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Sintered samples of MgB₂ were irradiated in a fission reactor. Defects in the bulk microstructure are produced during this process mainly by the ¹⁰B(n,α)⁷Li reaction while collisions of fast neutrons with the lattice atoms induce much less damage. Self-shielding effects turn out to be very important and lead to a highly inhomogeneous defect distribution in the irradiated samples. The resulting disorder enhances the normal state resistivity and the upper critical field. The irreversibility line shifts to higher fields at low temperatures and the measured critical current densities increase following irradiation.

We study the temperature dependence of the upper critical field H_c2(T) for superconducting magnesium diboride, MgB₂ in the vicinity of T_c using a two-band Ginzburg-Landau (G-L) theory. The temperature dependence of H_c2(T) near T_c exhibits a positive curvature. In addition, the calculated temperature dependence and its higher-order derivatives are also shown to be in good agreement with the experimental data. In analogy with the multi-band character of the Eliashberg microscopic theory, the positive curvature of H_c2(T) is described reasonably by solving the two-band G-L theory.

The high quality Er-doping Y_0.8Er_0.2Ba₂Cu₃O_7−δ (YErBCO) crystal is grown by the melted-textured-growth (MTG) method. The microstructure photograph shows that the MTG-YErBCO crystal has an obvious layered structure. Its magnetization loops show that the irreversibility field is decreased by Er-doping for H//c and that there is an anomalous behaviour, a second peak, in the magnetization curve for H//ab. The critical current density along the ab-plane (J_c^ab(77 K, 0 T) = 4.75 × 10⁴ A cm⁻²) is higher than that along the c-axis (J_c^c(77 K, 0 T) = 470 A cm⁻²). The field dependence of the irreversibility temperature T_irr follows a power law, H = H₀(1 − T_irr(H)/T_irr(0))^1.3, with H₀ = 550 T for H//ab and 51 T for H//c, respectively. The field dependence of the activation energy U_e shows a different behaviour for H//ab and for H//c, namely a linear relation, U = 49131–3006H, for H//ab and a power law, U ∝ H^−0.68, for H//c, respectively. The anisotropy factor γ = 15 was obtained from the angular dependences of the activation energy and the irreversibility temperature. A possible reason for the effect of Er addition on the flux-pinning behaviour has been discussed.

The microstructural dynamics of Bi-2223 tapes are investigated in situ during annealing in 8% O₂ by means of 100 keV x-ray diffraction. A green mono- and a green multi-filamentary tape are annealed at 829.5 °C. During ramp-up (Ca,Sr)₂PbO₄ decomposes above 750 °C, resulting in an incorporation of Pb in Bi-2212. The associated grain growth of Bi-2212 is the main cause of the strain relief and the c-axis grain alignment of the Bi containing phases. Above 825 °C the Bi-2212 partly dissociates into (Ca,Sr)₁₄Cu₂₄O_x and a liquid phase. The linewidth of Bi-2212 is constant during the transformation to Bi-2223, indicating no strain or finite-size broadening. The most probable transformation mechanism is found to be nucleation and growth with a fast decomposition of the individual Bi-2212 grain, followed by a growth of Bi-2223 from the Bi-2212 melt reacting with (Ca,Sr)₁₄Cu₂₄O_x. The multi-filamentary tape transforms faster and exhibits a much better degree of c-axis alignment. Results are compared to similar studies in air. During a final annealing of a fully converted tape no change is found in the average concentration, stoichiometry and grain misalignment. A post-annealing experiment at 650 °C on quenched tapes shows a reaction over 3 weeks, whereby randomly oriented 3222 grains are formed and Bi-2223 is depleted of Pb. Simultaneously, the critical current decreases by a factor of 2.

