Table of contents

A novel hybrid microwave procedure for the synthesis of MgB₂ superconductor was demonstrated. This procedure has advantages over other methods using sealed tubes or high pressure. Using silicon carbide (SiC) powder as the susceptor, the MgB₂ samples can be synthesized in about 10 min by microwave heating. The superconducting transition width of the MgB₂ sample was less than 0.3 K, and its zero-resistance temperature was 39.5 K. The critical current density J_c estimated from the magnetization hysteresis is 4.5 × 10⁴ A cm⁻² at 20 K and 1 T, which is close to that of the samples prepared by high-pressure sintering.

Biomagnetic signals are up to six orders of magnitude smaller than the usual level of urban magnetic interference and noise. SQUID gradiometers and magnetically shielded rooms (MSRs) are therefore commonly used to reduce the disturbances and to record magnetic physiological signals. MSRs are very expensive and are inconvenient for many patients due to their narrow dimensions. We have developed an active magnetic shield with the aim of compensating for the urban magnetic field and its first-order spatial field gradient to allow biomagnetic measurements outside an MSR. Two separate feedback circuits measure the magnetic field and the field gradient and feed two sets of compensation coils. We tested two field gradient compensation configurations which made use of different gradiometer designs and different separations between the reference and measurement sensors. The two types of shield were found to achieve interference suppressions of 43 and 26 dB, respectively. We were able to measure a human magnetocardiogram (MCG) signal in an open laboratory, using the less effective shielding configuration.

In this paper we report the generation of higher harmonics in a bulk polycrystalline MgB₂ superconductor when it is subjected to ac and dc magnetic field. The MgB₂ sample shows the generation of higher odd harmonics when an ac field is applied. The application of a small dc field ( gauss) in addition to an ac field also leads to the generation of second and fourth harmonics, which is an indication of the presence of some weak link grain boundaries in a MgB₂ superconductor. We have studied the variation of the amplitude of higher harmonics with temperature for different values of amplitude of ac field. The observed results are interpreted in terms of weak link grain boundaries and a critical state model.

We fabricated MgB₂/AlN/NbN trilayer Josephson junctions on c-plane sapphire substrates, and investigated the dependence of the MgB₂ surface morphology on MgB₂ growth temperature. AFM measurement showed that the grain size was about 150 nm and the surface roughness was about 10 nm when deposition was done at the highest critical temperature. The critical temperature of the MgB₂ layer after SIS junction fabrication remained the same as that for bare MgB₂ film. The current–voltage characteristics of the MgB₂/AlN/NbN junctions showed a very clear Josephson current and a gap structure. The critical current density was over 1 kA cm⁻², and the ratio of the sub-gap resistance to the normal resistance was 16.6 when the AlN insulator thickness was 0.14 nm. The critical current was ideally modulated by applying a magnetic field, indicating that showed that the Josephson current flowed uniformly in the junctions.

The harmonic responses of granular YBCO were obtained via mutual inductance measurements. Two samples, one with and another without intergranular coupling, were investigated in terms of the harmonic components of magnetization at various field amplitudes and frequencies. By comparing the behaviour of the features in the harmonics to that of the peaks in the fundamental response, we explicitly identified which features in the harmonics originate from intragranular harmonic generation and which arise due to a contribution of intergranular coupling. Harmonic responses were obtained despite the absence of vortices and even harmonics were detected in a purely AC magnetic field.

Different values of T_c were found for the 1:2:3 tetragonal superconductors (Ca_xLa_1−x)(La_0.25+xBa_1.75−x)Cu₃O_y (these compounds have previously been denoted as CLBLCO, CLBCO or CaLaBaCuO) prepared under various conditions but having the same x and y. The x = 0.1 ceramic samples prepared using various grindings differ in T_c by 8 K and the x = 0.4 ones prepared at various temperatures differ in T_c by 4 K. The different ratios between the unit cells and the various cation distributions might be the causes of this phenomenon.

