Table of contents

Strong evidence for high intergranular critical current densities and large bulk magnetic flux pinning in superconducting polycrystalline MgB₂ has been observed. The presence of strongly-coupled grain boundaries in this material has been confirmed by a dramatic collapse of the magnetic hysteresis loop when a bulk specimen is ground into a fine powder and re-measured under similar conditions. Further evidence for strong intergrain links in polycrystalline MgB₂ is provided by the continuous variation of the remanent magnetic moment up to the full penetration field of a bulk sample. The absence of weak-link nature in this material has profound implications for its potential in a wide range of engineering applications.

An experimental study of the axial strain on I_c degradation of multifilamentary Bi(2223)/Ag tapes by different methods has been performed. The tapes were stressed by three kinds of tensioning set-up (expanded turn, short straight sample and U-shape spring) and the transport current versus strain I_c(ε) was measured. A comparison of I_c(ε) curves showed that the applied tensioning system may influence the measured sensitivity of Bi-2223 filaments, apparently against the tension strain. The lowest strain limit for critical current degradation (ε_irr) was observed for the expanded turn and the highest limit for the U-shape spring system. The observed differences in I_c(ε) curves are discussed and analysed.

This review deals with the properties of superconductors with competing electron spectrum instabilities, namely, charge-density waves (CDWs) and spin-density waves (SDWs). The underlying reasons of the electron spectrum instability may be either Fermi surface nesting or the existence of Van Hove saddle points for lower dimensionalities. CDW superconductors include layered dichalcogenides, NbSe₃, and compounds with the A15 and C15 structures among others. There is much evidence to show that high-T_c oxides may also belong to this group of materials. The SDW superconductors include URu₂Si₂ and related heavy-fermion compounds, Cr-Re alloys and organic superconductors. We review the experimental evidence for CDW and SDW instabilities in a wide range of different superconductors, and assess the competition between these instabilities of the Fermi surface and the superconducting gap. Issues concerning the superconducting order parameter symmetry are also touched upon. The accent is put on establishing a universal framework for further theoretical discussions and experimental investigations based on an extensive list of available and up-to-date references.

Rapid advances in the performance of superconducting cavities have made RF superconductivity a key technology for accelerators that fulfil a variety of physics needs: high-energy particle physics, nuclear physics, neutron spallation sources and free-electron lasers. New applications are forthcoming for frontier high-energy physics accelerators, radioactive beams for nuclear astrophysics, next-generation light sources, intense proton accelerators for neutron and muon sources. There are now nearly one kilometre of superconducting cavities installed in accelerators around the world, providing more than 5 GV of acceleration. The most recent installation of 20 m for a free-electron laser realized an average gradient a factor of four higher than existing applications. Improved understanding of the physics of RF superconductivity, together with advances in technology, are responsible for the spectacular increases in performance. RF superconductivity is a mature science going well beyond technological know-how and trial-and-error approaches to genuine understanding of the underlying physics. Research continues to push performance levels towards the theoretical limit, which is another factor of two higher than the levels yet achieved.

A detailed theoretical qualitative, as well as quantitative, analysis of the influence of the dimensions of Bi(2223)/Ag based cylindrical magnets on the energy stored in the magnet's winding was performed for various operating temperatures. The results achieved can be used to consider the potential suitability of the high-temperature superconductivity magnets for the purposes of micro-superconducting magnetic energy storage applications. A mathematical model which enables one to calculate the values of basic parameters, such as the critical current and stored energy of cylindrical magnets consisting of the set of Bi(2223)/Ag pancake coils, was developed with respect to the real distribution of the magnetic field in the winding and the angular dependence (anisotropy) of the I_c(B) characteristic of the tape. An example of a detailed analysis of the influence of the winding geometry, which is changed within the same overall length of 1, 2 and 5 km of the multifilamentary Bi(2223)/Ag tape, was performed at the temperatures of 77, 65 and 4.2 K. The most interesting and important result achieved is that the geometry of the winding that corresponds to the maximum stored energy differs according to the temperature. The disc-shaped magnets, which consist of a very low number of pancake coils, are the most suitable solution at 77 and 65 K. Simultaneously, the value of the stored energy is practically independent of the bore diameter of the magnet. On the other hand, when looking for the optimum winding geometry at 4.2 K, the magnets with the smallest bore diameter are more suitable, while the value of the energy stored does not depend on the number of pancake coils.

In the present work, the harmonic magnetic susceptibilities of the high-T_c superconductors are examined and corresponding harmonic susceptibilities are calculated employing a model by Kim and coworkers. According to Kim's model, the critical-current density, J_c is a function of the internal magnetic field H_i, J_c(H) = k/(H_o + |H_i|) where k and H_o are constants. In the model calculations, full penetrations field, H_p, is taken to be H_o[(1 + p²)^1/2-1] where p = (2ka)^1/2/H_o. As a result of the comparison of model calculations with experiments, we conclude that the harmonic susceptibilities for lower p values (p<10) correspond to the similar characteristic magnetic behaviour of granular superconductors, while the harmonic susceptibilities for higher p values correspond to the behaviour of homogeneous superconductors such as single crystal or samples with contiguous oriented grains.

For numerical magnetostatic calculations involving superconductors in the perfect Meissner state, we replace the traditional boundary condition of zero normal magnetic field at a superconducting surface by that of zero tangential field at the inside of any such surface.