Bulk samples of MgB₂ have been formed by hot isostatic pressing (HIPping) of commercial powder at 100 MPa and 950 °C. The resulting material is 100% dense with a sharp superconducting transition at 37.5 K. Microstructural studies have indicated the presence of small amounts of second phases within the material, namely MgO- and B-rich compositions, probably MgB₄. Magnetization measurements performed at 20 K have revealed values of J_c = 1.3 × 10⁶ A cm⁻² at zero-field and 9.3 × 10⁵ A cm⁻² at 1 T. Magneto-optical (MO) studies have shown direct evidence for the superconducting homogeneity and strong intergranular current flow in the material.

The effects of Nd₂O₃ doping (from 0.25 to 1% wt of Nd₂O₃) of top-seeding-melt-texture-growth-processed YBa₂Cu₃O_7−δ (Y123) ceramics are investigated. The texturing thermal cycle of the pellets is similar to the one used to texture neodymium-free Y123 samples. The SEM study of the microstructures of the doped samples does not show significant differences compared to those of textured neodymium-free Y123 samples; the size of the Y₂Ba₁Cu₁O₅ particles (Y211) does not change and no secondary phase appears. From EDS analysis, neodymium is found in both the Y123 matrix and the Y211 particles (about 1.80% atom of neodymium in the Y211 particles and about 0.7% atom of neodymium in the Y123 matrix for 1% wt of Nd₂O₃). After the usual oxygen annealing process used for neodymium-free Y123 samples, the samples show poor superconducting properties. A second similar annealing does not improve these properties. A new annealing process at lower temperatures (between 430 and 300 °C) leads to samples with good superconducting properties, but only similar to those usually observed for neodymium undoped compositions.

We present measurements of the unloaded quality factor of superconducting microstrip resonators at 77 K and 3.86 GHz. The resonators were made of 0.3 μm thick YBa₂Cu₃O₇ films with a transition temperature of 90 K on both sides of ceria-buffered 3 inch sapphire wafers. In particular, we investigated the effect of a 0.3 μm thick gold contact layer on the resonator performance. It was found that the gold layer decreases the quality factor by a factor of almost 5. This result is due to an additional microwave loss in the gold film which can be quantitatively described by the impedance transformation rules for transmission lines. On the basis of the quantitative analysis, we suggest a selective removal of the contact layer by appropriate patterning in order to eliminate the extra loss without deterioration of the low-ohmic galvanic contact to the microwave housing. The experimental results demonstrate the usefulness of the proposed method. Their comparison with the calculated surface current density in the ground plane shows that the contact layer has to be in an area where the current is zero. Furthermore, the results reveal that the surface current density distribution in the ground plane and microstrip depends on the microwave power.

The morphological and compositional changes of the Pt-based phase in melt-grown Y-Ba-Cu-O samples grown on the Al₂O₃ substrate were studied. It was shown that Al dissoluted in the melt is effectively consumed by the Pt-based phase, by replacing Cu with Al until it becomes fully converted to the Pt, Al-based phase. Al substitution changes irregular shape of Pt-based particles into Pt, Al-based crystals with well-developed habits. Ag addition hinders Al diffusion into the Y-Ba-Cu-O sample during melt processing.

The Ca-doped YBa₂Cu₃O_7−δ/Y₂BaCuO₅ bulk-textured superconductors were prepared by using the top-seeding melt-textured growth process. Composition analyses were made for such prepared samples. We found that Ca concentration in the YBa₂Cu₃O_7−δ grains decreased along the growing direction; Ca preferred to substitute Y in the YBa₂Cu₃O_7−δ grains, rather than to stay at the grain boundary. The accumulation of extra CuO at the growth front was considered as one of the causes of the inhomogeneous distribution of Ca.

Quantized voltage across the unbiased terminals of a bicrystal YBa₂Cu₃O₇ dc SQUID exposed to microwave radiation has been observed at 77 K. Using this voltage as an indication of the microwave power coupled to the junctions, we identified three distinct regions in the I–V curves as the microwave power was tuned, similar to those reported for low-T_c SQUID. Pronounced double period steps were observed when the microwave radiation power was high enough to suppress the dc Josephson current completely. Half-integer steps also appeared as the applied dc-magnetic flux was close to half-integers of flux quanta. The experimental phenomenon was explained based on the resistively shunted Josephson junction model.

