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We have developed a numerical technique to estimate the current carrying capability of HTS Roebel cable composed from coated conductor strands. The influence of self-field on the critical current density is studied computationally for a Roebel cable using a realistic field and angle dependence behaviour of critical current. The computations are carried out for N /2 (number of strands/strand width in mm), and N /5 Roebel cable for N = 2–15. The local current distribution in each strand satisfies the self-consistent criteria J = J c ( B ( J )) except for a small region where the current density is set to zero to maintain the condition of equal currents in all strands. The variation of critical current with vertical and horizontal separation between the strands is also investigated. Results are compared with the measured values of critical current for some of our cables. The comparison shows an error of up to 10% which we attribute mostly to the model not accounting for the spread in I c values of the constituent strands.

We report measurements of the anisotropic thermopower in the normal state of YBa 2 Cu 3 O 7 − δ thin films with three different orientations: a -axis perpendicular to the plane of the substrate; c -axis perpendicular; and coherently tilted on a (305) SrTiO 3 substrate respectively. We have also measured the thermopower of magnetically aligned YBa 2 Cu 3 O 7 − δ ceramic samples. The thermopower in the direction of the c -axis is found to be positive and increases as the temperature increases. The thermopower along the b -axis direction has only a small temperature dependence, while the averaged thermopower in the ab -plane in c -axis films decreases as temperature increases. These results are consistent with those made on single crystals.

Y 2 Ba 4 Cu 7 O 15−δ superconductors with similar oxygen contents but different T c values have been produced by varying the reaction conditions. Resistivity, 89 Y NMR and thermopower measurements were used to ascertain the origin of the different T c 's. We find that the sample with the lowest T c has a lower hole concentration on the CuO 2 planes and the normal-state energy gaps are similar to those found in oxygen-depleted YBa 2 Cu 3 O 7−δ superconductors. Thus the electronic behaviour of Y 2 Ba 4 Cu 7 O 15−δ is similar to that of YBa 2 Cu 3 O 7−δ and different T c values can be attributed to different hole concentrations on the CuO 2 planes and not to different pairing potentials or pair-breaking. The difference in the temperature dependence of the thermopower between these samples can be understood in terms of the different relative CuO chain and CuO 2 plane contributions.

Far-infrared reflectivity measurements on ceramic samples of YBa 2 Cu 4 O 8 , a high- T c superconductor with T c = 80 K, were made over the temperature range of 20–300 K and frequency range of 20–1000 cm −1 . Six phonons were observed at 130 cm −1 , 192 cm −1 , 300 cm −1 (doublet), 500 cm −1 , and 603 cm −1 and their temperature dependences were studied. Through comparisons with 1–2–3 compounds, these phonons were identified and assigned to the respective lattice vibrations of B 1u symmetry (i.e. along the c -axis). The observed temperature dependences of phonon frequencies, lineshapes and reflectivity ratios all point to the existence of two superconducting energy gaps, one in the c direction at about 150 cm −1 and another (somewhat more speculatively) in the ab plane at about 450 cm −1 . While the c -axis gap occurs at a nearly BCS-like value of ∼ 2.7 kT c , the a−b plane gap corresponds to ∼ 8 kT c , in agreement with the work of others on single crystals of YBa 2 Cu 3 O 7 .

We report the preparation and characterization of the homologous series of high temperature superconductors, Y2Ba4Cu6+nO14+n−ξ with n=0 (123), n=1 (247) and n=2 (124) as a function of Ca and La substitution through measurement of Tc, lattice parameters, selected Rietveld refinements and thermogravimetric analysis. Solubility limits of Ca and La are investigated and found to be widely different for each member. Data is presented showing that Ca substitutes on both the Y- and Ba- sites and as the Ca concentration increases the relative fraction on the Y site decreases. Systematic trends indicate that for n=0 123 is slightly overdoped, 247 is slightly underdoped and 124 is well underdoped relative to Tc(max) such that 0.06 La substitution, 0.04 Ca substitution and 0.15 Ca substitution is required to maximise Tc in 123, 247 and 124, respectively. Differences in the orthorhombicity between the members of the series as a function of substitution are interpreted in terms of differences in hole doping and in oxygen stoichoimetry for each member. For instance, the observed rapid decrease in the orthorhombicity in 124 at a high La content is interpreted as evidence that oxygen loads into the interchain site. This destabilises the ribbons leading to the formation of tetragonal 123.

Most high- T c superconductors require loading or unloading of oxygen after synthesis, in order to maximise the transition temperature, T c . The rate of oxygen diffusion in dense, bulk material, is so slow as to be prohibitive, or at best calls for a lengthy manufacturing process. We report here that by a variety of aliovalent atomic substitutions in YBa 2 Cu 3 O 7−δ , the rate of oxygen loading can be increased by up to 100 times allowing a large decrease in processing time for bulk material.

We present the thermopower (TEP) within the ab plane and c axis of magnetically aligned YBa 2 Cu 3 O 7−δ ceramics with different oxygen contents. For the sample with almost fully loaded oxygen content ( δ = 0.05), TEP exhibits strong anisotropy, namely, the ab -plane TEP behaves distinctly from that along the c axis. The ab -plane TEP bears remarkable similarities to that of the b -axis in an untwinned YBa 2 Cu 3 O 7−δ ( δ = 0.08) single crystal. The c -axis TEP is nearly identical to that of a twinned YBa 2 Cu 3 O 7−δ single crystal. These measurements make available the anisotropic property measurements of ceramic samples by aligning polycrystallites using magnetic forces.

The universal phase behaviour of high T c superconductors is considered in a space of temperature, field and hole-concentration and its essential features are explained in terms of several energy and length scales.

Temperature-dependent infrared reflectivity of ceramic samples of YBa 2 Cu 4 O 8 and substituted YBa 2 Cu 4 O 8 are presented. Substitutions include rare earth ions for Y, and part substitutions of Zn for Cu, Ca for Y and for Ba. It is observed that the phonon near 300 cm −1 softens as the temperature is lowered and that the softening begins well above T c . The magnitude of the softening and the softening onset temperature change with T c reflecting a dependence on the hole concentration.

Phonon modes for a series of La substituted YBa 2 Cu 4 O 8 ceramic samples are found to be sensitive to the level of charge on the Cu-O planes. In addition a mode involving mainly motion of the apical oxygen is found to display a strong coupling to the electronic background that increases with decreasing hole concentration.