic flux evolution in HTS films and coated conductors in the high-frequency current regime.

The oscillatory dynamics and quasi-static Campbell regime of the vortex lattice (VL) in twinned YBa₂Cu₃O₇ single crystals have been explored at low fields near the peak effect (PE) region by linear and non-linear ac susceptibility measurements. We show evidence that the PE is a dynamic anomaly observed in the non-linear response, and is absent in the Labusch constant derived from the linear Campbell regime. Static properties play a major role however, and we identify two H(T) lines defining the onset and the end of the effect. At H₁(T) a sudden increase in the curvature of the pinning potential wells with field coincides with the PE onset. At a higher field, H₂(T), a sudden increase in linear ac losses, where dissipative forces overcome pinning forces, marks the end of the Campbell regime and, simultaneously, the end of the PE anomaly. Vortex dynamics was probed in frequency dependent measurements, and we find that in the PE region, vortex dynamics goes beyond the description of a power law with a finite creep exponent for the constitutive relation.

The intergranular properties of Cu_0.5Tl_0.5Ba₂Ca_2−yMg_yCu₃O_10−δ superconductor have been studied by resistivity and AC magnetic susceptibility measurements. Magnesium substitution has been found to improve the interplane coupling in the unit cell of Cu_0.5Tl_0.5Ba₂Ca_2−yMg_yCu₃O_10−δ, which in turn would enhance the intergranular coupling. The main objective of the present studies was to observe any possible role played by Mg doping in developing and enhancing the intergranular coupling and flux pinning properties of this compound. Any improvement to the intergrain coupling may be promoted by a change in the oxygen content in the final compound. Since the loss and/or intake of oxygen occurs at T_a≥350 °C, the post-annealing experiments were carried out at 500 °C for 3 h in air, nitrogen and oxygen atmospheres. It was observed from these studies that the oxygen contents decreased in all the samples after post-annealing in air, nitrogen and oxygen atmospheres. The most prominent effects in terms of enhanced superconductivity in the intergranular regions are observed in Mg-doped samples after post-annealing in air. The Mg-doped samples have shown enhanced granular connectivity, since the intergranular coupling peak observed in χ'' in AC susceptibility measurements is shifted to higher temperatures with the increase of Mg concentration. The enhanced intergranular coupling and flux pinning in magnesium-substituted samples is also observed in AC susceptibility measurements in external magnetic fields. The enhancement of intergranular coupling increases the transport critical current density of the samples.