We study the angular dependence of E–J characteristics and dissipative properties due to thermal disturbance in the Bi-2223/Ag multifilamentary tape in the low-voltage region. In order to simulate the dissipative properties, we take into account the analytical expressions of the E–J characteristics as a function of temperature, external magnetic field and field direction, which agree well with the experimental data. It is shown that the simulation results of the dissipative properties agree with the experimental data semi-quantitatively. The relationship between the current sharing rate and its contribution to the temperature increase is also discussed, based on the simulation. These results are crucial for high-T_c superconducting power application systems with a persistent current mode where even low-power dissipation is detrimental to the performance.

Copper, iron and silver MgB₂ sheathed tapes have been manufactured under different conditions. It has been found that copper-sheathed tapes can show a higher critical current density than iron-sheathed tapes if heat-treated at temperatures below 850 °C. The influence of different overall mechanical deformation rates has been studied for tapes sheathed by all three types of metals. By increasing the deformation rate the critical current density was improved by about an order of magnitude in the case of the copper-sheathed tapes, while the critical current density of the iron-sheathed tapes remained constant.

Powder-in-tube (PIT) processing of BiSrCaCuO superconductor is widely used to introduce textured microstructure to high temperature superconductor tapes, thus effectively minimizing the weak-link effects caused by grain boundary misorientations. Although it was reported that PIT tapes have parabolic critical current density (J_c) distribution across the tape width, the role played by texture in this is not clearly understood. In this study, both the micro- and the mesotexture of PIT processed (Bi,Pb)₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) superconductor tapes were analysed in the transverse direction of Bi2223 tapes. Microtexture and mesotexture of PIT processed Bi2223 tapes were characterized using angle-axis pairs and Rodrigues–Frank vectors. The results of microtexture evaluation indicates that a/b axes texture did exist in PIT processed tapes while the mesotexture RF plot exhibits that majority of the grain boundaries were formed by grains with non-parallel c-axis. These grain boundaries generally had low mismatch angles of up to ∼10°. High-angle misorientation boundaries ranging up to 45° were generally associated with c-axis twist boundaries. The dominating misorientation angle for both sample sides and centre was found to be 4°. It is believed that the micro- and mesotexture distribution characteristics has influence over the J_c distribution in the transverse direction of Bi2223 tapes.

Harmonic ac susceptibilities, χ_n = χ_n' + i χ_n'', and ac magnetization of a metallic granular superconductor of MgB₂ have been measured as a function of temperature, ac field amplitude and additional small dc field in the temperature range between 15 and 45 K. Temperature- and amplitude-dependent measurements are similar to those obtained for a typical high-temperature superconductor. In addition, MgB₂ has a very narrow transition temperature width and high critical current densities. The experimental results have been compared to the numerical solutions of a model based on Bean's critical state. Odd harmonic susceptibilities (n = 3, 5 and 7) are in good agreement with those obtained numerically from the model. In contrast, the measured even harmonic susceptibilities exhibit a peak-like behaviour below T_c, with only small magnitudes of about 10⁻⁴ (SI). The results are interpreted in terms of flux penetration to the bulk of superconductor MgB₂.

A comparison of the history effects in weakly pinned single crystals of a high T_c YBa₂Cu₃O_7−δ (for H || c) and a low T_c Ca₃Rh₄Sn₁₃, which show anomalous variations in critical current density J_c(H), is presented via the tracings of the minor magnetization hysteresis loops using a vibrating sample magnetometer. The sample histories focussed are: (i) the field cooled (FC), (ii) the zero field cooled (ZFC) and (iii) an isothermal reversal of field from the normal state. An understanding of the results in terms of the modulation in the plastic deformation of the elastic vortex solid and supercooling across the order–disorder transitions is sought.

