K.N.R. Taylor

Two classes of growth spirals on single crystals of YBCO

Abstract A detailed examination of the (001) surfaces of CuO-flux grown single crystals of YBCO (YBa 2 Cu 3 O 7− x ) has shown the presence of two types of growth feature. The first of these consists of relatively large growth steps which can be in the form of spirals, arcs, instabilities or complex intergrowths which are similar to those reported by a number of workers. The second type involves much more conventional growth spirals with a step height close to the limit of optical DIC detection and which only become readily visible after decoration. The nature of these two classes of growth feature is discussed.

Enhanced critical current density in melt-textured (Y1−xPrx)Ba2Cu3Oy

Abstract We have investigated the dependence of the magnetisation critical density J c of Pr-substituted YBa 2 Cu 3 O y on the percentage praseodymium content (for 0, 2, 4 and 8% substitution) using a vibrating sample magnetometer. Although the critical temperatur of these pseudo-quaternary compouds Y 1− x Pr x Ba 2 Cu 3 O y decreases gradually as x increases, a maximum in the critical current density is observed for x = 0.04. At this composition the critical current density is about three times higher than that in a pure YBa 2 Cu 3 O y sample prepared in the same way. From a discussion of the possible mechanisms it is suggested that J c increases as a result of pinning force enhancement due to Pr substitution. This may be associated with the fact that the atomic volume of Pr is larger than that of Y. It is also possible that Pr-containing compound particles (123 or 211) are finely dispersed in the Y 123 superconducting phase leading to increased pinning, and consequently to higher J c values.

Flux trapping energies in YBCO in the presence of a transport current

Abstract The time dependence of the residual voltage in specimens of YBCO driven into the mixed state by the simultaneous presence of an external magnetic field and a transport current, has been used to determine the flux pinning energies for various sample types in the presence of a transport current. The time decays vary strongly from specimen and there is a good correlation with the critical current density. However, the derived values of the pinning energies are remarkably similar and only for the highest critical current density smaple was a significant current dependence observed.

Josephson behaviour for high critical current density YBCO+Ag thick films on YSZ substrates

Abstract High critical current density thick films of yttrium barium copper oxide plus 10 wt.% silver have been fabricated on yttria stablised zirconia substrates. The films are approximately 10 μm thick, superconducting with T c ( R = 0) in the range 90.5 to 91.0 K and are found to have j c s close to 3000 A cm -2 at 77 K in zero applied magnetic field. The thermal cycle for the production of these films requires the use of temperatures in excess of the peritectic and therefore involves melt processing. This has a number of advantages, including film texturing, which is significantly improved with the addition of Ag, and the formation of an inert BaZrO 3 barrier layer which has fine CuO needles dispersed throughout. This layer prevents atomic diffusion and film poisoning. At 77 K, the j c values decreased markedly for applied magnetic fields increasing from 0–20 Oe, and then decreased slowly for increasing fields, being almost constant for field values ∥#62; 150 Oe. The temperature variation, near T c , of the critical current i c was best described by i c £ (1 − T / T c ) 1.52 , the Ginzburg-Landau 3 2 power law characteristic, and indicates that the granular films may be modelled as an array of Josephson-coupled grains which have a short coherence length. Variable thickness bridges patterned into the thick films showed very clear DC and AC characteristics that were very similar to those expected from a single Josephson junction.

Intersections of twins and of optical domains in crystalline YBa2Cu3O7-δ

Abstract A study of the optical domain and twin structure in single crystals of YBa 2 Cu 3 O 7-δ has shown the presence of interesting features of the microstructure in regions where two sets of orthogonal twins intersect. The observed features are interpreted in terms of a simple model in which the resultant microstructure of the intersection region is related directly to the oxygen ordering, and hence to the orthorhombic strain in the individual optical domains. It is shown that the detailed structure might be anticipated on energetic grounds. Convergent beam electron diffraction (CBED) techniques have been used to establish the variations in structure in this area.

High current capacity textured thick films of YBCO on YSZ obtained by melt processing

Abstract By processing thick films of YBCO on stabilized zirconia at temperatures in excess of the peritectic value critical current densities in excess of 103 A cm−2 have been obtained at 77 K in film thicknesses of ≈ 40 μm. The microstructures of these films include extensive, crystalline sheets which appear to have been produced from a liquid phase system. These sheets are strongly textured with the c-axes perpendicular to the substrate. With increasing processing temperature, severe microcracking appears, resulting in a major limitation on the current carrying capacity. Variable thickness microbridges cut into the films exhibit many features characteristic of Josephson junctions. The addition of silver to these thick films during processing greatly improves their surface morphology, increases the degree of texture obtained and results in critical current densities in excess of 1300 A cm−2 at 77 K.

Effects of helium absorption on the superconducting mechanism of YBa2Cu3Oy

Abstract During the course of a series of measurements of the electrical conductivity and Hall effect in the YBa2Cu3Oy superconductors, it was observed that the presence of an helium atmosphere during the measurements led to reversible changes in the electrical properties including increases in the critical temperature. The results of a series of measurements to investigate this effect are described and possible mechanisms considered. It is suggested that helium enters the lattice and leads to different internal pressure effects in different parts of the unit cell.

Neutron scattering study of the structure of Y2BaCuO5 at 294 K and 77 K

Abstract Single phase Y 2 BaCuO 5 has been studied using neutron diffraction at 294 K and 77 K. We found the lattice parameters to a = 7.1343 (2) A, b = 12.1820 (4) A and c = 5.6607 (2) A at 294 K and a = 7.1225 (3) A, b = 12.1651 (4) A and c = 5.6542 (2) A at 77 K using the Pbnm space group. We found no evidence for any structural transition in this temperature range.

Josephson behaviour of variable thickness bridges in textured YBa2Cu3O7−δ

Abstract The i - V characteristics of textured yttrium barium cuprate have been measured using variable thickness bridges (VTBs) fabricated from bulk granular material. The results show the characteristic DC Josephson effect and, using microwave irradiation, the presence of integral and sub-integral Shapiro steps in the characteristics. In addition a strong field dependence of the critical Josephson current is also found. The V - H variation at various junction currents shows a well developed periodic structure with features that may be associated with flux trapping in itergranular regions. These features, combined with the temperature dependence of the Josephson current support the view that we are seeing the behaviour of a small number of Josephson junctions in these VTBs. Comparable measurements on non-aligned material do not show these effects, probably because of the wide range of intergranular misorientation across the bridge.

High critical density thick films of YBCO on YSZ

Abstract High critical current density thick films of yttrium barium copper oxide have been fabricated on yttrium stabilized zirconia substrates. The films are approximately 40 μm thick, superconducting with Tc(R=0) ≈91.5 K and are found to have jcs in the range 750 to 1200 A cm-2 at 77 K in zero applied magnetic fields. This jc range was found to correspond to a 5°C variation in the highest processing temperature. At 77 K, jc decreased rapidly for applied magnetic fields, 0–20 Oe, but was almost constant for field values 100 Oe.