The voltage signal on a composite superconducting wire transporting AC beyond its critical current is analysed. The wire is replaced by a simplified model consisting in the parallel combination of superconductor, standing for the properties of all the filaments, and the normal resistor, representing the metallic matrix. At low currents, the main dissipation mechanism is the flux pinning. When AC amplitudes are comparable or higher than the critical current, a 'resistive' voltage appears that causes the bulk of dissipation. Seeing that this voltage is caused mainly by the highly non-linear current–voltage dependence in the superconductor, its waveform will also contain higher harmonics in the case of a sinusoidal AC. We present here a procedure that allows the calculation of all Fourier coefficients of this voltage. For the first time, complete theoretical formulae for higher harmonics in resistive voltage are presented. We also discuss how the harmonic spectrum of the voltage signal develops with AC amplitude.

Carbon has been substituted into the MgB₂ lattice, forming the series Mg(B_1−xC_x)₂, by heat treatments in sealed Ta tubes starting with elemental constituents. The superconducting transition temperature, T_C, was measured by diamagnetic susceptibility. The superconducting transitions are sharp and the x-ray diffraction on the samples show only trace amounts of impurity phases. The C fraction that substitutes the B atoms into the MgB₂ lattice, as determined from the now well established correlation between the 'a' lattice parameter and x in Mg(B_1−xC_x)₂ is found to be very close to the targeted values. The temperature dependence of the normalized resistance is similar to that seen in C doped MgB₂ single crystals along the ab plane. R_n/R(300 K), where R_n is the resistance just above T_C, increases with x.

We have performed atomic-scale high-resolution scanning tunnelling microscopy and spectroscopy measurements on the cleaved surface of single crystal Bi₂Sr₂CaCuO_8+δ superconductors with high critical current density J_c. The samples exhibited rugged structure about 15 nm in period, larger than the modulation of the BiO layer, which corresponded well to the energy gap distribution at 77 K. The presence of inhomogeneity from a nanometre to a micrometre scale, in the energy gap distribution, the structural modulation and the chemical composition fluctuation, may play an important role in improving J_c values in the Bi₂Sr₂CaCuO_8+δ crystals.

Axial tensile stress was applied to four different composite wires containing the superconductors NbTi, Nb₃Sn, Bi-2223 and MgB₂ at room temperature. The basic mechanical properties obtained from the stress-strain characteristics of these wires are compared and discussed. The wire samples were then tested by transport critical current measurements at 4.2 and 77 K. The results obtained show that the critical current degradation of all composite wires occurs at the stress value corresponding to the beginning of the plastic deformation. These data have important implications for the technological application of such superconducting wires.

We have studied in detail the kinetics aspects of Y Ba₂Cu₃O_7−x (YBCO)thin film growth on LaAlO₃(100) single crystals by the sol–gel TFA-MOD method, using an in situ fluoride selective electrode. Kinetics curves show that an intermediate step exists before the YBCO formation. This step, which starts at a lower temperature than YBCO formation, consists in a partial elimination of F from the BaF₂ precursor to form an oxyfluoride intermediate compound. The total YBCO formation appears at temperatures as high as 700 °C and the activation energy of the reaction is . We have found that at low flow carrier gas rates, the reaction is controlled by diffusion mechanisms and the apparent order of YBCO formation is n = 1 with respect to the stirring rate, but at higher flow rates the YBCO formation is controlled chemically, then the apparent order is zero. The apparent E_a for the oxyfluoride formation at lower temperatures corresponding to the intermediate step is only and the oxyfluoride rate formation increases with the stirring rate, indicating that this intermediate reaction is controlled by diffusion mechanisms.

Despite great success in the TFA methods of depositing Y Ba₂Cu₃O_x (YBCO) thin films for coated conductors, critical issues involved in removing BaCO₃ have not entirely been settled. There could be other possible ways of dealing with carbon that remains in the film. We have recently developed a fluorine-free sol–gel synthesis with several important advantages including precursor solution stability, improved film density, and elimination of HF during processing. With this approach, high-quality YBCO films have been developed on single crystal substrates with the transport J_c s up to 10⁶ A cm⁻². In this study, the precursor solution stoichiometry was altered and its effects on superconducting properties were studied. The fluorine-free sol–gel-derived films on the LaAlO₃ (LAO) substrate exhibited epitaxial growth with excellent in- and out-of-plane texture. Experimental details are reported on the sol–gel synthesis chemistry and XRD and TEM characterization of the YBCO thin films. Also discussed is the underlying formation mechanism of the YBCO phase during the synthesis.