The vortex phase diagram in a weakly pinned crystal of YBa₂Cu₃O_7−δ (YBCO) for H || c is reviewed in the light of a recent elucidation of the process of 'inverse melting' in a bismuth cuprate system and the imaging of an interface between the ordered and the disordered regions across the peak effect in 2H-NbSe₂. In the given YBCO crystal, a clear distinction can be made between the second magnetization peak and the peak effect in between 65 K and 75 K. The field region between the peak fields of the second magnetization peak (H^m_smp) and the onset fields of the peak effect (H^on_pe) is not only continuously connected to the Bragg glass phase at lower fields, but it is also sandwiched between the higher temperature vortex liquid phase and the lower temperature vortex glass phase. Thus, an ordered vortex state between (H^m_smp) and (H^on_pe) can get transformed to the (disordered) vortex liquid state on heating as well as to the (disordered) vortex glass state on cooling, a situation analogous to the thermal melting and the inverse melting phenomenon seen in a bismuth cuprate.

Precursor films were deposited by the co-evaporation technique using Y, BaF₂ and Cu as evaporation sources. Then, the films were annealed at low-pressure oxygen atmosphere without the introduction of water vapour, which was different from the so-called BaF₂ex situ process. Previous studies have indicated that the c-axis-oriented YBCO films can be prepared by annealing at low-pressure oxygen atmosphere, and that the reaction of the precursor films during sample temperature elevation is very important in the present process according to reflection high-energy electron diffraction (RHEED) observation. Therefore, two kinds of samples, rapidly cooled down from 400 °C and 630 °C, were prepared for microstructural study. YBCO film growth in this process is discussed on the basis of the results of x-ray diffraction (XRD), RHEED and transmission electron microscopy (TEM).

In situ and ex situ MgB₂ wires were prepared by the powder-in-tube method. Copper and silver tubes were used as a cladding material. AC susceptibility measurements revealed a small anomalous decrease with onset around 50 K. This effect persisted also when the wires were ground into powders. Electron microscopy and x-ray studies were performed on copper clad samples. Spectroscopic measurements in a SEM showed that regions contained either Cu or Mg and B. X-ray diffraction gave the major crystalline phases as Cu, MgCu₂ and MgB₂. Diffraction evidence for Cu substituting in the Mg position was inconclusive.

A demonstration sub-system for mobile communications was developed. In this system, a high-temperature superconducting (HTS) microstrip bandpass filter on r-plane sapphire substrate was designed to have a quasi-elliptic function response, which was aimed at covering the whole receive band of DCS 1800 base station, i.e. 1710–1785 MHz. This filter was then integrated with a low noise amplifier (LNA) and a pulse tube cryocooler, forming a demonstration sub-system. Satisfactory performance of this system was achieved.

We fabricated and tested a high-temperature superconducting (HTS) sigma–delta modulator using interface-engineered Josephson junctions. Proper dc-characteristics were obtained with a sampling frequency of 100 GHz at 20 K. To achieve high-speed sampling, Josephson junctions with a higher critical-current normal-resistance product were used. Circuit simulations with thermal noise were performed for the analysis of switching characteristics and SFQ dynamics.

For the ferromagnetic metal/superconductor (FM/S) superlattices, the new πmagnetic states with antiferromagnetic ordering of the FM layers magnetizations are predicted. If the S layers thickness d_s is less than the threshold value d_s^π, these new states have a higher critical temperature T_c than the earlier known ferromagnetic states (the 0 magnetic states). Therefore, the T_c oscillations origin at d_s < d_s^π is due to the transitions cascade between the 0–π–0 types of superconductivity at π magnetism conditions. A new type of logical device combining the advantages of the superconducting and magnetic recording channels in one sample is offered on the FM/S superlattices base.