Table of contents

Thick films of yttrium barium copper oxide have been prepared by spinning a suspension of YBCO powder in an organic medium onto a substrate. Films produced have a superconducting onset of 90 K and zero resistance at 79 K. The effects of substrate composition, barrier layers and process parameters have been investigated.

The oxidation of cold copper films in vacuum using a flux of oxygen radicals is reported. Also atomic hydrogen can be usefully employed to remove surface contaminants and to reduce the oxide controllably.

The authors have used differential scanning calorimetry and X-ray diffraction to study absorption and desorption of oxygen by superconducting Y₁Ba₂Cu₃O_7-x. They show that these reactions are reversible and discuss them in terms of underlying thermodynamic and kinetic processes. In particular, they measure the heat evolved during absorption of oxygen and correlate the forms of absorption peaks on DSC traces with the rate of ingress of oxygen. This rate is presumed to be limited by the speed with which oxygen atoms can diffuse through the superconducting phase. They find values for the activation energy and pre-exponential factor for diffusion of oxygen through the superconducting material and suggest guidelines for annealing treatments intended to optimise the oxygen content and superconducting properties of Y₁Ba₂Cu₃O_7-x.

The authors have fabricated bulk samples of high-T_c oxide superconductors with narrow inductive T_c transitions ( approximately 3 K) and low resistivities ( rho (100 K):250-1000 mu Omega cm). They find such samples to have improved transport critical current density (J_ct) properties. Although J_ct decreases rapidly with increasing field for fields <1 T, J_ct is then approximately independent of field over the range 2-15 T at 4.2 K; at 77 K the field dependence is also much reduced. J_ct is about the same whether the applied current is parallel to or perpendicular to the applied field. The high-field values of J_ct are, however, still small (<or approximately=50 A cm^-2), but are much higher than previous samples in which S-N-S tunnelling controlled J_ct, reducing it to less than 1 A cm^-2 in fields of approximately 1 mT at 77 K. DC magnetisation data demonstrate that the local critical current density within the individual grains is as high as 10⁷ A cm^-2 in fields up to 20 T at 4.2 K. The evidence that the low J_ct is the result of easy flux shear along the grain boundaries, as well as that suggesting percolative processes operate, is discussed.

The absorption of microwave radiation by the high-temperature superconducting ceramic Y₁Ba₂Cu₃O_y in nearly zero magnetic field is extremely sensitive to the applied magnetic field. The spectra reported in the literature are very noisy. The authors show that this noise is not inherent to the superconducting system but is due to environmental magnetic noise. This noise can easily be removed by proper magnetic shielding which allows the ceramic to be used, without any further materials processing, as a very low magnetic field sensor. They also report the first measurements of this absorption in a single-crystal sample and show why certain explanations about the nature of this absorption can be ruled out.

YBa₂Cu₃O_{7- delta} multi-crystalline ceramics were shaped into one-turn coils. The RF performance of the coils was evaluated by measuring the Q-value at 77 K from 1.3 to 270 MHz and comparing it with that of a Cu coil of the same geometry. The coils were also tested as NMR detection coils. In a poorly magnetically shielded environment the multicrystalline YBa₂Cu₃O_{7- delta} performed less well than Cu in the frequency range studied. Q was significantly lowered by applying a magnetic field. The temperature dependence of the Q-value also was measured at 100 MHz, from 77 K to well above the superconducting phase transition temperature.

The resistivity of superconducting single-crystal Bi₂Sr₂Ca₁Cu₂O_y, with its zero-resistance temperature at 85.8 K, has been found to possess a quasi-two-dimensional anisotropy characteristic, with a typical metallic behaviour in the direction parallel to the a-b plane, and a 'semiconducting-like' temperature dependence in the c direction.

Magnetisation hysteresis measurements have been performed at T=4.2 K and T=77 K on tubular samples of YBa₂Cu₃O_7-y. There is clear evidence of two hysteresis loops. The first, at very low magnetic fields, represents a total-body superconducting behaviour with a shielding current circulating in the whole sample; the second one, at high magnetic fields, represents the hysteresis of single grains (or superconducting regions of smaller dimensions). This model emerges also from various bulk magnetisation and AC susceptibility measurements, as has been pointed out elsewhere. The particular geometry used assigns to the identified contributions opposite signs and it is potentially able to separate them. The critical currents related to the two hysteresis loops are very different in value (about four orders of magnitude at T=4.2 K and two-to-three orders of magnitude at 77 K). A preliminary evaluation of some critical fields related to both the hysteresis loops is given.

The effective Hamiltonian of strongly correlated electrons on a square lattice is replaced by a renormalised Hamiltonian and the factors that renormalise the kinetic energy of holes and the Heisenberg spin-spin coupling are calculated using a Gutzwiller approximation scheme. The accuracy of this renormalisation procedure is tested numerically and found to be qualitatively excellent. Within the scheme a resonant valence bond (RVB) wavefunction is found at half-filling to be lower in energy than the antiferromagnetic state. If the wavefunction is expressed in fermion operators, local SU(2) and U(1) invariance leads to a redundancy in the representation. The introduction of holes removes these local invariances and the authors find that a d-wave RVB state is lowest in energy. This state has a superconducting order parameter whose amplitude is linear in the density of holes.

The temperature dependence of the complex AC diamagnetic susceptibility chi = chi '-j chi " of an inhomogeneous Y-Ba-Cu-O superconducting sample with multiphase composition Y_1.2Ba_0.8Cu₁O_y has been measured from 4 K to room temperature in the presence of DC magnetic fields in the range 3.2*10³ to 2.4*10⁴ A m^-1 (40 to 300 Oe). For small AC field values H_ac<1 A m^-1 RMS, the onset of superconducting diamagnetism is apparent at 87 K, while zero-field DC resistivity measurements indicate a broad superconducting transition from 90 to 75 K, where rho vanishes completely. The behaviour of chi ' and chi " and the effects of applied DC fields indicate two distinct contributions to the diamagnetic response of the sample. The first is associated with a peak in chi " at T approximately=60 K and is due to the transition to superconductivity and the Meissher effect in the high-T_cYBa₂Cu₃O_7-x phase. The second contribution is associated with a peak in chi " at T approximately=20 K and is believed to be due to proximity effects in the inhomogeneous sample.

The AC losses are calculated for superconducting filaments in the regime where the filaments are smaller than lambda ' and the amplitude of the oscillation is low. In a bias field analytic expressions can be obtained by assuming that the effect of pinning forces on the field distribution is small. In zero applied field the losses are still further decreased due to an increase in the effective value of B_c1. The results are in qualitative agreement with the experimental results of Sumiyoshi and co-workers (1986).

The authors have calculated the total energy and equilibrium volumes of La₂CuO₄ and La_1.5Sr_0.5CuO₄ using the local density approximation to density functional theory (DFT). The calculations were performed using the ASA LMTO method under the constraint of a fixed c/a ratio. The calculated equilibrium volumes are in good agreement with experiment indicating that DFT provides a useful starting point for a discussion of the energetics of this class of superconductor. In the strontium compound they find local charge redistributions that result in holes in the p band of the out-of-plane oxygen atoms. These local charge rearrangements invalidate the use of the rigid band and virtual crystal models for describing alloying.