Monocore Bi-2223/Ag and Bi-2223/Ag–Mg tapes have been made by two axial (TAR) and eccentric rolling (ER). The pole figures gave a closer to reality distribution of c-axis orientation of crystallites for monocore Bi-2223 ceramics in comparison to standard evaluation by 'rocking curves'. Texturization of crystallites in the 'transverse' direction of the monocore Bi-2223 tape had a minor effect on the tape performance. For rolled tapes with relatively high aspect ratios () the crystallites' texturization in the 'rolling' direction was better then that in the 'transverse' direction. SEM observations at the Bi-2223/Ag interface were not objective for the judging of texturization, since the supercurrents flow beneath that interface.

The EPR g factors for the tetragonal NdBa₂Cu₃O_7−δ high-T_c superconductor are theoretically investigated on the basis of the perturbation formulae of the g factors for a 4f³ ion in tetragonal symmetry. The contributions to the g factors arising from the second-order perturbation terms, the admixture of various states and the covalency effect are included in these formulae. The relevant crystal-field parameters are determined from the superposition model and the local structural data of the tetragonal Nd³⁺ site in NdBa₂Cu₃O_7−δ. The calculated g factors are in reasonable agreement with the observed values. The validity of the results is discussed.

Structural and superconducting properties of magnesium diboride thin films grown by pulsed laser deposition on zirconium diboride buffer layers were studied. We demonstrate that the ZrB₂ layer is compatible with the MgB₂ two step deposition process. Synchrotron radiation measurements, in particular anomalous diffraction measurements, allowed us to separate MgB₂ peaks from ZrB₂ ones and revealed that both layers have a single in plane orientation with a sharp interface between them. Moreover, the buffer layer avoids oxygen contamination from the sapphire substrate. The critical temperature of this film is near 37.6 K and the upper critical field measured at the Grenoble High Magnetic Field Laboratory up to 20.3 T is comparable with the highest ones reported in literature.

A radically new approach to the fabrication of electrical machines based on an HTS coated conductor has been developed. Instead of long lengths of coated conductor tape being wound into coils in a conventional manner, we propose to use a combination of thin film deposition and patterning techniques with an essentially coaxial cylindrical geometry to produce superconducting multilayer coils. These patterned coated conductor cylinders can subsequently be configured in different ways to form a variety of superconducting electrical machines all based on highly manufacturable 'standard' cylindrical modules with high engineering current density. The cost benefits of such standardized manufacturing may be very significant in future applications of coated conductor. Multi-layer thin films of YBCO on buffered, curved Ni-based, textured substrates have been grown by pulsed laser deposition in order to demonstrate the feasibility of fabricating, in situ, a multi-turn coil on a cylindrical former by continuous deposition of sequential layers of superconductor and insulator.

We develop an effective two-dimensional dynamic interaction potential embodying the screening of holes as carriers by charge density fluctuations and by optical phonons leading to superconductivity in magnesium diborides. The MgB₂ system is viewed as an ionic solid containing boron layers with holes as carriers in a unit cell and a polarization function is set up that fulfils the appropriate sum rules on the electronic and ionic polarizabilities. The value of the electron–phonon coupling strength and of the Coulomb screening parameter obtained from the static dielectric function within the two-dimensional model indicates that the superconductor is in the strong coupling regime. The estimated superconducting transition temperature T_c of MgB₂ is about 43 K and the pressure derivative of T_c is negative. The isotope effect, coherence length, magnetic penetration depth and volume derivative of T_c are also estimated to be consistent with the earlier results. The implications of the effective interactive potential and its analysis are discussed.