A new class of `powder-in-tube' Mg-B superconducting conductors has been prepared using two different methods: an in situ technique where an Mg + 2B mixture was used as a central conductor core and reacted in situ to form MgB₂, and an ex situ technique where fully reacted MgB₂ powder was used to fill the metal tube. Conductors were prepared using silver, copper and bimetallic silver/stainless steel tubes. Wires manufactured by the in situ technique, diffusing Mg to B particles experienced ~25.5% decrease in density from the initial value after cold deformation, due to the phase transformation from Mg + 2(β-B)→MgB₂ all with hexagonal structure.

A comparative study of the intergranular current and grain connectivity in wires was conducted by AC susceptibility measurements and direct four point transport measurements. Using a SQUID magnetometer, magnetization versus magnetic field (M-H) curves of the round wires before and after sintering and reactive diffusion were measured at 5 K and in magnetic fields up to 5 T to define the J_cmag. The direct current measurements were performed in self field at 4.2 K. A comparison between zero-field-cooled (ZFC) and field-cooled (FC) susceptibility measurements for sintered Ag/MgB₂, and reacted Cu/Mg + 2B conductors revealed systematic differences in the flux pinning in the wires which is in very good agreement with direct high transport current measurements.

High-quality Tl₂Ba₂CaCu₂O₈ films for microwave filter application are prepared on both sides of 2 inch lanthanum aluminate substrates as well as on double-sided CeO₂ coated r-cut sapphire substrates. The films have thicknesses as high as 1 µm in the case of lanthanum aluminate substrates. On CeO₂ buffered sapphire substrates it is possible to prepare crack-free superconducting films as thick as 400 nm. For both substrate materials we reach high critical current densities of J_c>2×10⁶ A cm^-2 at 77 K. The microwave surface resistances R_s were measured to fall short of 100 µΩ at 5.6 GHz and 77 K in the low-field case, comparable to the best films made of YBa₂Cu₃O_7-x material. The films were able to tolerate high microwave power levels corresponding to magnetic field amplitudes of B_RF>2 mT. The critical temperature exceeds 102 K. This excellent performance is maintained even after several thermal cycles. The films were used to produce four-pole band-pass filters for the C-band which were operated without any detectable loss in their transfer characteristics up to temperatures of about 85 K. This is the first time that large area (2 inch) double-sided Tl₂Ba₂CaCu₂O₈ films have been prepared on sapphire with excellent critical current density in conjunction with low surface resistance.

The granular Gd_1-x-zPr_xCa_zBa₂Cu₃O_7-δ (GdPrCa-123) high-temperature cuprate superconductor samples were prepared and characterized. The electrical resistivity of Pr-free samples show a reduction in T_c with increasing Ca content. For these samples, the T_c(z) curve is nonlinear and a plateau appears at z≈0.05-0.15. With increasing Ca concentration, the lattice parameter a increases. Moreover, it is observed that the in-plane Cu(2)-O(2) bond distance increases with increasing Ca content. On the other hand, the Cu(2)-O(2)-Cu(2) angle increases and the CuO₂ plane becomes flatter with successive Ca doping, which causes the suppression of superconductivity. The same bond distances in Pr-Ca substituted samples decrease slowly with increasing Ca doping, but increase with increasing Ca concentration after an optimum value. In addition, the electrical resistivity of Pr-Ca-doped samples shows an increase of T_c with Ca doping up to an optimum value, whereafter it begins to decrease. We suggest that the competition of two effects, the number of holes and the flattening of CuO₂ planes by Ca doping, results in a nonlinear decrement in the T_c(z) curve.

Y-BaF₂-Cu precursor films for Y-Ba₂Cu₃O_7-y (YBCO) coated conductors have been deposited on both uncoated nickel and Rolling Assisted Biaxially Textured Substrate (RABiTS^TM) tapes by electron beam coevaporation. The water partial pressure was kept constant at 1×10^-5 Torr during the deposition in order to control the oxygen content in the precursor films. Rutherford backscattering spectroscopy measurements indicate an average composition of Y:Ba:Cu of 1.05:2.10:3.0 with a standard deviation of less than 2.0%. This result was obtained for both moving and stationary tapes. The thickness variation of such precursor films is less than 2.5% over lengths of up to 1 m. Films up to 5 µm thick have been deposited on a stationary tape. An end-to-end J_c of 100 000 A cm^-2 from a 1 m length of RABiTS^TM sample has been obtained with the highest J_c of 774 000 A cm^-2 in a short section within the tape. The present research demonstrates a possible route for industrial scale-up using electron beam coevaporation of YBCO precursors.

Flux pinning and the peak effect of Y_xHo_1-xBa₂Cu₃O_y melt-textured samples fabricated by a powder-melting-process method are investigated by a SQUID magnetometer in a magnetic field parallel to the c-axis of the samples. It has been found that the partial substitution of Ho for Y can lead to a strong enhancement of flux pinning over a wide temperature and field range in melt-textured YBa₂Cu₃O_y samples. A J_c value as high as 1.3×10⁵ A cm^-2 at 77 K and in self-field has been achieved in the Y_0.4Ho_0.6Ba₂Cu₃O_y sample. The enhancement of flux pinning may be attributed to the reduction of Y₂BaCuO₅ particles, the higher density of stacking faults and the paramagnetic moment of Ho³⁺. Contrary to the pure YBa₂Cu₃O_y sample, a peak effect can be observed below 77 K in H∥c in the Ho-added superconductors.