The thermal conductivity κ(B,T) of polycrystalline superconducting MgB₂ has been studied as a function of temperature (from 4 up to 50 K) and (up to a rather large 14 T) applied magnetic field. No anomaly in the thermal conductivity κ(B,T) is observed around the superconducting transition in the absence or presence of magnetic fields up to 14 T; at that field value the superconductivity of MgB₂ persisted. The thermal conductivity in zero field shows a T-linear increase up to 50 K. The thermal conductivity is found to increase with increasing field at high fields. We interpret the findings as if there are two subsystems of quasiparticles with different field-dependent characters in a two- (L- and S-) band superconductor reacting differently with the vortex structure. The unusual enhancement of κ(B,T) at low temperature but higher than a ( T) critical field is interpreted as a result of the overlap of the low energy states outside the vortex cores in the S band. An order of magnitude of the B_c2S temperature dependence is given, in agreement with recent results by other authors under different experimental conditions.

The large cross-sectional aspect ratio of YBCO coated conductors leads to a large magnetization loss in an AC transverse magnetic field. In this work, the magnetization loss of a multifilamentary YBCO coated conductor was studied experimentally. A 100 mm length of striated multifilamentary YBCO coated conductor was prepared where the conductor and filaments were 10 mm wide and 0.4 mm wide, respectively. Striations in the sample were accomplished by laser ablation. The magnetization loss of this striated conductor as well as a reference non-striated conductor was measured in AC transverse magnetic fields normal to the conductor at various frequencies. The measured magnetization loss of the 100 mm striated conductor was less than 9% of the measured loss of the non-striated conductor at f = 11.3 Hz and H/H_cc = 8.8 (H_cc = I_c/πw_c; I_c: critical current, w_c: conductor width). Even though the coupling loss component increases the magnetization loss in the striated conductor, the AC loss reduction by striation is still apparent even at 171.0 Hz. The transverse resistance between filaments estimated by a four-probe measurement was 38 µΩ for 1 m at 80 K. The coupling length estimated using this transverse resistance is much longer than the sample length even at 171.0 Hz, suggesting that the filaments in the striated conductor are far from 'completely coupled' or 'saturated'.

The free energy of a two-band superconductor is obtained by taking into account all actual electron–phonon and Coulomb interactions. The temperature dependences of the superconducting gaps, the specific heat with its jump at T = T_c (T_c is the critical temperature) are calculated by minimizing the free energy. The theory is applied to the MgB₂ superconductor. The T_c isotope effect is estimated and the accordance with the experiment is obtained.

The low-frequency susceptibility of six Bi-2223/Ag superconducting tapes with the same 61 filaments but different cross-sectional aspect ratios is measured at 77 K as a function of the amplitude of the perpendicularly applied field, H_m, using a carefully designed and calibrated high-field ac susceptometer with necessary sample-length corrections. The measured results are compared with two-dimensional calculations performed based on the critical state model with constant critical current density or the −1 internal susceptibility at low fields. It is shown that although the fill fraction is about 0.55 for the filament-containing core, the measured low-field agrees well with the calculated critical state when the core is assumed to be completely superconducting, when the aspect ratio of the core ranges from 13 to 32. This indicates a strong magnetostatic coupling among the filaments. The same coupling is theoretically shown to reduce the field where maximum occurs, . However, the experimental is much higher than that even without coupling when the transport critical current density is used for the model calculation. This suggests that ac magnetization arises from all superconducting grains and the links between them, whereas the transport current is limited by the filament cross-section where grains are the worst linked, and the great discrepancy between magnetic and transport results is a consequence of the non-uniformity of the superconducting structure of the tape. Therefore, transport critical current can be significantly improved by ensuring the uniformity of the grain links.

The basic premise of a recent rapid communication (2004 Supercond. Sci. Technol. 17 L17) is that if a rectangular coil is in a homogeneous magnetic field and one leg of the coil is covered with a paramagnetic coating, there is then a net force on the coil. This premise is shown to be incorrect. Such a coil will experience no net force.

The PDF file provided contains web links to all articles in this volume.